EX BIBLIOTHECA FRANCES A. YATES THE Story of the Heavens. 1 Digitized by the Internet Archive in 2014 https://archive.org/details/storyofheavensOOball_0 PLATE I. CASSELL &COM?ANV. LIMITED, LrTH.LO.VnON. THE PLANET SATURN, IN 18 7 2. THE Story of the Heavens BY SIR ROBERT STAWELL BALL LL.D. Author of " Star -Land" FELLOW OF THE ROYAL SOCIETY OF LONDON, HONORARY FELLOW OF THE ROYAL SOCIETY EDINBURGH, FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY, HONORARY MEMBER OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY, VICE-PRESIDENT OF THE ROYAL IRISH ACADEMY, SCIENTIFIC ADVISER TO THE COMMISSIONERS OF IRISH LIGHTS, ANDREWS PROFESSOR OF ASTRONOMY IN THE UNIVERSITY OF DUBLIN, AND ROYAL ASTRONOMER OF IRELAND. SSUtI) €tg|)teen. Coloured pates ani numerous 31llustrations NEW AND REVISED EDITION CASSELL and COMPANY Limited LONDON PARIS & MELBOURNE 1893 ALL RIGHTS RESERVED PREFACE TO ORIGINAL EDITION. I have to acknowledge the kind aid which I have received in the preparation of this book. Mr. Nasmyth has permitted me to use some of the beautiful drawings of the Moon, which have appeared in the well-known work published by him in conjunction with Mr. Carpenter. To this source I am indebted for Plates vil, viii., ix., x., and Figs. 26, 27, 28. Professor Pickering has allowed me to copy some of the drawings made at Harvard College Observatory by Mr. Trouvelot, and I have availed myself of his kindness for Plates I., iv., XT., XII., xv. I am indebted to Professor Langley for Plate ii., to Mr. De la Rue for Plates in. and xiv., to Mr. T. E. Key for Plate xvii., to Professor Schiaparelli for Plate xviii., to Dr. Huggins for Fig. 16, to Professor C. Piazzi Smyth for Fig. 89, to Mr. Chambers for Fig. 6, which has been borrowed from his " Hand- book of Descriptive Astronomy," to Dr. Stoney for Fig. 65, to Mr. Grub for Fig. 4, and to Dr. Copeland and Dr. Dreyer for Fio-. 59. I have to acknowledge the valuable assistance derived from Professor Newcomb's " Popular Astronomy," and Professor Young's " Sun." In revising the volume I have had the kind aid of the Rev. Maxwell Close. I have also to thank Dr. Copeland and Mr. Steele for their kindness in reading through the entire proofs ; while I have also occasionally availed myself of the help of Mr. Cathcart. ROBERT S. BALL. Observatory, Dunsink, Co. Dublin. 12th May, 1886. \ NOTE TO THIS EDITION. 1 have taken the opportunity in the present edition to revise the work in accordance with the progress of astronomy during the last four years. ROBERT S. BALL. Observatory, Dunsink, Co. Dublin. 22 nd December, 1890. TABLE OF CONTENTS. PAGE Introduction , „ . l CHAPTER I. THE ASTRONOMICAL OBSERVATORY. Early Astronomical Observations — The Observatory of Tycho Brahe — The Pupil of the Eye — Vision of Faint Objects— The Telescope — The Object- Glass — Advantages of Large Telescopes — The Equatorial — The Obser- vatory — The Power of a Telescope — Reflecting Telescopes — Lord Rosse's Great Reflector at Parsonstown— How the mighty Telescope is used — Instruments of Precision — The Meridian Circle — The Spider Lines — Delicacy of pointing a Telescope — Precautions necessary in mak- ing Observations— The Ideal Instrument and the Practical one — The Elimination of Error — The ordinary Opera-Glass as an Astronomical Instrument — The Great Bear — Counting the Stars in the Constellation — How to become an Observer 9 CHAPTER II. THE SUN. The vast Size of the Sun — Hotter than Fusing Platinum — Is the Sun the Source of Heat for the Earth ?— The Sun is 92,700,000 miles distant- How to realise the Magnitude of this Distance — Day and Night — Luminous and Non-Luminous Bodies — Contrasts between the Sun and the Stars— The Sun a Star — The Spots on the Sun— Changes in the Form of a Spot — They are Depressions on the Surface — The Rotation of the Sun on its Axis — The Size and Weight of the Sun — Is the Sun a Solid Body? — View of a Typical Sun-Spot— Periodicity of the Sun -Spots — Connection between the Sun-Spots — Terrestrial Magnetism — The FaculaB — The Granulated Appearance of the Sun — The Prominences surround- ing the Sun— Total Eclipse of the Sun — Size and Movement of the Prominences — The Corona surrounding the Sun — The Heat of the Sun 26 CHAPTER III. THE MOON. The Moon and the Tides — The Use of the Moon in Navigation — The Changes of the Moon — The Moon and the Poets — Whence the Light of the Moon ? — Sizes of the Earth and the Moon — Weight of the Moon — Changes in viii CONTENTS. Apparent Size — Variations in its Distance — Influence of the Earth on the Moon — The Path of the Moon — Explanation of the Moon's Phases- Lunar Eclipses — Eclipses of the Sun, how produced— Visibility of the Moon in a Total Eclipse — How Eclipses are Predicted — Uses of the Moon in finding- Longitude — The Moon not connected with the Weather — Topography of the Moon — Nasmyth's Drawing of Triesnecker — Volcanoes on the Moon — Normal Lunar Crater — Plato — The Shadows of Lunar Mountains — The Micrometer — Lunar Heights — Former Activity on the Moon — Nasmyth's View of the Formation of Craters- — Gravitation on the Moon — Varied Sizes of the Lunar Craters — Other features of the Moon — Is there Life on the Moon 1 — Absence of Water and of Air- Explanation of the Rugged Character of Lunar Scenery — Possibility of Life on Distant Bodies in Space ........ 49 CHAPTER IV. THE SOLAR SYSTEM. Exceptional Importance of the Sun and Moon — The Course to be pursued — The Order of Distance — The Neighbouring Orbs — How are they to bo discriminated ? — The Planets Venus and J upiter attract notice by their brilliancy — Sirius not a neighbour — The Planets Saturn and Mercury — Telescopic Planets — The Criterion as to whether a Body is to be ranked as a neighbour — Meaning of the word Planet — Uranus and Neptune — Comets — The Planets are Illuminated by the Sun — The Stars are not— The Earth is really a Planet — The four Inner Planets, Mercury, Venus, the Earth, and Mars — Velocity of the Earth — The Outer Planets, Jupiter, Saturn, Uranus, Neptune — Light and Heat received by the Planets from the Sun — Comparative Sizes of the Planets^— The Minor Planets — The Planets all Revolve in the same direction — The Solar System— An Island Group in Space .......... 81 CHAPTER V. THE LAW OF GRAVITATION. Gravitation — The Falling of a Stone to the Ground — All Bodies fall Equally, Sixteen Feet in a Second — Is this True at Great Heights ?— Fall of a Body at a height of a Quarter of a Million Miles — How Newton obtained an Answer from the Moon — His great Discovery — Statement of the Law of Gravitation — Illustrations of the Law — How is it that all the Bodies in the Universe do not rush together ? — The Effect of Motion — How a Circular Path can be produced by Attraction — General Account of the Moon's Motion — Is Gravitation a Force of great intensity? — Two Weights of 50 lbs. — Two Iron Globes, 53 yards in diameter, and a mile apart, attract with a force of 1 lb. — Characteristics of Gravitation — Orbits of the Planets not strictly Circles — The Discoveries of Kepler — Construction of an Ellip.se — Kepler's First Law — Does a Planet move uniformly 1 — CONTENTS. ix Law of the Changes of Velocity — Kepler's Second Law — The Relation between the Distances and the Periodic Times — Kepler's Third Law — Kepler's Laws and the Law of Gravitation— Movement in a straight line — A Body unacted on by disturbing Forces would move in a straight line with constant Velocity— Application to the Earth and the Planets— The Law of Gravitation deduced from Kepler's Laws — Universal Gravitation 95 CHAPTER VI. THE PLANET OF ROMANCE. Outline of the Subject — Is Mercury the Planet nearest the Sun ? — Transit of an Interior Planet across the Sun— Has a Transit of Vulcan ever been seen ? — Visibility of Planets during a Total Eclipse of the Sun — Professor Watson's Researches in 1878 122 CHAPTER VII. MERCURY. The Ancient Astronomical Discoveries — How Mercury was first found ? — Not easily seen — Mercury was known in 265 B.C. — Skill necessary in the Discovery — The Distinction of Mercury from a Star — Mercury in the East and in the West — The Prediction — How to Observe Mercury — Its Telescopic Appearance — Difficulty of Observing its Appearance — Orbit of Mercury — Velocity of the Planet — Can there be Life on the Planet? — Changes in its Temperature — Transit of Mercury over the Sun — Gassendi's Observations — Atmosphere around Mercury — The Weight of Mercury 127 CHAPTER VIII. VENUS. Interest attaching to this Planet — The Unexpectedness of its Appearance — The Evening Star — Visibility in Daylight — Only lighted by the Sun — The Phases of Venus — Why the Crescent is not visible to the unaided Eye — Variations in the Apparent Size of the Planet — Resemblance of Venus to the Earth— The Transit of Venus— Why of such especial Interest — The Scale of the Solar System — Orbits of the Earth and Venus not in the same Plane — Recurrence of the Transits in Pairs — ■ Appearance of Venus in Transit— Transits of 1874 and 1882— The Early Transits of 1631 and 1639 — The Observations of Horrocks and Crabtree —The Announcement of Halley — How the Track of the Planet differs from different places— Illustrations of Parallax— Voyage to Otaheite — The result of Encke— Probable Value of the Sun's Distance — Observa- tions of the recent Transit of Venus at Dunsink — The Question of an Atmosphere to Venus — Dr. Copeland's Observations — Utility of such Researches — Other Determinations of the Sun's Distance — Statistics about Venus .139 * X CONTENTS. CHAPTER IX. THE EARTH. PAGE The Earth is a great Globe — How the Size of the Earth is Measured — The Base Line— The Latitude found by the Elevation of the Pole — A Degree of the Meridian — The Earth not a Sphere — The Pendulum Experiment — Is the Motion of the Earth slow or fast ? — Coincidence of the Axis of Rotation and the Axis of Figure — The Existence of Heat in the Earth — The Earth once in a Soft Condition — Effects of Centrifugal Force — Comparison with the Sun and Jupiter — The Protuberance of the Equator — The Weighing of the Earth — Comparison between the Weight of the Earth and an equal Globe of Water — Comparison of the Earth with a Leaden Globe— The Pendulum — Use of the Pendulum in Mea- suring the Intensity of Gravitation — The Principle of Isochronism — Shape of the Earth Measured by the Pendulum . . . , .162 CHAPTER X. MARS. Our nearer Neighbours in the Heavens— Surface of Mars can be Examined in the Telescope — Remarkable Orbit of Mars — Resemblance of Mars to a Star — Meaning of Opposition — The Eccentricity of the Orbit of Mars— Different Oppositions of Mars — Apparent Movements of the Planet — Effect of the Earth's Movement — Measurement of the Distance of Mars —Theoretical Investigation of the Sun's Distance — Drawings of the Planet— Is there Snow on Mars ? — The Rotation of the Planet- Gravitation on Mars — Has Mars any Satellites ? — Mr. Asaph Hall's great Discovery — The Revolutions of the Satellites — Deimos and Phobos — "Gulliver's Travels" 17c CHAPTER XL THE MINOR PLANETS. The Lesser Members of our System— Bode*s Law — The Vacant Region in the Planetary System — The Research — The Discovery of Piazzi — Was the small Body a Planet ? — The Planet becomes Invisible — Gauss under- takes the Search by Mathematics — The Planet Recovered— Further Discoveries — Number of Minor Planets now known— The Region to be Searched— The Construction of the Chart for the Search for Small Planets— How a Minor Planet is Discovered— Physical Nature of the Minor Planets— Small Gravitation on the Minor Planets— The Berlin Computations — How the Minor Planets tell us the Distance of the Sun — Accuracy of the Observations — How they may be Multiplied — Victoria and Sappho — The most perfect Method 193 CHAPTER XII. JUPITER. The great size of Jupiter — Comparison of his Diameter with that of the Earth— Dimensions of the Planet and his Orbit — His Rotation— Com- CONTENTS, xi parison of his Weight and Bulk with that of the Earth — Relative Lightness of Jupiter — How explained — Jupiter still probably in a Heated Condition — The Belts on Jupiter — Spots on his Surface— Time of Rotation of different Spots various — Storms on Jupiter— Jupiter not Incandescent — The Satellites — Their Discovery — Telescopic Ap- pearance — Their Orbits — The Eclipses and Occultations — A Satellite in Transit— The Velocity of Light Discovered — How is this Velocity to be measured experimentally ? — Determination of the Sun's Distance by the Eclipses of Jupiters Satellites— Jupiter's Satellites demonstrating the Copernican System 208 CHAPTER XIII. SATURN. The Position of Saturn in the System— Saturn one of the Three most Inter- esting Objects in the Heavens— Compared with Jupiter— Saturn to the Unaided Eye— Statistics relating to the Planet — Density of Saturn — Lighter than Water — The Researches of Galileo — What he found in Saturn— A Mysterious Object— The Discovery made by Huyghens half a Century later — How the Existence of the Ring was Demon- strated—Invisibility of the Rings every Fifteen Years — The Rotation of the Planet — The Celebrated Cypher— The Explanation — Drawing of Saturn— The Dark Line — W. Herschel's Researches— Is the Division in the Ring really a Separation ?— Possibility of Deciding the Question — The Ring in a Critical Position — Are there other Divisions in the Ring ?— The Third Ring — Has it appeared but recently ? — Physical Nature of Saturn's Rings — Can they be Solid ?— Can they even be Slender Rings ?— A Fluid —Probable Nature of the Rings — A Multi- tude of Small Satellites— Analogy of the Rings of Saturn to the G-roup of Minor Planets— Problems Suggested by Saturn — The Group of Satellites to Saturn — The Discoveries of Additional Satellites— The Orbit of Saturn not the Frontier of our System 220 CHAPTER XIV. URANUS. Contrast between Uranus and the other great Planets— William Hersohel — His Birth and Parentage— Herschel's Arrival in England— His Love of Learning— Commencement of his Astronomical Studies— The Con- struction of Telescopes— Reflecting Telescopes— Construction of Mir- rors—The Professor of Music becomes an Astronomer— The Methodical Research— The 13th March, 1781— The Discovery of Uranus— Delicacy of Observation— Was the Object a Comet ?— The Significance of this Discovery— The Fame of Herschel— George III. and the Bath Musician — The King's Astronomer at Windsor — Caroline Herschel — The Planet Uranus — Numerical Data with reference thereto — The Four Satellites of Uranus — Their Circular Orbits— Early Observations of Uranus — Flamsteed's Observations — Lemonnier saw Uranus — Utility of their Measurements — The Elliptic Path — The great Problem thus Suggested. 253 Xll CONTENTS. CHAPTER XV. NEPTUNE. Discovery of Neptune — A Mathematical Achievement — The Sun's Attraction — All bodies Attract — Jupiter and Saturn — The Planetary Perturb- ations — Three Bodies — Nature has Simplified the Problem — Approxi- mate Solution — The Sources of Success — The Problem Stated for the Earth — The Discoveries of Lagrange — The Eccentricity — Necessity that all the Planets Revolve in the same Direction — Lagrange's Discoveries have not the Dramatic Interest of the more Recent Achievements — The Irregularities of Uranus — The Unknown Planet must Revolve outside the path of Uranus — The Data for the Problem — Le Verrier and Adams both Investigate the Question — Adams Indicates the Place of the Planet— How the Search was to be Conducted — Le Verrier also Solves the Problem— The Telescopic Discovery of the Planet— The Rival Claims — Early Observation of Neptune — Difficulty of the Tele- scopic Study of Neptune — Numerical Details of the Orbit — Is there any Outer Planet 1 — Contrast between Mercury and Neptune CHAPTER XVI. COMETS. Comets contrasted with Planets in Nature as well as in their Movements — Coggia's Comet — Periodic Returns — The Law of Gravitation— Parabolic and Elliptic Orbits — Theory in Advance of Observations — Most Comet- ary Orbits are sensibly Parabolic — The Labours of Halley — The Comet of 1682 — Halley 's Memorable Prediction — The Retardation Produced by Disturbance — Successive Returns of Halley's Comet — Encke's Comet — Effect of Perturbations— Orbit of Encke's Comet — Attraction of Mercury and of Jupiter — How the Identity of the Comet is secured — How to weigh Mercury — Distance from the Earth to the Sun found by Encke's Comet— The Disturbing Medium— The Comets of 1843 and 1858 Passage of a Comet between the Earth and the Stars — Comets not composed of Gas of appreciable Density— Can the Comet be weighed ? — Evidence of the Small Mass of the Comet derived from the Theory of Perturbation — The Tail of the Comet — Its Changes — Views as to its Nature— Carbon present in Comets . CHAPTER XVIL SHOOTING STARS. Small Bodies of our System— Their Numbers— How they are Observed— The Shooting Star— The Theory of Heat -A great Shooting Star— The November Meteors — Their Ancient History— The Route followed by the Shoal— Diagram of the Shoal of Meteors— How the Shoal becomes Spread out along its Path— Absorption of Meteors by the Earth— The Discovery of the Relation between Meteors and Comets— The remark- able Investigations concerning the November Meteors— Two Showers in Successive Years— No Particles have ever been Identified from the CONTENTS. xiii great Shooting Star Showers— Meteoric Stones— Chladni's Researches —Early Cases of Stonefalls— The Meteorite at Ensisheim— Collections of Meteorites— The Rowton Siderite— Relative Frequency of Iron and Stony Meteorites— Fragmentary Character of Meteorites— No Reason to connect Meteorites with Comets— Tschermak's Theory— Effects of Gra- vitation on a Missile ejected from a Volcano— Can they have come from the Moon ?— The Claims of the Minor Planets to the Parentage of Meteorites— Possible Terrestrial Origin— The Ovifak Iron . . 325 CHAPTER XVIII. THE STARRY HEAVENS. Whence the Importance of the Solar System ?— Home— View in Space- Other Stellar Systems— The Sun a Star— Stars are Self -Luminous — We see the Points of Light, but nothing else— The Constellations— The Great Bear and the Pointers— The Pole Star— Cassiopeia— Andro- meda, Pegasus, and Perseus— The Pleiades : Auriga, Capella, Alde- baran— Taurus, Orion, Sirius ; Castor and Pollux— The Lion— Bo6tes ; Corona, and Hercules— Virgo and Spica— Vega and Lyra— The Swan 363 CHAPTER XIX. THE DISTANT SUNS. Comparison between the Sun and the Stars— Sirius Contrasted with the Sun— Stars can be Weighed, but not Measured— The Companion of Sirius— Determination of the Weights of Sirius and his Companion- Dark Stars — Variable Stars — Enormous Number of Stars . . . 382 CHAPTER XX. DOUBLE STARS. Interesting Stellar Objects— What is a Double Star?— Stars Optically Double -The Great Discovery of the Binary Stars made by Herschel — The Binary Stars describe Elliptic Paths — Why is this so import- ant ? — The Law of Gravitation — Special Double Stars — Castor — Mizar —The Pole Star— The Coloured Double Stars— £ Cygni, 7 Andromeda . 393 CHAPTER XXI. THE DISTANCES OF THE STARS. Sounding-line for Space— The Labours of W. Herschel — His Reasonings Illustrated by Vega — Suppose this Star to recede 10 times, 100 times, or 1,000 times— Some Stars 1,000 times as far as others— Herschel's Method incomplete— The Labours of Bessel— Meaning of Annual Paral- lax — Minuteness of the Parallactic Ellipse Illustrated — The case of 61 Cygni — Different Comparison Stars used — Difficulty owing to Refrac- tion — How to be avoided — The Proper Motion of the Star — Bessel's Preparations — The Heliometer— Struve's Investigations — Can they be xiv CONTENTS. Eeconciled 1 — Researches at Dunsink — Conclusion obtained — Accuracy which such Observations admit — Examined— How the Results are Dis- cussed — The Proper Motion of 61 Cygni — The Permanence of the Sidereal Heavens — Changes in the Constellation of the Great Bear since the time of Ptolemy — The Star Groombridge 1,830 — Large Proper Motion— Its Parallax— Velocity of 200 Miles a Second— The New Star in Cygnus — Its History — No Appreciable Parallax — A Mighty Outburst of Light — The Movement of the Solar System through Space — Herschel's Discovery — Journey towards Hercules — Probabilities — Conclusion . 401 CHAPTER, XXII. THE SPECTROSCOPE. A New Department of Science — The Materials of the Heavenly Bodies — Meaning of Elementary Bodies — Chemical Analysis and Spectroscopic Analysis— The Composite Nature of Light — Whence Colours? — The Rainbow — The Prism — Passage of Light through a Prism — Identifica- tion of Metals by the Rays they emit when Incandescent — The great discovery of the Identity of the D-Lines with Sodium — The Dark Lines in the Solar Spectrum Interpreted — Metals present in the Sun — Examination of Light from the Moon or the Planets — The Prominences surrounding the Sun — Photographs of Spectra — Measurement of the Motion of the Stars along the Line of Sight . . . . . .434 CHAPTER XXIII. STAR CLUSTERS AND NEBULyE. Interesting Sidereal Objects — Stars not Scattered uniformly — Star Clusters — Their Varieties — The Cluster in Perseus — The Globular Cluster in Hercules — The Milky Way— A Cluster of Minute Stars — Nebulee dis- tinct from Clouds — Number of known Nebulas — The Constellation of Orion— The Position of the Great Nebula — The Wonderful Star 0 Orionis— The Drawing of the Great Nebula in Lord Rosse's Telescope — Photographs of this Wonderful Object — The Importance of Accurate Drawings— The Great Survey — Photographs of the Heavens — Magni- tude of the Nebula— The Question as to the Nature of a Nebula — Is it composed of Stars or of Gas ?— How Gas can be made to glow— Spectroscopic Examination of the Nebula— The Great Nebula in Andro- meda—Its Examination by the Spectroscope— The Annular Nebula in Lyra— Resemblance to Vortex Rings— Planetary Nebulas— Drawings of Several Remarkable Nebulas— Distance of Nebulas— Conclusion 447 CHAPTER XXIV. THE PRECESSION AND NUTATION OF THE EARTH'S AXIS. The Pole is not a Fixed Point— Its Effect on the Apparent Places of the Stars— The Illustration of the Peg-Top— The disturbing Force which acts on the Earth— Attraction of the Sun on a Globe— The Protuber- ance at the Equator— The Attraction of the Protuberance by the San CONTENTS. xv ' and by the Moon produces Precession — The Efficiency of the Preces- sional Agent varies inversely as the Cube of the Distance — The Rela- tive Efficiency of the Sun and the Moon — How the Pole of the Earth's Axis revolves round the Pole of the Ecliptic 467 CHAPTER XXY. THE ABERRATION OF LIGHT. The Real and Apparent Movements of the Stars — How they can be Dis- criminated — Aberration produces Effects dependent on the Position of the Stars — The Pole of the Ecliptic — Aberration makes Stars seem to Move in a Circle — An Ellipse or a Straight Line according to Position — All the Ellipses have Equal Major Axes — How is this Movement to be Explained ? — How to be Distinguished from Annual Parallax — The Apex of the Earth's Way— How this is to be Explained by the Velocity of Light — How the Scale of the Solar System can be Mea- sured by the Aberration of Light 477 CHAPTER XXVI. THE ASTRONOMICAL SIGNIFICANCE OF HEAT. Heat and Astronomy — Distribution of Heat — The Presence of Heat in the Earth — Heat in other Celestial Bodies — Varieties of Temperature — The Law of Cooling — The Heat of the Sun — Can its Temperature be Measured ? — Radiation connected with the Sun's Bulk — Can the Sun b3 Exhausting his Resources ? — No marked Change has occurred — Geological Evidence as to the Changes of the Sun's Heat Doubtful — The Cooling of the Sun — The Sun cannot be merely an Incandescent Solid Cooling — Combustion will not Explain the matter — Some Heat is obtained from Meteoric Matter, but this is not Adequate to the Main- tenance of the Sun's Heat — The Contraction of a Heated Globe of Gas — An Apparent Paradox — The Doctrine of Energy — The Nebular Theory — Evidence in support of this Theory— Sidereal Evidence of the Nebular Theory — Herschel's View of Sidereal Aggregation — The Ne- bulas do not Exhibit the Changes within the Limits of our Observation 487" CHAPTER XXVII. THE TIDES. Mathematical Astronomy — Recapitulation of the Facts of the Earlier Re- searches — Another great Step has been taken — Lagrange's Theories, how far they are really True— The Solar System not Made of Rigid Bodies — Kepler's Laws True to Observation, but not Absolutely True when the Bodies are not Rigid— The Errors of Observation— Growth of certain Small Quantities— Periodical Phenomena — Some Astro- nomical Phenomena are not Periodic — The Tides — How the Tides were Observed — Discovery of the Connection between the Tides and the Moon — Solar and Lunar Tides — Work done by the Tides — Whence XVI CONTENTS. do the Tides obtain the Power to do the Work? — Tides are Increasing the Length of the Day — Limit to the Shortness of the Day — Early History of the Earth-Moon System — Unstable Equilibrium— Ratio of the Month to the Day: — The Future Course of the System — Equality of the Month and the Day — The Future Critical Epoch — The Constant Face of the Moon accounted for — The other Side of the Moon — The Satellites of Mars — Their Remarkable Motions — Have the Tides Pos- sessed Influence in Moulding the Solar System generally 1 — Moment of Momentum — Tides have had little or no appreciable Effect on the Orbit of Jupiter — Conclusion 505 Appendix. — Astronomical Quantities 535 LIST OF ILLUSTRATIONS. PLATES. PLATE I. Saturn Frontispiece II. A Typical Sun-spot ' . . To face page 9 III. Spots and Faculfe on the Sun „ „ 41 IV. Solar Prominences or Flames „ „ 45 V. The Solar Corona .. . „ ' „ 48 VI. Chart of the Moon's Surface „ „ 60 VII. The Lunar Crater Triesnecker „ „ 68 VIII. A Normal Lunar Crater 73 IX. The Lunar Crater Plato „ ., 80 X. The Lunar Crater Tycho „ 88 XL The Planet Jupiter „ ., 215 XII. Coggia's Comet ., - 293 XIII. Spectra of the Sun and of three Stars . . . . „ „ 441 XIV. The Great Nebula m Orion „ ., 456 XV. The Great Nebula in Andromeda „ 462 XVI. Nebulfe observed with Lord Posse's Telescope . . „ 465 XVII. The Comet of 1882 322 XVIII. Schiaparelli's Map of Mars „ . „ 187 ENGRAVINGS. FIG. PAGE 1. Principle of the Refracting Telescope . 11 2. Dome of the South Equatorial at Dunsink Observatory, Co. Dublin . 12 3. Section of the Dome of Dunsink Observatory . . . '. . 13 4. The Great Vienna Telescope 15 5. Principle of Herschei's Reflecting Telescope 16 6. Lord Rosse's Telescope . , . 17 7. Meridian Circle ...... 19 8. The Great Bear . . ' . . . . . ..... .. 24 9. Comparative Sizes of the Earth and the Sun 27 10. The Sun, photographed Sept. 22, 1870 30 11. An ordinary Sun-spot 31 xviii LIST OF ILLUSTRATIONS. FIG. PAGE 12. Successive Appearances of a Sun-spot . . . ... . 32 13. Schemer's Observations on Sun-spots 33 14. Zones on the Sun's surface in which spots appear 36 15. Texture of the Sun and a small spot 38 16. Dr. Huggins' Drawing of a remarkable arrangement of Solar Granules 39 17. Willow-leaf Texture of the Sun's Surface 40 18. Prominences seen in Total Eclipse . . . . . . . • . 41 19. View of the Corona in a Total Eclipse . 45 20. The Zodiacal Light in 1874 47 21. Comparative Sizes ©f the Earth and the Moon ..... 52 22. The Moon's Path around the Earth .• 55 23. The Phases of the Moon .......... 55 24. The Earth's Shadow and Penumbra . 57 25. Key to Chart of the Moon (Plate VI.) ........ 60 26. Lunar Volcano in Activity : Nasmyth's Theory 73 27. „ „ Subsequent Feeble Activity 73 28. „ „ Formation of the Level Floor by Lava ... 74 29. Orbits of the Four Interior Planets 88 30. The Earth's Movement .90 31. Orbits of the Four Giant Planets 91 32. Apparent Size of the Sun from various Planets 92 33. Comparative Sizes of the Planets . 93 34. Illustration of the Moon's Motion . 103 35. Drawing an Ellipse . 109 36. Varying Velocity of Elliptic Motion 112 37. Equal Areas in Equal Times . . .113 38. Transit of the Planet of Romance ........ 124 39. Variations in Phase and apparent Size of Mercury 132 40. Mercury as a Crescent ,133 11. Different Aspects of Venus in the Telescope 112 42. Venus on the Sun at the Transit of 1874 . . . . . . . 147 43. Paths of Venus across Sun in the Transits of 1874 and 1882 . . .149 44. A Transit of Venus, as seen from Two Localities ..... 153 45. Orbits of the Earth and of Mars 18C 46. Apparent Movements of Mars in 1877 182 47. Relative Sizes of Mars and the Earth 186 48. Views of Mars =187 49. The Zone of Minor Planets between Mars and Jupiter .... 197 50. Relative Dimensions of Jupiter and the Earth 209 51. Jupiter and his Four Satellites . . 218 52. Disappearances of Jupiter's Satellites 219 53. Mode of Measuring the Velocity of Light 225 54. Relative Sizes of Saturn and the Earth ....... 233 55. Parabolic Path of a Comet 295 LIST OF ILLUSTRATIONS. FIG. 56. Orbit of Encke's Comet .... 57. Resisting Medium around the Sun . 58. Tail of a Comet directed from the Sun . 319. Bredichin's Theory of Comets' Tails 60. Tails of the Comet of 1858 61. Cheseaux's Comet of 1744 . 62. Path of the Fire-ball of November 6, 1S09 63. The Orbit of a Shoal of Meteors 64. Radiant Point of Shooting' Stars 65. The History of the Leonids 66. Section of the Chaco Meteorite 67. The Great Bear and Pole Star . 68. The Great Bear and Cassiopeia 69. The Great Square of Pegasus . 70. Perseus and its Neighbouring Stars 71. The Pleiades 72. Orion, Sirius, and Neighbouring Stars . 73. Castor and Pollux . . . 74. The Great Bear and the Lion . 75. Bootes and the Crown . . . . 76. Virgo and Neighbouring Constellations . 77. The Constellation of Lyra 78. Vega, the Swan, and the Eagle 79. The Parallactic Ellipse .... 80. 61 Cygni and the Comparison Stars . 81. Parallax in Declination of 61 Cygni 82. The Prism 83. Dispersion of Light by the Prism . 84. Globular Star-cluster in Hercules . 85. Position of the Great Nebula in Orion . 86. The Multiple Star 6 Orionis ... 87. Lyra, with the Annular Nebula 88. The Annular Nebula in Lyra . 89. Star-Map, showing Precessional Movement 90. Illustration of the Motion of Precession THE Story of the Heavens. " The Story of the Heavens " is the title of our book We have indeed a wondrous story to narrate ; and could we tell it ade- quately it would prove of boundless interest and of exquisite beauty. It leads to the contemplation of grand phenomena in nature and great achievements of human genius. Let us enumerate a few of the questions which will be naturally asked by one who seeks to learn something of those glorious bodies which adorn our skies : What is the Sun — how hot, how big, and how distant? whence comes its heat ? What is the Moon ? What are its landscapes like ? How does our satellite move ? How is it related to the earth ? Are the planets globes like that on which we live ? how large are they, and how far off ? What do we know of the satellites of Jupiter and of the rings of Saturn ? How was Uranus discovered ? and what Avas the intellectual triumph which brought the planet Neptune to light ? Then, as to the other bodies of our system, what are we to say of those mysterious objects, the comets ? can we discover the laws of their seemingly capricious movements ? do we know anything of their nature and of the marvellous tails with which they are often decorated ? What can be told about the shooting-stars which so often dash into our atmosphere and perish in a streak of splendour ? What is the nature of those constellations of bright stars which have been recognised from all antiquity, and of the host of smaller stars which our telescopes disclose ? Can it be true that these countless orbs are really majestic suns, sunk to an appalling depth in the abyss of 1 2 THE STORY OF THE HEAVENS. unfathomable space? What have we to tell of the different varieties of stars — of coloured stars, of variable stars, of double stars, of multiple stars, of stars that seem to move, and of stars that seem at rest ? What of those glorious objects, the great star clusters ? What of the milky way ? And lastly, what can we learn of the marvellous nebulae which our telescopes disclose, poised at an immeasurable distance on the very confines of the universe of things visible to us ? Such are a few of the questions which occur when we ponder on the mysteries of the heavens. The history of Astronomy is, in one respect, only too like many other histories. The earliest part of it is completely and hopelessly lost. The stars had been studied, and some great astronomical discoveries had been made, untold ages before those to which our earliest historical records extend. For example, the observation of the apparent movement of the sun, and the discrimination between the planets and the fixed stars, are both to be classed among the discoveries of pre-historic ages. Nor is it to be said that these achievements related to matters of an obvious character. Ancient astronomy may seem very< ele- mentary to those of the present day who have been familiar from childhood with the great truths of nature, but, in the in- fancy of science, the men who made such discoveries as we have mentioned must have been sagacious philosophers. Of all the phenomena of Astronomy the first and the most obvious is that of the rising and the setting of the sun. W T e may assume that in the dawn of human intelligence these daily occurrences would form one of the first problems to engage the attention of those whose thoughts rose above the animal anxieties of everyday existence. A sun sets and disappears in the west. The following morning a sun rises in the east, moves across the heavens, and it too disappears in the west ; the same appear- ances recur every day. To us it is obvious that the sun, which appears each day, is the same sun ; but this would not seem reasonable to one who thought his senses showed him that the earth was a flat plane of indefinite extent, and that around the inhabited regions on all sides extended, to vast distances, either desert wastes or trackless oceans. How could that same sun, which plunged into the ocean at a fabulous distance in the west, EARLIEST IDEAS OF THE STARS. 3 reappear the next morning at an equally great distance in the east ? The old mythology asserted that after the sun had dipped in the western ocean at sunset (the Iberians, and other ancient nations, actually imagined that they could hear the hissing of the waters when the glowing globe was plunged therein), it was seized by Vulcan and placed in a golden goblet. This strange craft with its astonishing cargo navigated the ocean by a northerly course, so as to reach the east again in time for sunrise the following morning. Among the more sober physi- cists of old, as we are told by Aristotle, it was believed that in some manner the sun was conveyed by night across the northern regions, and that darkness was due to lofty mountains, which screened off the sunbeams during the voyage. In the course of time it was thought more rational to suppose that the sun actually pursued his course below the solid earth during the course of the night. The early astronomers had, more- over, learned to recognise the fixed stars. It was noticed that, like the sun, many of these stars rose and set in consequence of the diurnal movement, while the moon obviously followed a similar law. Philosophers thus taught that the various heavenly bodies were in the habit of actually passing- beneath the solid earth. When it was acknowledged that the whole contents of the heavens performed these movements, an important step in com- prehending the constitution of the universe had been decidedly taken. It was clear that the earth could not be a plane extending to an indefinitely great distance. It was also obvious that there must be a finite depth to the earth below our feet. Nay, more, it became certain that whatever were the shape of the earth, it was at all events something detached from all other bodies, and poised without visible support in space. When this discovery was first announced it must have appeared a very startling truth. It was found difficult to realise that the solid earth on which we stand reposed on nothing ! What was to keep it from falling ? How could it be sustained without tangible support, like the legendary coffin of Mahomet ? But difficult as it may have been to receive this doctrine, yet its necessary truth in due time commanded assent, and the science of Astronomy began to exist. 4 THE STORY OF THE HEAVENS. The changes of the seasons and the recurrence of seed-time and harvest must, from the earliest times, have been associated with certain changes in the position of the sun. In the summer at mid-day the sun rises high in the heavens, in the winter it is always Ioav Our luminary, therefore, performs an annual movement up and down in the heavens, as well as a diurnal movement of rising and setting. But there is a third species of change in the sun's position, which is not quite so obvious, though it is still capable of being detected by a few careful observations, if combined with a philosophical habit of reflection. The very earliest observers of the stars can hardly have failed to notice that the constellations visible at night varied Avith the season of the year. For instance, the brilliant figure of Orion, though so well seen on winter nights, is absent from the summer skies, and the place it occupied is then taken by quite different groups of stars. The same may be said of other con- stellations. Each season of the year can thus be characterised by the sidereal objects that are conspicuous by night. Indeed, in ancient days, the time for commencing the cycle of agri- cultural occupations was sometimes indicated by the position of the constellations in the evening. By reflecting on these facts the early astronomers were enabled to demonstrate the apparent annual movement of the sun. There could be no rational explanation of the changes in the constellations with the seasons, except by supposing that the place of the sun was altering, so as to make a complete circuit of the heavens in the course of the year. This movement of the sun is otherwise confirmed by looking at the west after sunset, and watching the stars. As the season progresses, it will be noticed each evening that the constellations seem to sink lower and lower towards the west, until at length they become in- visible from the brightness of the sky. The disappearance is explained by the supposition that the sun appears to be con- tinually ascending from the west to meet the stars. This motion is of course not to be confounded with the ordinary diurnal rising and setting, in which all the heavenly bodies participate. It is to be understood that besides being affected by the com- mon motion our luminary has a slow independent movement ANCIENT DISCOVERIES. 5 in the opposite direction ; so that though the sun and a star may set at the same time to-day, yet as by to-morrow the sun will have moved a little towards the east, it follows that the star must then set a few minutes before the sun. * The patient observations of the early astronomers enabled the sun's track through the heavens to be ascertained, and it was found that in its circuit amid the stars and constellations our luminary invariably followed the same path. This is called the ecliptic, and the constellations through which it passes form a belt around the heavens known as the zodiac. It was anciently divided into twelve equal portions or " signs," so that the stages on the sun's great journey could be conveniently indicated. The duration of the year, or the period required by the sun to run its course around the heavens, seems to have been first ascertained by astronomers whose names are unknown. The skill of the early Oriental geometers was further evidenced by their determination of the position of the ecliptic with regard to the equator, and by their success in the measurement of the angle between these two important circles on the heavens. The principal features of the motion of the moon have also been noticed with intelligence at an antiquity more remote than history. The attentive observer perceives the important truth that the moon does not occupy a fixed position in the heavens. During the course of a single night the fact that the moon- has moved from west to east across the heavens can be perceived by noting its position relatively to adjacent stars. It is indeed probable that the motion of the moon was a discovery prior to that of the annual motion of the sun, inasmuch as it is the immediate consequence of a simple observation, and involves but little exercise of any intellectual power. In pre-historic times also the time of revolution of the moon had been ascer- tained, and the phases of our satellite had been correctly at- tributed to the varying aspect under which the sun-illuminated side is turned towards the earth. But we are far from having exhausted the list of great discoveries which have come down from unknown antiquity. * It may, however, be remarked that a star is never seen to set, as, owing 1 to our atmosphere, it ceases to be visible before it reaches the horizon. 6 THE STORY OF THE HEAVENS. Correct explanations had been given of the striking phe- nomenon of a lunar eclipse, in which the brilliant surface is plunged temporarily into darkness, and also of the still more imposing spectacle of a solar eclipse, in which the sun itself undergoes a partial or even a total obscuration. Then, too, the acuteness of the early astronomers had detected the five wandering stars or planets : they had traced the movements of Mercury and Venus, Mars, Jupiter, and Saturn. They had observed with awe the various configurations of these planets ; and just as the sun, and in a lesser degree the moon, were intimately associated with the affairs of daily life, so in the imagination of these early investigators the movements of the planets were thought to be pregnant with human weal or human woe. At length a certain order was perceived to govern the apparently capricious movements of the planets. It was found that they obeyed certain laws. The cultivation of the science of geometry went hand in hand with the study of astronomy ; and as we emerge from the dim prehistoric ages into the historical period, we find that the theory of the phenomena of the heavens possessed some degree of coherence. Ptolemy perceived that the earth's figure was globular, and he demonstrated it by the same arguments that Ave employ at the present day. He also saw how this mighty globe was poised, in what he believed to be the centre of the universe. He ad- mitted that the diurnal movement of the heavens could be accounted for by the revolution of the earth upon its axis, but unfortunately he assigned reasons for the deliberate rejection of this view. The earth, according to him, was a fixed body ; it possessed neither rotation round an axis nor translation through space, but remained constantly at rest in what he supposed to be the centre of the universe. According to Ptolemy's theory the sun and the moon moved in circular orbits around the earth in the centre. The explanation of the movements of the planets he found to be more complicated, because it was necessary to account for the fact that a planet sometimes advanced and that it sometimes retrograded. The ancient geometers refused to believe that any movement, except revolution in a circle, was possible for a celestial body, accordingly a contrivance COPERNICUS. 7 was devised by which each planet was supposed to revolve in a circle, of which the centre described another circle around the earth. Although the Ptolemaic doctrine is now known to be framed on quite an extravagant estimate of the importance of the earth in the scheme of the heavens, yet it must be admitted that the apparent movements of the celestial bodies can be thus accounted for with considerable accuracy. This theory is described in the great work known as the " Almagest," which was written in the second century of our era, and was regarded for fourteen centuries as the final authority on all questions of astronomy. Such was the system of Astronomy which prevailed during the Middle Ages, and was only discredited at an epoch nearly simultaneous with that of the discovery of the New World by Columbus. The true arrangement of the solar system was then expounded by Copernicus in the great work to which he devoted his life. The first principle established by these labours showed the diurnal movement of the heavens to be due to the rotation of the earth on its axis. Copernicus pointed out the fundamental difference between real motions and apparent motions ; he proved that the appearances presented in the daily rising and setting of the sun and the stars could be accounted for by the supposition that the earth rotated just as satisfactorily as by the more cumbrous supposition of Ptolemy. He showed moreover that the latter supposition must attribute an almost infinite velocity to the stars, so that the rotation of the entire universe around the earth was clearly a preposterous supposition. The second great principle, which has conferred immortal glory on Copernicus, assigned to the earth its true position in the universe. Coper- nicus transferred the centre, about which all the planets revolve, from the earth to the sun ; and he established the some- what humiliating truth, that our earth is merely a planet pursuing a track between the paths of Venus and of Mars, and subordinated like all the other planets to the supreme sway of the Sun. This great revolution swept from astronomy those dis- torted views of the earth's importance which arose perhaps not 8 THE STORY OF THE HEAVENS. unnaturally from the fact that we happen to be domiciled on that particular planet. The achievements of Copernicus were soon to be followed by the invention of the telescope, that wonderful instrument by which the modern science of astronomy has been created. To the consideration of this important subject we shall devote the first chapter of our book. PLATE II. A TYPICAL SUN-SPOT. 'AFTER LANGLEY.) CHAPTER I. THE ASTRONOMICAL OBSERVATORY. Early Astronomical Observations — The Observatory of Tycho Brahe — The Pupil of the Eye — Vision of Faint Objects — The Telescope — The Object-Glass — Advantages of Large Telescopes — The Equatorial — The Observatory — The Power of a Telescope— Reflecting 1 Telescopes — Lord Rosse's Great Reflector at Parsonstown — How the mighty Telescope is used — Instruments of Precision— The Meridian Circle— The Spider Lines — Delicacy of pointing a Telescope — Precautions necessary in making Observations— The Ideal Instrument and the Practical one— The Elimination of Error— The ordinary Opera-Glass as an Astronomical Instrument — The Great Bear — Counting the Stars in the Constellation — How to become an Observer. The earliest rudiments of the Astronomical Observatory are as little known as the earliest discoveries in astronomy itself. Probably the first application of instrumental observation to the heavenly bodies consisted in the simple operation of measuring the shadow of a post cast by the sun at noonday. The variations in the length of this shadow enabled the primitive astronomers to investigate the movements of the sun. But even in very early times special astronomical instruments were employed which possessed sufficient accuracy to add to the amount of astronomical knowledge, and displayed considerable ingenuity on the part of the designers. Professor Xewcomb * thus writes : " The leader was Tycho Brahe, who was born in 1546, three years after the death of Copernicus. His attention was first directed to the study of astronomy by an eclipse of the sun on August 21st, 1560, which was total in some parts of Europe. Astonished that such a phenomenon could be predicted, he devoted himself to a study of the methods of observation and calculation by which the * "Popular Astronomy," p. 66. 10 THE STORY OF THE HEAVENS. prediction was made. In 1576 the King of Denmark founded the celebrated observatory of Uraniberg, at which Tycho spent twenty years assiduously engaged in observations of the positions of the heavenly bodies with the best instruments that could then be made. This was just before the invention of the telescope, so that the astronomer could not avail himself of that powerful instrument. Consequently, his observations were superseded by the improved ones of the centuries following, and their celebrity and importance are principally due to their having afforded Kepler the means of discovering his celebrated laws of planetary motion." The direction of the telescope to the skies by Galileo gave a wonderful impulse to the study of the heavenly bodies. This extraordinary man is prominent in the history of astronomy, not alone for his connection with this supreme invention, but also for his achievements in the more abstract parts of astronomy. He was born at Pisa in 1564, and in 1609 the first telescope used for astronomical observation was constructed. Galileo died in 1642, the year in which Newton was born. It was Galileo who laid with solidity the foundations of that science of Dynamics, of which astronomy is the most splendid illustration ; and it was he who chiefly promulgated the doctrines taught by Copernicus, and thereby incurred the wrath of the Inquisition. The structure of the human eye in so far as the exquisite adaptation of the pupil is concerned presents us with an apt illustration of the principle of the telescope. To see an object, it is necessary that the light from it should enter the eye. The portal through which the light is admitted is the pupil. In day- time, when the light is brilliant, the iris decreases the size of the pupil, and thus prevents too much light from entering. At night, or, indeed, whenever the light is scarce, the eye requires to grasp all it can. The pupil then expands ; more and more light is admitted according as the pupil grows larger. The illumination of the image on the retina is thus effectively con- trolled in accordance with the requirements of vision. A star sends its feeble rays of light to us, and from those rays the image is formed. Even with the most widely-opened pupil, it may, however, happen that the image is not bright THE ASTRONOMICAL OBSERVATORY. 11 Rays of light from the Star enough to excite the sensation of vision. Here the telescope comes to our aid : it catches all the rays in a beam of dimensions far too large to enter the pupil, and concentrates those rays into a stream slender enough to pass through the narrow opening. We thus find the image on the retina intensified in brightness ; in fact, it is illuminated with nearly as much light as would be obtained from the same ob- ject through a pupil as large as the great lenses of the telescope. In astronomical observatories we employ telescopes of two entirely different classes. The more familiar forms are those known as refractors, in which the operation of condens- ing the rays of light is conducted by refrac- tion. The character of the refractor is shown in Fig. 1. The rays from the star fall upon the object-glass at the end of the telescope, and on passing through they become refracted into a converging beam, so that all intersect at the focus. Diverging from thence, the rays encounter the eye-piece, which has the effect of restoring them to parallelism. The large cylindrical beam which poured down on the object-glass has been thus condensed into a small one, which can enter the pupil. It should, however, be added that the composite nature of light requires a more complex form of object-glass than the simple lens here shown. In a refracting telescope we have to employ what is known as the achromatic combination, consisting of one lens of flint glass and one of crown glass, ad- justed to suit each other with extreme care. It will now be sufficiently apparent that the larger the object- glass the greater will be the quantity of light grasped, and the greater will be the success of the telescope in revealing very faint objects. Hence it is that in the efforts to increase the powers of their telescopes, each succeeding race of astronomers have sought to obtain larger object-glasses than those used by their predecessors. I Focus IJI ! m To the Eye Fig. 1. — Principle of the Refracting Telescope. 12 THE STORY OF THE HEAVENS. The appearance of an astronomical observatory, designed to accommodate an instrument of moderate dimensions, is shown in the adjoining figures. The first (Fig. 2) represents the dome Fig. 2.— The Dome of the South Equatorial at Dunsink Observatory, Co. Dubli: erected at Dunsink Observatory for the equatorial telescope, the object-glass of which was presented to the Board of Trinity College, Dublin, by the late Sir James South. The main part of the building is a cylindrical wall, on the top of which reposes a hemispherical roof. In this roof is a shutter, which can be THE ASTRONOMICAL OBSERVATORY. 13 opened so as to allow the telescope in the interior to obtain a view of the heavens. The dome is capable of revolving so that the opening may be turned towards that part of the sky where the object happens to be situated. The next view (Fig. 3) exhibits a section through the dome, showing the machinery Fig. 3. — Section of the Dome of Dunsink Observatory. by which the attendant causes it to revolve, as well as the telescope itself. The eye of the observer is placed at the eye- piece, and he is represented in the act of turning a handle, which has the power of slowly moving the telescope, in order to adjust the instrument accurately on the celestial body which it is desired to observe. The two lenses which together form the object-glass of this instrument are twelve inches in diameter, and the quality of the telescope mainly depends on the accuracy with which these lenses have been wrought. The 14 THE STORY OF THE HEAVENS. eye-piece is a comparatively simple matter. It consists merely of one or two small lenses ; and various eye-pieces can be employed, according to the magnifying power which may be desired. It is to be observed that for many purposes of astronomy high magnifying powers are not desirable. The object-glass can only collect a certain quantity of light from the star ; and if the magnifying power be too great, this limited amount of light will be thinly dispersed over too large a surface, and the result will be found unsatisfactory. A telescope mounted in the manner here shown is called an equatorial. The convenience of this peculiar style of supporting the instrument consists in the ease with which the telescope can be moved so as to follow a star in its apparent journey across the sky. The necessary movements of the tube are given by a special mechanism driven by a weight, so that, once the instru- ment has been correctly pointed, the star will remain in the observer's held of view, and the effect of the apparent diurnal movement will be neutralised. The last refinement in this direction is the application of an electrical arrangement by which the driving*- of the instrument is controlled from the standard clock of the observatory. The power of a refracting telescope — so far as the expression has any definite meaning — is to be measured by the diameter of its object-glass. There has, indeed, been some honourable rivalry between the various civilised nations as to which should possess the greatest refracting telescope. Among the largest instruments that have been successfully completed is that erected a few years ago by Sir Howard Grubb, of Dublin, at the splendid observatory at Vienna. This gigantic refractor is represented in Fig. 4. Its dimensions may be estimated from the fact that the object-glass is two feet and three inches in diameter. Many in- genious contrivances help to lessen the inconvenience incident to the use of an instrument possessing such vast proportions. Among them we may here notice the method by which the graduated circles attached to the telescope are brought within view of the observer. These circles are necessarily situated at parts of the instrument which lie remote from the eye-piece where the observer is stationed. The delicate marks and figures Fig. 4.— The Great Vienna Telescope. 16 THE STORY OF THE HEAVENS. Numerous refracting *o^ e Rays of light from the Star 9 1 are, however, easily read from a distance by a small auxiliary telescope (shown in the figure, close to the eye-piece), which, by suitable reflectors, conducts the rays of light from the circles to the eye of the observer. telescopes of exquisite perfection have been produced by Messrs. Alvan Clark, of Cambridgeport, Boston, Mass. The size of the instruments turned out from their works has been gradually increasing. One of their most famous telescopes is the great Lick Refractor now in use on Mount Hamilton in California. The diameter of this object-glass is thirty-six inches, and its focal length is fifty-six feet two inches. A still greater effort has been made by the same firm in the Yerkes refractor of forty inches aperture for the University of Chicago. The limit to the size of the re- fractor is connected with the ma- terial of the object-glass. Glass manufacturers seem to experience unusual difficulties in their attempts to form large discs of optical glass pure enough and uniform enough to be suitable for telescopes. These difficulties are apparently enhanced with every increase in the size of the discs. It may be mentioned in illus- tration that the price paid for the object-glass of the Lick tele- scope exceeded ten thousand pounds. There is, however telescope, in which the difficulty we have just mentioned does not arise. The principle of the simplest form of reflector is shown in Fig. 5, which represents what is called the Herschelian instrument. The rays of light from the star under observation Mirror Fig. 5. — Principle of Herschel's Reflecting Telescope. an alternative method of constructing a 2 18 THE STORY OF THE HEAVENS. fall on a mirror which is both carefully shaped and highly- polished. After reflection, the rays proceed to a focus, and diverging from thence, fall on the eye-piece, by which they are restored to parallelism, and thus become adapted for reception in the eye. It was essentially on this principle, though with a secondary mirror at the upper end of the tube, that Sir Isaac Newton constructed that little reflecting telescope which is now treasured by the Royal Society. A famous instrument of the Newtonian type was built, half a century ago, by the late Earl of Rosse at Parsonstown. It is represented in Fig. 6. The colossal dimensions of this instrument have never been surpassed ; they have, indeed, never been rivalled. The mirror or speculum, as it is often called, is a thick metallic disc, composed of a mixture of two parts of copper with one of tin. This alloy is so hard and brittle as to make the necessary mechanical operations difficult to manage. The material admits, however, of a brilliant polish, and of receiving and retaining an accurate figure. The Rosse speculum — six feet in diameter and three tons in weight — reposes at the lower end of a telescope sixty feet long. The tube is suspended between two massive castellated walls, which form an imposing feature on the lawn at Birr Castle. This instrument cannot be turned about towards any part of the sky, like the equatorials we have recently been considering. The great tube is only capable of elevation in altitude along the meridian, and of a small lateral movement east and west of the meridian. Every star or nebula visible in the latitude of Parsonstown can, however, be observed in the great telescope, if looked for at the right time. Before the object reaches the meridian, the telescope must be adjusted at the right elevation. The necessary power is trans- mitted by a chain from a winch at the northern end of the walls to a point near the upper end of the tube. By this contrivance the telescope can be raised or lowered, and an ingenious system of counterpoises renders the movement equally easy at all altitudes. The observer then takes his station in the lofty gallery which gives access to the eye-piece ; and when the right moment has arrived, the star enters the field of view. Power- ful mechanism drives the great instrument, so as to counteract THE ASTRONOMICAL OBSERVATORY. 19 the diurnal movement, and thus the observer can retain the object in view until he has made his measurements or finished his drawing. Fig. 7. — Meridian Circle. Notwithstanding the stupendous size of a telescope large enough for a tall man to walk through without stooping, it is comparatively easy to manipulate. We must not, however, assume that for the general work in an observatory an in- strument so colossal is the most suitable. The mighty reflec- tor is chiefly of use where unusually faint objects are being 20 THE STORY OF THE HEAVENS. examined. For work in which accurate measurements are made of objects not particularly difficult to see, telescopes of smaller dimensions are more suitable, Among the great reflectors constructed in recent years, we may specially mention those of Mr. Common, which possess great optical perfection and have done excellent astronomical work. The fundamental facts about the heavenly bodies have been chiefly learned from observations obtained with instruments of moderate optical power, specially furnished so as to enable precise measures of position to be secured. Indeed, in the early stages of astronomy, important determinations of position were effected by contrivances which showed the direction of the object without any telescopic aid. Perhaps the most valuable measurements obtained in our modern observatories are yielded by that instrument of precision known as the meridian circle. It is impossible, in any adequate account of the Story of the Heavens, to avoid some reference to this indispensable aid to astronomical research, and therefore we shall give a brief account of one of its simpler forms, choosing for this purpose a great instrument in the Paris Observatory, which is represented in Fig. 7. The telescope is attached at its centre to an axis at right angles to its length. Pivots at each extremity of this axis rotate upon fixed bearings, so that the movements of the tele- scope are completely restricted to the plane of the meridian. Inside the eye-piece of the telescope extremely fine vertical fibres are stretched. The observer watches the moon, or star, or planet, enter the field of view ; and he notes by the clock the exact time, to the fraction of a second, at which the object passes over each of the lines. A silver band on the circle attached to the axis is divided into degrees and subdivisions of a degree, and as this circle moves with the telescope, the elevation at which the instrument is pointed will be indicated. For reading the delicately engraved marks and figures on the silver, micro- scopes are necessary. These are shown in the sketch, each one being fixed into an aperture in the wall which supports one end of the instrument. At the opposite side is a lamp, the light from which passes through the perforated axis of the pivot, THE ASTRONOMICAL OBSERVATORY. 21 and is thence ingeniously deflected by mirrors so as to provide the requisite illumination for the lines at the focus. The fibres which the observer sees stretched over the field of view of the telescope demand a few words of explanation. We require for this purpose a material which shall be very fine and fairly durable, as well as somewhat elastic, and of no appreciable weight. These conditions cannot be completely fulfilled by any metallic wire, but they are exquisitely realised in the beautiful thread which is spun by the spider. The well-known fibres of gossamer are stretched with nice skill across the field of view of the telescope, and cemented in their proper places. With instruments so beautifully appointed we can understand the precision attained in modern observations. The telescope is directed towards a star, and the imao-e of the star is a minute point of light. When that point coincides with the intersection of the two central spider lines, the telescope is properly sighted We use the word sighted designedly, because we wish to suggest a comparison between the sighting of a rifle at the target and the sighting of a telescope at a star. Instead of the ordinary large bull's-eye, suppose that the target only consisted of a watch- dial, which the rifleman could not even see at the distance of any ordinary range. But with the telescope of the meridian circle the watch-dial would be visible even at the distance of a mile. The meridian circle has, indeed, such precision as a sighting instrument that it could be pointed separately to each of two stars which subtend at the eye an angle no greater than that subtended by an adjoining pair of the sixty minute dots around the circumference of a watch dial a mile away. This power of directing the instrument so accurately would be of but little avail unless it were combined with arrangements by which, when once the telescope has been pointed correctly, its position can be ascertained and recorded. One element in the determination of the position is secured by the astronomical clock, which gives the moment when the object crosses the central vertical wire ; the other element is given by the graduated circle which reads the zenith distance. Superb meridian instruments adorn our great observatories, and are nightly devoted to those measurements upon which the 22 THE STORY OF THE HEAVENS. great truths of astronomy are mainly based. These instruments have been constructed with refined skill ; but it is the duty of the painstaking astronomer to distrust the accuracy of his instrument in every conceivable way. The great tube may be as rigid a structure as mechanical engineers can produce ; the graduations on the circle may have been engraved by the most perfect of dividing machines ; but the conscientious astronomer will not be content with mere mechanical precision. That meridian circle which, to the uninitiated, seems a marvellous piece of workmanship, possessing almost illimitable accuracy, is viewed in a very different light by the astronomer who makes use of it. No one can appreciate, indeed, so fully as he the skill of the artist who has made it, and the beautiful contrivances for illumi- nation and reading off' which give the instrument its perfection ; but while the astronomer recognises the beauty of the actual machine he is using, he has always before his mind's eye an ideal instrument of absolute perfection, to which the actual meridian circle only makes an approximation. Contrasted with this ideal instrument, the finest meridian circle is little more than a mass of imperfections. The ideal tube is perfectly rigid, the actual tube is flexible ; the ideal divisions of the circle are perfectly uniform, the actual divisions are not uniform. The ideal instrument is a geometrical embodiment of perfect circles, perfect straight lines, and perfect right angles ; the actual instrument can only show approximate circles, approximate straight lines, and approximate right angles. Perhaps the spider's part of the work is on the whole the best ; she gives us the nearest mechanical approach to a perfectly straight line ; but we mar her work by not being able to insert her beautiful threads with perfect uniformity, while our attempts to stretch two of them across the field of view at right angles do not succeed in producing an angle of exactly ninety degrees. Nor are the difficulties encountered by the meridian observer solely due to his instrument. He has to contend with his own want of skill ; he has often to allow for personal pecu- liarities of an unexpected nature ; the troubles that the atmo- sphere can give are notorious ; while the levelling of his instrument warns him that he cannot even rely on the solid earth itself. We learn that the earthquakes, by which the THE ASTRONOMICAL OBSERVATORY. 23 solid ground is sometimes disturbed, are merely the more con- spicuous instances of incessant and universal movements in the earth which every night in the year derange the delicacy of the instrument. When the existence of these errors has been recognised, the first great step has been taken. By an alliance between the astronomer and the mathematician it is possible to measure the discrepancies between the actual meridian circle and the in- strument that is ideally perfect. Once this has been done, we can estimate the effect which the irregularities produce on the observations, and finally, we succeed in purging the observations from the grosser errors by which they are contaminated. We thus obtain results which are not indeed mathematically accurate, but are nevertheless close approximations to those which would be obtained by a perfect observer, using an ideal instrument of geometrical accuracy, standing on an earth of absolute rigidity, and viewing the heavens without the inter- vention of the atmosphere. It is not, however, necessary to use such great instruments as those just described in order to obtain some idea of the aid the telescope will afford in showing the celestial glories. The most suitable instrument for commencing astronomical studies is within ordinary reach. It is the well-known binocular that a captain uses on board ship ; or if that cannot be had, then the common opera-glass will answer nearly as well. This is, no doubt, not so powerful as a telescope, but it has some compen- sating advantages. The opera-glass will enable us to survey a large region of the sky at one glance, while a telescope, generally speaking, only presents a much smaller field of view. Let us suppose that the observer is provided with an opera- glass and is about to commence his astronomical studies. The first step is to become acquainted with the renowned group of seven stars represented in Fig. 8. It is often called the Plough, or Charles' Wain, but astronomers prefer to regard it as a portion of the constellation of the Great Bear (Ursa Major). There are many features of interest in this constellation, and the beginner should learn as soon as possible to identify the seven stars which are its chief characteristics. Of these the two marked a and /3, 24 THE STORY OF THE HEAVENS. at the head of the Bear, are generally called the " pointers. * They are of special use in astronomy, because they serve to guide the eye to that most important star in the whole sky, known as the " pole star." We shall return in a later chapter to the study of the different constellations. Our present object is a simpler one ; it is merely to employ the Great Bear as a means of illustrating how vast is the richness of the heavens in stars. Every beginner in astronomy is recommended to make a simple observation on the Great Bear, which will give him a THE GREAT BEAR This is a Beautiful^ -L --"Telescopic"-^ € Double "t*-^ Star c --IT- V Fig. 8.— The Great Bear. conception of the power of the telescope, and reveal the glories of the starry heavens in an impressive manner. Fix the attention on that region in the Great Bear, which forms a sort of rectangle, of which the stars a j3 7 8 are the corners. The next fine night try and count how many stars are visible within that rectangle. None are conspicuous, but there are two or three sufficiently bright to be easily seen with the unaided eye. On a very fine night, without a moon, perhaps a dozen might be perceived, or even more, according to the keen- ness of the eyesight. But when the opera-glass is directed to the same part of the constellation an astonishing sight is witnessed. A hundred stars can now be seen with the greatest ease. The opera-glass will, indeed, plainly disclose more than ten times as many stars as could be seen with the unaided eye. But the opera-glass will not show nearly all the stars in this THE ASTRONOMICAL OBSERVATORY. '25 region. Any good telescope will reveal many hundreds too faint for the feebler instrument. The greater the telescope the more numerous the stars ; so that through one of the colossal instru- ments the number would have to be reckoned in thousands. We have chosen the Great Bear because it is more generally known than any other constellation. But the Great Bear is not exceptionally rich in stars. Other parts of the sky would equally demonstrate the grand truth that the stars which our unaided eyes disclose are only an exceedingly small fraction of the entire number with which the whole heaven is teeming. To tell the number of the stars is a task which no man has accomplished ; but various estimates have been made. Our great telescopes can probably show at least 50,000,000 stars. There would be a star apiece for every man, woman, and child in the United Kingdom, and there would still remain a liberal margin for distribution elsewhere. The student of the heavens who uses a good refracting 1 telescope, having an object-glass not less than three inches in diameter, will find ample and delightful occupation for many a fine evening. He should, however, be provided with an efficient atlas of the heavens, which will direct his attention to suitable objects, and afford him the requisite information as to the times and seasons at which they may be sought. CHAPTER II. THE SUN". The vast Size of the Sun — Hotter than Fusing Platinum — Is the Sun the Source of Heat for the Earth? — The Sun is 92,700,000 miles distant — How to realise the magnitude of this distance — Day and Night — Luminous and Non-Lumin- ous Bodies— Contrast between the Sun and the Stars — The Sun a Star — The Spots on the Sun — Changes in the Form of a Spot — They are Depressions on the Surface — The Rotation of the Sun on its Axis — The Size and Weight of the Sun — Is the Sun a Solid Body 1 — View of a Typical Sun-Spot — Periodicity of the Sun-Spots — Connection between the Sun-Spots — Terrestrial Magnetism — The Facuhe — The Granulated Appearance of the Sun — The Prominences surrounding the Sun — Total Eclipse of the Sun — Size and Movement of the Prominences — The Corona Surrounding the Sun — The Heat of the Sun. In commencing our examination of the orbs which surround us, we naturally begin with our peerless sun. His splendid brilliance gives him the pre-eminence over all other celestial bodies. The dimensions of our luminary are commensurate with his importance. Astronomers have succeeded in the difficult task of as- certaining the exact figures, but they are so gigantic that the results are hard to realise. The diameter of the orb of day, or the length of the axis, passing through the centre from one side to the other, is 865,000 miles. Yet this bare statement of the dimensions of the great globe fails to convey an adequate idea of its vastness. If a railway were laid round the sun, and if we were to start in an express train moving sixty miles an hour, we should have to travel for five years without intermission night or day before we had accomplished the journey. When the sun is compared with the earth the bulk of our luminary becomes still more striking. Suppose his globe were cut up into one million parts, each of these parts would appreciably exceed the bulk of our earth. Were the sun placed THE SUN. 27 in one pan of a mighty weighing balance, and were 300,000 bodies as heavy as our earth placed in the other, the luminary would turn the scale. Fig. 9 exhibits a large circle and a very small one, marked S and E respectively. These circles show the comparative sizes of the two bodies. The sun has a temperature far surpassing any that we arti- ficially produce, either in our chemical laboratories or our metallurgical establishments. We can send a galvanic current Fig. 9. — Comparative Sizes of the Earth and the Sun. through a piece of platinum wire. The wire first becomes red hot, then white hot ; then it glows with a brilliance almost dazzling until it fuses and breaks. The temperature of the melting platinum wire could hardly be surpassed in the most elaborate furnaces, but it does not attain the temperature of the sun. It must, however, be admitted that there is an apparent discrepancy between a well-known physical fact and the ex- tremely high temperature that we find it necessary to attribute to the sun. " If the sun were hot," it has been said, " then the nearer we approach to him the hotter we should feel ; yet this « does not seem to be the case. On the top of a high mountain we are nearer to the sun, and yet everybody knows that it is 28 THE STORY OF THE HEAVENS. much colder up there than in the valley beneath. If the mountain be as high as Mont Blanc, then we are certainly two or three miles nearer ; yet, instead of additional warmth, we find eternal snow." A simple illustration will lessen this difficulty. Go into a greenhouse on a sunshiny day, and we find the temperature much hotter there than outside. The glass will permit the hot sunbeams to enter, but it refuses to allow them out again with equal freedom, and consequently the temperature rises. The earth may, from this point of view, be likened to a greenhouse, only, instead of the panes of glass, our globe is enveloped by an enormous coating of air. When on the earth's surface, we are, as it were, inside the greenhouse, and we benefit by the interposition of the atmosphere ; but when we climb very high mountains, Ave gradually pass through some of the protecting medium, and then we suffer from the cold. If we could imagine the earth to be deprived of its coat of air, then eternal frost would reign over whole continents as well as on the tops of the mountains. The actual distance of the sun from the earth is about 92,700,000 miles ; but by merely reciting the figures we do not receive a vivid impression of the real magnitude. 92,700,000 is indeed a very large number. It would be necessary to count as quickly as possible for three days and three nights before one million was completed ; yet this would have to be repeated nearly ninety-three times before we had counted all the miles between the earth and the sun. Every clear night we see a vast host of stars scattered over the sky. Some are bright, some are faint, some are grouped into remarkable forms. With regard to this multitude of brilliant points we have now to ask an important question. Are they bodies which shine by their own light like the sun, or do they only shine with borrowed light like the moon ? The answer is easily stated. Most of these bodies shine by their own light, and they are properly called stars. If, then, our sun and the multi- tude of stars, properly so called, are each and all self-luminous brilliant bodies, what is the great distinction between the sun and the stars ? There is, of course, a vast and obvious difference between the unrivalled splendour of the sun and the feeble THE SUN. 29 twinkle of the stars. Yet this distinction does not necessarily indicate that our luminary has an intrinsic splendour superior to that of the stars. The fact is that we are nestled up compara- tively close to the sun for the benefit of his warmth and light, while Ave are separated from even the nearest of the stars by a mighty abyss. If the sun were gradually to retreat from the earth, his light would decrease, so that when he had penetrated the depths of space to a distance comparable with that by which we are separated from the stars, his glory would have utterly departed. No longer would the sun seem to be the majestic orb with which we are familiar. No longer would he be a source of genial heat, or a luminary to dispel the darkness of night. Our great sun would have shrunk to the insignificance of a star, not so bright as many of those which we see every night. Momentous indeed is the conclusion to which we are now led. That myriad host of stars which studs our sky every night has been elevated into vast importance. Each one of those stars is itself a mighty sun, actually rivalling, and in many cases surpass- ing, the splendour of our own luminary. We thus open up a majestic conception of the vast dimensions of space, and of the dignity and splendour of the myriad globes by which that space is tenanted. There is another aspect of the picture not without its utility. We must from henceforth remember that our sun is only a star, and not a particularly important star. If the sun and the earth, and all which it contains, were to vanish, the effect in the universe would merely be that a tiny star had ceased its twinkling. Viewed simply as a star, the sun must retire to a position of in- significance in the mighty fabric of the universe. But it is not as a star that we have to deal with the sun. To us his comparative proximity gives him an importance incalculably transcending that of all the other stars. We imagined our- selves to be withdrawn from the sun to obtain his true perspec- tive in the universe ; let us now imagine that we draw near, and give to him that attention which his supreme importance to us merits. To the unaided eye the sun appears to be a flat circle. If, 30 THE STORY OF THE HEAVENS. however, it be examined with the telescope, taking care of course to interpose a piece of deep-coloured glass, or to employ some similar precaution to screen the eye from injury, it will then be perceived that the sun is not a flat surface, but a veritable glowing globe, of which one hemisphere is presented to us. Fig. 10. — The Sun, photographed on September 22, 1870. The first question which we must attempt to answer inquires whether the glowing matter which forms the globe is a solid mass, or, if not solid, which is it, liquid or gaseous? At the first glance we might think that the sun cannot be fluid, and we might naturally imagine that it was a solid ball of some white-hot substance. But this view is not correct ; for we can show that the sun is certainly not a solid body in so far at least as its superficial parts are concerned. Look at the general view of the sun shown by a telescope of THE SUN. 31 moderate dimensions. It is represented in Fig. 10.* We observe the circular outline and the general bright surface, but the brilliancy of the face of the sun is not quite uninterrupted. There are, here and there, small, dark objects called spots, which can be made to render a great deal of information with respect to the sun. These spots vary both as to size and as to number — indeed, the sun seems sometimes almost devoid of them. In the early days of telescopes the discoverers of the sun's spots were laughed at. They were told that our luminary was far too perfect to have any blemishes, and that it would be absurd to suppose that " the eye of the universe could suffer from ophthalmia." The general character of a sun-spot, as seen in a moderate telescope under ordinary circumstances, is illustrated in Fig. 11, Fig. 11.— An Ordinary Sun-Spot. in which the dark central part is shown in sharp contrast with the lighter margin. Various theories have been propounded as to the nature of these curious features. One of the early astronomers suggested that they were merely objects situated between the earth and the sun, and shown projected on the brilliant background. It is easy to prove that this cannot be the case, by carefully watching the same spot for a few days. Look first at Fig. 12, which exhibits a small portion of the sun and a part of its edge as seen in a good telescope ; the object marked a represents a sun-spot of the ordinary type. The central portion seems black by contrast with the brilliant back- ground which surrounds it. Around the black centre there is a shaded region of a somewhat lighter hue. We carefully observe the spot and measure how far it appears to be from the edge of the sun. The next day we repeat the observation, and find that * This picture is taken from a photograph of the sun obtained by Mr, Rutherford at New York on the 22nd September, 1870. 32 THE STORY OF THE HEAVENS. the spot is no longer in its original position. It has travelled nearer the edge of the sun, to the place marked b. Kepeat the same observation the third day, and it will be found that the little object has attained the position c, which is still nearer to the edge of the sun. It will also be noticed that its appearance has decidedly changed. The shaded portion at one side has diminished, and, indeed, disappeared. Day after day the spot gradually approaches the margin of the sun, indeed, it is on rare Fig. 12. — Successive Appearances of a Suu-Spot. occasions actually seen on the very edge, though it is not so represented in this drawing. By such observations we learn that the spot cannot be a body floating aloft above the luminous surface, for then a space between this object and the sun would be seen after the spot had reached the edge. Yet, though we must admit that the spots are veritable solar objects, we cannot agree with those old philosophers who said that they are to be regarded as blemishes which detract from the perfection of the eye of the heavens. We ought rather, in these days of scientific activity, to feel thankful to the spots ; for they teach us much about the nature of the sun of which we must otherwise have remained in ignorance. An artist who was viewing the changes in the appearance ol the spot as it approached the margin of the sun would rightly conclude that the apparent alterations of its form were chiefly due to what he would call the effect of fore-shortening, and he THE SUN. 33 would draw the conclusion that the spot must be a cavity in the surface of the sun. If the interior parts of the luminary were much darker than the exterior, and if the spots were really basin- shaped apertures through the outer portions of the sun, then some of the changes in the appearances of the spots could be c D Fig. 13. — Scheiner's Observations on Sim-Spots. readily explained. When the region is turned directly towards the observer he sees the bottom of the basin which exposes the dark interior of the sun. This view of the spot is represented at a. As it is carried nearer to the limb of the sun, one side of the basin becomes much fore-shortened, and the appearance is represented at b, in which we observe that the shaded edge of the spot on the left is narrower than that on the other side. Finally, when the object approaches extremely close to the limb, then one side of the basin would be entirely hid from view, 3 34 THE STORY OF THE HEAVENS. and only a glimpse could be obtained of the dark interior ; while trie opposite side is distorted into undue prominence. The apparent movement of the spots across the face of the sun is well illustrated in the drawing shown in Fig. 13, which was made nearly three centuries ago. On the 2nd March, 1627, the skilful astronomer Schemer observed an object in the posi- tion marked 2, just on the edge of the sun. By the next day the spot had moved to the position marked 3. It appears that Schemer was favoured with a succession of fine weather, for day after day he followed it until the 11th, when clouds interrupted his observations. On this day, therefore, he outlined a mark on his drawing, at the place where he. reasonably con- jectured that the spot must have been. The following day he resumed his observations. The 13th seems also to have been cloudy, but on the 14th another view of the feature was obtained, just before its disappearance at the edge of the sun. In the same month it so happened that a second conspicuous spot could be observed, and the faithful Scheiner recorded its place also from clay to day, as is duly shown on the drawing. Again we find interruptions due to the clouds on the 11th and 13th. In each case the object travelled in the same direction and com- pleted the journey across the face of the sun in a period of about twelve or thirteen days. It is invariably found that these objects move in the same direction. It is also noticed that when the spots disappear at the edge of the sun, they certainly remain invisible for twelve or thirteen days, after which it often happens that the same objects reappear at the other edge. It is therefore obvious that they must move round behind the sun in about the same time that they occupy in crossing his face. Further inquiries on this subject have enabled the speed of the spots to be ascertained with some accuracy, and it has been shown that they accomplish a complete revolution around the sun in about twenty-five days and five hours. So remarkable a characteristic of the movements of the spots demands satisfactory explanation. How does it come to pass that all these objects— whether large, small, regular or irregular —accomplish their revolutions in nearly the same time ? A THE SUN. 35 simple explanation of the phenomenon conducts to an important discovery. We know that the sun is a globe, and that our earth is also a globe. We also know that the earth performs a daily rotation on its axis, and it is natural to inquire whether it may not be like- wise possible for the sun to rotate. If the sun turned round in a period of twenty-five days and five hours, then the hemi- sphere directed towards the earth would be completely turned to the other side in a fortnight, and we should have a ready method of accounting for the apparent movement of the spots. This explanation is so simple, and so satisfactory — it is con- firmed by so many other lines of reasoning — that doubt can no longer attach to it. Hence we have, as the first fruits of the study of the spots, ascertained the remarkable fact of the rotation of the sun on its axis. It has, however, been shown that the time of rotation of each sun-spot varies slightly with its position on the sun's face. Observations made with spots at the Equator give for the period of rotation a value of 25^ days, while, judging 'from spots at the latitude of 30°, the period of rotation is a day longer, while for higher latitudes it has been lately shown to be longer still. Of course if the sun were really a solid body all these periods would have to be equal. It must not, however, be imagined that the only changes in the spots are those variations of perspective which arise from the rotation of the sun. In this movement all such features partici- pate ; but there are other movements and changes constantly going on in the individual spots. Some of these objects may last for days, for weeks, or for months, but they are in no sense per- manent ; and after an existence of greater or less duration, those on one part of the sun may disappear, while as frequently fresh marks of the same kind become visible in other places. The inference from these various facts is irresistible. They tell us that the visible surface of the sun is not a solid mass — is not even a liquid mass — but that the globe, as far as we can see it, consists of matter in the gaseous or vaporous condition. It often happens that a large spot divides into two or more separate portions, and these have been sometimes seen to fly apart with a velocity in some cases not less than a thousand 3G THE STORY OF THE HEAVENS. miles an hour. " At times, though very rarely " (I quote here Professor Young,* to whom I am so frequently indebted), " a different phenomenon of the most surprising and startling character appears in connection with these objects ; patches of intense brightness suddenly break out, remaining visible for a few minutes, moving, while they last, with velocities as great as one hundred miles a second " One of these events has become classical. It occurred on the forenoon (Greenwich time) of September 1st, 1859, and was independently witnessed by two well-known and reliable ob- Fig. 14. — Zones on the Sun's surface in which Spots appear. servers — Mr. Carrington and Mr. Hodgson — whose accounts of the matter may be found in the monthly notices of the Royal Astronomical Society for November, 1859. Mr. Carrington at the time was making his usual daily observations upon the position, configuration, and size of the spots by means of an image of the solar disk upon a screen — being then engaged upon that eight years' series of observations which lies at the foundation of so much of our present solar science. Mr. Hodgson, at a distance of many miles, was at the same time sketching details of sun-spot structure by means of a solar eye-piece and shade- glass. They simultaneously saw two luminous objects, shaped something like two new moons, each about eight thousand miles in length and two thousand wide, at a distance of some twelve thousand miles from each other. These burst suddenly into * "The Sun," p. 119. THE SUN. 37 sight at the edge of a great sun-spot with a dazzling brightness at least live or six times that of the neighbouring portions of the photosphere, and moved eastward over the spot in parallel lines, growing smaller and fainter, until in about five minutes they disappeared, after traversing a course of nearly thirty-six thousand miles." We have still to note an extraordinary feature, which points to an intimate connection between the phenomena of sun-spots, and the purely terrestrial phenomena of magnetism. It has been often noticed that a maximum abundance of sun-spots occurs simultaneously with an unusual amount of disturbance in the magnetic needle. Mr. Elias Loomis has illustrated this in a striking manner from an examination of 135 magnetic storms recorded at Greenwich in twenty-three years. He has shown that an abnormal development of the earth's magnetism is accompanied on the very day of the storm by a marked dis- turbance of the sun's surface. It also appears that the sun-spots are specially abundant about three or four days before the out- break, though on the day immediately preceding the sun may seem perfectly quiescent. The earth's magnetism is well known to be connected with the phenomena of the aurora borealis, inasmuch as an unusual aurora seems to be invariably ac- companied by a great magnetic disturbance. It has also been shown that there is an almost perfect parallelism between the intensity of auroral phenomena and the abundance of sun-spots. Besides these general coincidences, there have been also special cases in which a peculiar outbreak on the sun has been associated with remarkable magnetic phenomena on the earth. Thus, the occurrence cited above, in 1859, was immediately followed by a magnetic storm of unusual intensity, as well as by splendid auroras, not only in Europe and America, but even in the Southern Hemisphere. A very interesting instance of a similar kind is recorded by Professor Young, who, when observing at Sherman on the 3rd August, 1872, perceived a very violent disturbance of the sun's surface. He was told the same day by the photographer of the party, who was engaged in magnetic observations, and who was quite in ignorance of what Professor Young had seen, that he had been obliged to desist from the 38 THE STORY OF THE HEAVEN'S. magnetic observations, in consequence of the violent fluctuations of the needle. Subsequent inquiry showed that a magnetic storm was also experienced in England on the same day. These observations demonstrate that there is some con- nection between solar phenomena and terrestrial magnetism, but what the nature of that connection may be is quite unknown. I have endeavoured to discuss the matter fully in " The Story of the Sun." Fig. 15. — The Texture of the Sun and a small Spot. Another mysterious law governs the sun-spots. Their number fluctuates from year to year, but it would seem that the epochs of maximum sun-spots succeed each other with a certain degree of regularity. The observations for nearly three centuries show that the recurrence of a maximum takes place, on an average, every eleven years. The course of one of these cycles may be described as follows : — For two or three years the sun-spots are both larger and more numerous than on the average ; then they begin to diminish, until in about five or six years from the maximum they decline to a minimum ; the number of the spots then begins to increase, and in another five or six years the maximum is once more attained. The cause of the periodicity in the area of the sun affected by sun-spots opens up a question THE SUN. 39 of profound interest, but at present the answer must be regarded as unknown. When our atmosphere is clear enough and steady enough to allow of exceptionally good vision, we can see that the sun's surface is mottled in a remarkable manner. This is well shown Fig. 16. — Dr. Huggins' drawing of a remarkable arrangement of Solar Granules. in Fig. 15, in which we perceive that the spot in the central part of the picture seems to be merely an enlargement of one of the minute pores with which the surface is marked. A remark- able illustration of the granulated appearance which the sun often presents is shown in a drawing made by the accurate pencil of Dr. W. Huggins (Fig. 16). Similar curious arrangements have also been witnessed by many other observers. Indeed, 40 THE STORY OF THE HEAVENS. photographs have been taken in which these brilliant granules seem to be disposed in singular patterns. It would thus appear as if the luminous surface of the sun was composed of intensely bright clouds suspended in a darker atmosphere. Some observers have thought that these floating objects are, occasionally at all events, of a characteristic size and shape, variously known as " willow leaves " or " rice grains." In Fig. 17 the willow-leaf texture is shown surrounding a sun-spot. The spots themselves generally suggest that the surface of the sun is in a state of violent disturbance. This is well shown in ■pig. 17.— The Willow-leaf Texture of the Sun's surface. Professor Langley's fine drawing (Plate II.) of an object which he observed on December 23-24, 1873. Near the edge of the sun, as represented in Plate III,, will be seen some of those brighter streaks or patches which are called facuke (little torches). They are often of enormous dimensions, and cover larger areas than any of our continents. The margin of the sun is fringed with objects of delicate beauty. They are so faint that in the full blaze of sunlight they cannot be readily observed. They are ordinarily invisible for the same reason that the stars are not to be seen in daylight. We can, however, see the remarkable solar fringe when the brilliant body of the sun is obscured during the rare occurrence of a total eclipse. When an eclipse of the sun takes place, the moon must actually come between the earth and the sun. The conditions necessary for the occurrence of an eclipse will be more fully con- THE SUN. 41 sidered in the next chapter. For the present it will be sufficient to observe that by the movement of the moon it may so happen that the moon completely hides the sun, and thus produces what we call a total eclipse. The few minutes during which a total eclipse lasts are of much interest to the astronomer. Darkness reigns over the earth, and in that darkness rare and beautiful sights can be witnessed. We have in Fig. 18 a diagram of a total eclipse, showing the position of some of those remarkable objects known as pro- minences (a, b, c, d, e), which project from the surface of the Fig. 18.— Prominences seen in Total Eclipse. sun. Objects of this character surround the sun at other times as well as at eclipses, but the great light of the sun renders them invisible to the unaided eye. From the obscurity which sur- rounds the sun during a total eclipse as a background, the phenomenon starts into brilliancy. It has been demonstrated that these objects are mighty masses of glowing gas ; and a beautiful arrangement has been discovered by which it has been made possible to view the pro- minences without waiting for the aid . of an eclipse. Suffice it now to observe that the principle of the method depends upon the peculiar character of the light radiated from the promi- nences, which the spectroscope enables us to isolate from the glare produced by the ordinary solar beams. It gives to astronomers the great advantage of looking at the prominences for hours together, instead of being limited to the few minutes during which an eclipse lasts. These objects appear to be 42 THE STORY OF THE HEAVENS. merely protuberant portions of a layer of red incandescent gas surrounding the sun. This gas has been shown to contain hydrogen and other substances. Majestic indeed are the proportions of some of those mighty prominences which leap from the luminous surface ; yet they nicker, as do our terrestrial names,, when we allow them time comparable to their gigantic dimensions. Drawings of the same prominence made at intervals of a few hours, or even less, often show great changes. The magnitude of the displacements that have been noticed sometimes attain many thousands of miles, and the actual velocity with which such masses move frequently exceeds 100 miles a second. Still more violent are the solar convulsions which some observers have been so fortunate as to behold, when from the sun's surface, as from a mighty furnace, vast incandescent masses are projected upwards. All indications show that the surface of the sun must be agitated bv tremendous storms and tempests, where the winds are intensely heated gases and the clouds are incandescent vapours. The remarkable facilities which the spectroscope places at our disposal for observing the prominences without a total eclipse, has been largely availed of in making drawings of these objects. Plate IV. gives a beautiful view of a number of them as seen by Trouvelot with the great telescope at Harvard College Observatory, Cambridge, U.S.A. These drawings show the red colour of the name-like bodies. Trouvelot has also succeeded in exhibiting in the different pictures the wondrous variety of aspect which these objects assume. The dimensions of the prominences may be inferred from the scale appended to the plate. The largest of those here shown is fully 80,000 miles high ; and trustworthy observers have recorded prominences of even much greater altitude. The rapid changes which these objects sometimes undergo are well illustrated in the two sketches on the left of the lowest line, which were drawn on April 27th, 1872. These are both drawings of the same promin- ence taken at an interval no greater than twenty minutes. This mighty flame is so vast that its length is ten times as great as the diameter of the earth, yet in this brief period it has com- pletely changed its aspect; the upper part of the flame has, THE SUN. 43 indeed, broken away, and is now shown in that part of the drawing between the two figures on the line above. The same plate also shows various instances of the remarkable spike- like objects, taken, however, at different times and at various parts of the sun. These spikes attain altitudes not generally greater than 20,000 miles, though sometimes they soar aloft to stupendous distances. We may quote one special object of this kind, the remarkable history of which has been chronicled by Professor Young.* On October 7th, 1880, a prominence was seen, at about 10.30 a.m., on the south-east limb of the sun. It was then about 40,000 miles high, and attracted no special attention. Half an hour later a marvellous transformation took place. During that brief interval the prominence became very brilliant, and doubled its length. For another hour the mighty flame still rose upwards, until it attained the un- precedented elevation of 350,000 miles — a distance more than one-third the diameter of the great luminary itself. At this climax the energy of the mighty outbreak seems to have at last become exhausted : the flame broke up into fragments, and by 12.30 — an interval of only two hours from the- time when it was first noticed — the phenomenon had completely faded away. No doubt this particular eruption was exceptional in its vehemence, and in the vastness of the changes of which it was an indication. The velocity of upheaval must have been at least 200,000 miles an hour, or, to put it in another form, more than fifty miles a second. This mighty flame leaped from the sun with a velocity more than 100 times greater than that of the swiftest bullet ever fired from a rifle. The most striking feature of a total eclipse of the sun is unquestionably the Corona, or aureole of light which is then seen to surround the sun. On such an occasion, when the sky * During- a visit to the United States in the autumn of 1884, the author was fortunate enough, by the kindness of Professor Young, to observe several solar prominences with the superb instruments at Princeton, New Jersey. Professor Hale has devised an ingenious method for photographing the prominences. His contributions to our knowledge of the sun are of much importance. (See " The Story of the Sun.") 44 THE STORY OF THE HEAVENS. is clear, the moon appears of an inky darkness, not like a flat screen, but like the huge black ball that it really is. " From behind it (I quote Professor Young) stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo with the black globe of the moon in its apparent centre. The portion nearest the sun is of dazzling brightness, but still less brilliant than the jDrominences which blaze through it like carbuncles. Generally this inner corona has a pretty uniform height, forming a ring three or four minutes of arc in width, separated by a somewhat definite outline from the outer corona, which reaches to a much greater distance, and is far more irregular in form ; usually there are several " rifts," as they have been called, like narrow beams of darkness, extending from the very edge of the sun to the outer night, and much resembling the cloud shadows which radiate from the sun before a thunder-shower. But the edges of these rifts are frequently curved, showing them to be something else than real shadows ; sometimes there are narrow bright streamers as long as the rifts, or longer. These are often inclined, occa- sionally are even nearly tangential to the solar surface, and frequently curved. On the whole, the corona is usually less extensive and brilliant on the solar poles, and there is a re- cognisable tendency to accumulation above the middle latitudes or spot zones, so that, roughly speaking, the corona shows a disposition to assume the form of a quadrilateral or four- rayed star, though, in almost every individual case, this form is greatly modified by abnormal streamers at some point or other." In Fig. 19 we present a view of the corona during a total eclipse, and we further present, in Plate V., the drawing of the corona made by Professor W. Harkness, from a comparison of a large number of photographs taken at different places in the United States during the total eclipse of July 29th, 1878. As to the precise nature of this wonderful appendage, we must for the present be content to wait for further knowledge. Probably when we understand the streamers of the aurora PIRATE IV. CAS SELL & COMPANY, J.IMITED.LITH LONDON. Scale of English Miles. 0 10 20 30 40 50 60 70 80 90 100,000 1 l i l i I I l i l I SOLAR PROMINENCES. (DRAWN BY TROUVELOT AT HARVARD COLLEGE, CAMBRIDGE, U.S., IN I072> THE SUN. 45 borealis and the tails of comets, we shall have learned something of those substances, well-nigh spiritual in texture, which con- stitute the solar corona A remarkable appendage to the sun, which extends to a distance much greater than that of the corona, produces the Fig. 19.— View of the Corona in a Total Eclipse. phenomenon of the zodiacal light. A pearly glow is sometimes seen to spread over a part of the sky in the vicinity of the point where the sun has disappeared after sunset. The same spectacle may also be witnessed before sunrise, and it would seem as if the material producing the zodiacal light, whatever it may be, had a lens-shaped form with the sun in the centre. The nature of this 46 THE STORY OF THE HEAVENS. object is still a matter of great uncertainty. A view of it is shown in Fig. 20. In all directions the sun pours forth, with the most prodigal liberality, its torrents of light and of heat. The greater part of this outlay seems quite wasted in the depths of space. The earth can only grasp the merest fraction, less than the 2,000,000,000th part of the whole. Our fellow planets and the moon also intercept a trifle ; but how small is the portion of the mighty flood which they can utilise ! The sip that a flying swallow takes from a river is as far from exhausting the water in hte river as are the planets from using all the heat which streams from the sun. Were the radiation from the sun to be intercepted, all life on this earth must cease. We learn from Professor Dewar that the atmosphere would have passed into a liquid, indeed probably into a solid, state. The ocean also would, of course, be solid, and the silence of death over the surface of the earth would only be broken by the occasional groans of a volcano. In my lectures addressed to a juvenile audience at the Eoyal Institution I have endeavoured to set forth in a simple fashion some account of the way in which the sun ministers to our daily wants. As these lectures have been published in my little volume known as " Star-land," I shall here merely terminate this chapter by a brief recital of what we at present enjoy by the benign influence of the sun. His gracious beams supply the magic power that enables the corn to grow and ripen. It is the heat of the sun which raises water from the ocean in the form of vapour, and then sends down that vapour as rain to refresh the earth and to fill the rivers which bear our ships down to the ocean. It is the heat of the sun beating on the large continents which gives rise to the breezes and winds that waft our vessels across the deep ; and when on a winter's evening we draw around the fire and feel its invigorating rays, we are only enjoying sunbeams which shone on the earth countless ages ago. The heat in those ancient sunbeams developed the mighty vegetation of the coal period, and in the form of coal that heat has slumbered for millions of years, till we now call it again into activity. It is the power of the sun stored up in coal 48 THE STORY OF THE HEAVERS. that urges on our steam-engines. It is the light of the sun stored up in coal that beams from every gas-light in our cities. For the power to live and move, for the plenty with which we are surrounded, for the beauty with which nature is adorned, we are immediately indebted to one body in the countless hosts of space, and that body is the sun. PLATE V. TOTAL SOLAR ECLIPSE, JULY 29th, 1878. THE CORONA FROM THE PHOTOGRAPHS.' (H A R K N E S 3.) CHAPTER III. THE MOON. The Moon and the Tides — The Use of the Moon in Navigation — The Changes of the Moon — The Moon and the Poets — Whence the Light of the Moon ?— Sizes of the Earth and the Moon — Weight of the Moon — Changes in Apparent Size — Variations in its Distance — Influence of the Earth on the Moon — The Path of the Moon — Explanation of the Moon's Phases — Lunar Eclipses — Eclipses of the Sun, how produced — Visibility of the Moon in a Total Eclipse — How Eclipses are Predicted— Uses of the Moon in finding Longitude — The Moon not connected with the Weather — Topography of the Moon — Nasmyth's Drawing of Triesnecker — Volcanoes on the Moon — Normal Lunar Crater — Plato — The Shadows of Lunar Mountains — The Micrometer — Lunar Heights — Former Activity on the Moon — Nasmyth's View of the Formation of Craters — Gravitation on the Moon — Varied Sizes of the Lunar Craters — Other features of the Moon — Is there Life on the Moon 1 — Absence of Water and of Air — Explanation of the Rugged Character of Lunar Scenery— Possibility of Life on Distant Bodies in Space. If the moon were suddenly struck out of existence, we should be immediately apprised of the fact by a Avail from every sea- port in the kingdom. From London and from Liverpool we should hear the same story — the rise and fall of the tide had almost ceased. The ships in dock could not get out ; the ships outside could not get in ; and the maritime commerce of the world would be thrown into dire confusion. The moon is the principal agent in causing the daily ebb and flow of the tide, and this is the most important work which our satellite has to do. The fleets of fishing boats around the coasts time their daily movements by the tide, and are largely indebted to the moon for bringing them in and out of harbour. Experi- enced sailors assure us that the tides are of the utmost service to navigation. The question as to how the moon causes the tides is postponed to a future chapter, in which we shall also sketch the marvellous part which the tides seem to have played in the past history of our earth. 4 50 THE STORY OF THE HEAVENS. Who is . there that has not watched, with admiration, the beautiful series of changes through which the moon passes every month ? We first see her as an exquisite crescent of pale light in the western sky after sunset. Night after night she moves further and further to the east, until she becomes full, and rises about the same time that the sun sets. From the time of the full the disc of light begins to diminish until the last quarter h reached. Then it is that the moon is seen hioii in the heavens o in the morning. As the days pass by, the crescent shape is again assumed. The crescent wanes thinner and thinner as the satellite draws closer to the sun. Finally she becomes lost in the over- powering light of the sun, again to emerge as the new moon, and again to go through the same cycle of changes. The brilliance of the moon arises solely from the light of the sun, which falls on the not self-luminous substance of the moon. Out of the vast Hood of light which the sun pours forth with such prodigality into space the dark body of the moon intercepts a little, and of that little it reflects a small fraction to illuminate the earth. The moon sheds so much light, and seems so bright, that it is often difficult at night to remember that the moon has no light except what falls on it from the sun. Never- theless, the actual surface of the brightest full moon is perhaps not much brighter than the streets of London on a clear sunshiny day. A very simple observation will suffice to show that the moon's light is only sunlight. Look some morning at the moon in daylight, and compare the moon with the clouds. The bright- ness of the moon and of the clouds are directly comparable, and then it can be readily comprehended how the sun which illu- minates the clouds has also illumined the moon. An attempt has been made to form a comparative estimate of the brightness of the sun and the full moon. If 600,000 full moons were shining at once, their collective brilliancy would equal that of the sun. The beautiful crescent moon has furnished a theme for many a poet. Indeed, if we may venture to say so, it would seem that some poets have forgotten that the moon is not to be seen every night. A poetical description of evening is almost certain to be associated with the appearance of the moon in some phase or other. We may cite one notable instance in which a poet, THE MOON. 51 describing an historical event, has enshrined in exquisite verse a statement which cannot be correct. Every child who speaks our language has been taught that the burial of Sir John Moore took place " By the struggling moonbeams' misty light." There is an appearance of detail in this statement which wears the garb of truth. We are not inclined to doubt that the night was misty, nor as to whether the moonbeams had to struggle into visibility ; the question at issue is a much more funda- mental one. We do not know who was the first to raise the point as to whether any moon shone on that memorable event at all or not ; but the question having been raised, the Nautical Almanac immediately supplies an answer. From it we learn in language, whose truthfulness constitutes its only claim to be poetry, that the moon was new at one o'clock in the morning of the day of the battle of Corunna (16th January, 1809). The ballad evidently implies that the funeral took place on the night following the battle. We are therefore assured that the moon can hardly have been a day old when the hero was consigned to his grave. But the moon in such a case is practically invisible, and yields no appreciable moonbeams at all, misty or otherwise. Indeed, if the funeral took place at the " dead of night," as the poet asserts, then the moon must have been far below the horizon at the time.* In alluding to this and similar instances, Mr. Nasmyth gives a word of advice to authors or to artists who desire to bring the moon on a scene without knowing as a matter of fact that our satellite was actually present. He recommends them to follow the example of Bottom in A Midsummer NigMs Dream, and consult " a calendar, a calendar ! look in the almanac ; find out moonshine, find out moonshine ! " Among the countless host of celestial bodies — the sun, the moon, the planets, and the stars — our satellite enjoys one special * Some ungainly critic has observed that the poet himself seems to have felt a doubt on the matter, because he has supplemented the dubious moonbeams by the " lantern dimly burning." The more generous, if somewhat sanguine remark, has been also made, that " the time will come when the evidence of this poem will prevail over any astronomical calculations." 52 THE STORY OF THE HEAVENS. claim on our attention. The moon is our nearest permanent neighbour. It is just possible that a comet may occasionally approach the earth more closely than the moon, but with this exception the other celestial bodies are all many hundreds or thousands, or even many millions, of times further from us than the moon. It is also to be observed that the moon is one of the smallest Fig. 21. — Comparative sizes of the Earth and Moon. visible objects which the heavens contain. Every one of the thousands of stars that can be seen with the unaided eye is enormously larger than our satellite. The brilliance and ap- parent vast proportions of the moon arise from the fact that it is only 240,000 miles away, which is a distance almost im- measurably small when compared with the distances between the earth and the stars. Fig. 21 exhibits the relative sizes of the earth and its attendant. The small globe shows the moon, while the larger globe represents the earth. When we measure the actual diameters of the two globes, we find that of the earth to be 7,918 THE MOON. 53 miles, and of the moon 2,160 miles, so that the diameter of the earth is nearly four times greater than the diameter of the moon. If the earth were cut into fifty pieces, all equally large, then one of these pieces rolled into a globe would equal the size of the moon. The superficial extent of the moon is equal to about one- thirteenth part of the surface of the earth. The hemisphere our neighbour turns towards us exhibits an area equal to about one twenty-seventh part of the area of the earth. This, to speak approximately, is about double the actual extent of the continent of Europe. The average materials of the earth are, however, much heavier than those contained in the moon. It would take more than eighty globes, each as ponderous as the moon, to weigh down the earth. Amid the changes which the moon presents to us, one ob- vious fact stands prominently forth. Whether our satellite be new or full, at first quarter or at last, whether it be high in the heavens or low near the horizon, whether it be in process of eclipse by the sun, or whether the sun himself is being eclipsed by the moon, the apparent size of the latter is nearly constant. We can express the matter numerically. A globe one foot in diameter, at a distance of 110 feet from the observer, would under ordinary circumstances be just sufficient to hide the disk of the moon ; occasionally, however, the globe would have to be brought in to a distance of only 101 feet, or occasionally it might have to be moved out to as much as 119 feet if the moon is to be exactly hidden. It is unusual for the moon to approach either of its extreme limits of position, so that the distance from the eye at which the globe must be situated so as to exactly cover the moon is usually more than 105 feet, and less than 115 feet. These fluctuations in the apparent size of our satellite are contained within such narrow limits, that in the first glance at the subject they may be overlooked. It will be easily seen that the apparent size of the moon must be connected with its real distance from the earth. Suppose, for the sake of illustra- tion, that the moon were to recede into space, its size would seem to dwindle, and long ere it had reached the distance of even the very nearest of the other celestial bodies, it would have shrunk into insignificance. On the other hand, if ,the 54 THE STORY OF THE HEAVENS. moon were to come nearer to the earth, its apparent size would gradually increase until, when close to our globe, it would seem like a mighty continent stretching over the sky. We find that the apparent size of the moon is nearly constant, and hence we infer that the average distance of the same body is also constant. The average value of that distance is 240,000 miles. It may approach under rare circumstances to a distance but little more than 220,000 miles ; it may recede under rare circumstances (to a distance hardly less than 260,000 miles, but the ordinary fluc- tuations do not exceed more than about 13,000 miles on either side of its mean value. From the moon's incessant changes we perceive that she is in constant motion, and we now further see that whatever these movements may be, the earth and the moon must at present remain at nearly the same distance apart. If we further add that the path pursued by the moon around the heavens lies in a plane, then we are forced to the conclusion that our satellite must be revolving in a nearly circular path around the earth at the centre. It can, indeed, be shown that the constant distance of the two bodies involves as a necessary condition the revolution of the moon around the earth. The attraction between the moon and the earth tends to bring the two bodies together. The only way by which such a catastrophe can be permanently avoided is by making the satellite move as we actually find it to do. The attraction between the earth and the moon still exists, but its effect is not then shown in bringing the moon in towards the earth. The attraction has now to exert its whole power in restraining the moon in its circular path ; were the attraction to cease the moon would start off in a straight line, and recede never to return. The fact of the moon's revolution around the earth is easily demonstrated by observations of the stars. The rising and setting of our satellite is of course due to the rotation of the earth, and this apparent diurnal movement the moon possesses in common with the sun and with the stars. It will, however, be noticed that the moon is continually changing its place among the stars. Even in the course of a single night, the displacement will be conspicuous to a careful observer, without the aid of a telescope. THE MOON. 55 The moon completes each revolution around the earth in a period of 27*3 days. In Fig. 22 we have a view of the relative positions of the earth, the sun, and the moon, but it is to be observed that, for the convenience of illustration, we have been obliged to represent the orbit of the moon on a much larger scale than it ought to Above the Plane Below the Plane Fig. 22.— The Moon's path around the Earth. be in comparison with the distance of the sun. That half of the moon which is turned towards the sun is brilliantly illuminated, and, according as we see more or less of that brilliant half, we say that the moon is more or less full, the several " phases " being visible in the succession shown by the numbers in Fig. 23. ) I € C 1 2 3 £ 5 6 7 8 9 Fig. 23.— The Phases of the Moon. A beginner sometimes finds considerable difficulty in under- standing how the light on the full moon at night can have been derived from the sun. " Is not," he will say, " the earth in the way ? and must it not intercept the sunlight from every object on the other side of the earth to the sun ? " A study of Fig. 22 will explain the difficulty. The plane in which the moon revolves does not coincide with the plane in which the earth revolves around the sun. The line in which the plane of the earth's motion is intersected by that of the moon divides the moon's path into two semicircles. We must imagine the moon's 5G THE STORY OF THE HEAVENS. path to be tilted a little, so that the upper semicircle is somewhat above the plane of the paper, and the other semicircle below. It thus follows that when the moon is in the position marked full, under the circumstances shown in the figure, the moon will be just above the line joining the earth and sun ; the sunlight will thus pass over the earth to the moon, and the moon will be illuminated. At new moon the moon will be under the line joining the earth and the sun. As the relative positions of the earth and the sun are changing, it sometimes happens that the sun does come exactly into the position of the line of intersection. When this is the case, the earth, at the time of full moon, lies directly between the moon and the sun ; the moon is thus plunged into the shadow of the earth, the light from the sun is intercepted, and we say that the moon is eclipsed. The moon sometimes only partially enters the earth's shadow, in which case the eclipse is a partial one. When, on the other hand, the sun is situated on the line of intersection at the time of new moon, the moon lies directly between the earth and the sun, and the dark body of the moon then cuts off the sunlight from the earth, producing a solar eclipse. Usually only a part of the sun is thus obscured, forming the well-known partial eclipse ; if, however, the moon pass centrally over the sun, then we must have one or other of two very remarkable kinds of eclipse. Sometimes the moon entirely blots out the sun, and thus is produced the sublime spectacle of a total eclipse, which tells us so much as to the nature of the sun, and to which we have already referred in the last chapter. Occasionally, however, even when the moon is placed centrally over the sun, a thin rim of sunlight is seen round the margin of the moon. We then have what is known as an annular eclipse. It is remarkable that the moon is some- times able to hide the sun completely, while on other occasions it fails to do so. It happens that the average apparent size of the moon is nearly equal to the average apparent size of the sun, but, owing to the fluctuations in their distances, the actual apparent sizes of both bodies undergo certain changes. On certain occasions the apparent size of the moon is greater than that of the sun. In this case a central passage produces a total THE MOON. 57 eclipse ; but it may also happen that the apparent size of the sun exceeds that of the moon, in which case a central passage can only produce an annular eclipse. There are hardly any more interesting celestial phenomena than the different de- scriptions of eclipses. The almanac will always give timely notice of the occur- rence, and the more striking features can be observed without a telescope. In an eclipse of the moon (Fig. 24) it is interest- ing to note the moment when the black shadow is first detected, to watch its gradual encroachment over the bright surface of the moon, to follow it, in case the eclipse is total, until there is only a thin crescent of moonlight left, and to watch the final extinction of that crescent when the whole moon is plunged into the shadow. But now a spectacle of great interest and beauty is often manifested ; for though the moon is so hidden behind the earth that not a single direct ray of the sunlight could reach its surface, yet we often find that the moon re- mains visible, and indeed actually glows with a copper-coloured hue bright enough to per- mit several of the markings on the surface to be discerned. Whence this light ? It is due to the sunbeams which have just grazed the edge of the earth. In doing so they have become bent by the refraction of the atmo- sphere, and have thus been turned inwards into the shadow. Such beams have passed through a prodigious thickness of the earth's atmosphere, and in this long journey through hundreds of miles of air they have become tinged with a ruddy or copper-like hue. Nor is this property of our atmosphere an unfamiliar one. The sun both at sunrise and at sunset glows with a light which is much more ruddy 1^ 8 J d xtl 58 THE STORY OF THE HEAVENS. than the beams it dispenses at noonday ? But at sunset or at sunrise the rays have much more of our atmosphere to pene- trate than they have at noon, and accordingly the atmosphere imparts to them that ruddy colour so characteristic and often so lovely. In the case of the eclipsed moon, the sunbeams have to take an atmospheric journey double as long as that at sunrise or sunset, and hence the ruddy glow of the eclipsed moon may be accounted for. The almanacs give the full particulars of each eclipse that happens in the corresponding year. These predictions are reli- able, because astronomers have been carefully observing the moon for ages, and have learned from these observations not only how the moon moves at present, but also how it Avill move for ages to come. The actual calculations are so complicated that we can- not here discuss them. There is, however, one leading principle about eclipses which is so simple that we must refer to it. The eclipses occurring this year have no very obvious relation to the eclipses that occurred last year, or to those that will occur next year. Yet, when we take a more extended view of the sequence of these phenomena, a very definite principle becomes manifest. If we observe all the eclipses in a period of eighteen years, or nineteen years, then we can predict, with at least an approximation to the truth, all the future eclipses for many years. It is only necessary to recollect that in 6,585J days after one eclipse, a nearly similar eclipse follows. For instance, a beautiful eclipse of the moon occurred on the 5th of December, 1881. If we count back 6,585 days from that date, or, that is, eighteen years and eleven days, we come to November 24th, 1863, and a similar eclipse of the moon took place then. Again, there were four eclipses in the year 1881. If we add 6,585| days to the date of each eclipse, it will give the dates of all the four eclipses in the year 1899. It was this rule which enabled the ancient astronomers to predict the recurrence of eclipses, at a time when the motions of the moon were not understood nearly so well as Ave now know them. During a long voyage, and perhaps under critical circum- stances, the moon will often render invaluable information to the sailor. To navigate a ship, suppose from Liverpool to THE MOON. China, the captain must frequently determine the precise posi- tion which his ship then occupies. If he could not do this, he would never find his way across the trackless ocean. Obser- vations of the sun give him his latitude and tell him his local time, but the captain further requires to know the Greenwich time before he can place his finger at a point of the chart and say, " My ship is here." To ascertain the Greenwich time the ship carries a chronometer which has been carefully rated before starting, and, as a precaution, two or three chronometers are usually provided to guard against the risk of error. An un- known error of a minute in the chronometer might perhaps lead the vessel fifteen miles from its proper course. It is therefore sometimes of importance to have the means of testing the chronometers during the progress of the voyage; and it would be a great convenience if every captain, when he wished, could actually consult some infallible standard of Greenwich time. We want, in fact, a Greenwich clock which may be visible over the whole globe. There is such a clock ; and, like any other clock, it has a face on which certain marks are made, and a hand which travels round that face. The great clock at Westminster shrinks into insignificance when compared with the mighty clock which the captain uses for setting his chrono- meter. The face of this stupendous dial is the face of the heavens. The numbers engraved on the face of a clock are replaced by the twinkling stars ; while the hand which moves over the dial is the beautiful moon herself. When the captain desires to test his chronometer, he measures the distance of the moon from a neighbouring star. For example, he may see that the moon is three degrees from the star Kegulus. In the Nautical Almanac he finds the Greenwich time at which the moon was three degrees from Regulus. Comparing this with the indications of the chronometer, he finds the required cor- rection. There is one widely-credited myth about the moon which must be regarded as devoid of foundation. The idea that our satellite and the weather bear some relation has no doubt been entertained by high authority, and appears to be an article in the belief of many an excellent mariner. Careful comparison 60 THE STORY OF THE HEAVENS. between the state of the weather and the phases of the moon has, however, quite discredited the notion that any connection of the kind does really exist. We often notice large blank spaces on maps of Africa and of Fig. 25.— Key to Chart of the Moon (Plate VI.). Australia, which indicate our ignorance of the interior of those great continents. We can find no such blank spaces in the map of the moon. Astronomers know the surface of the moon better than geographers know the interior of Africa. Every spot on the face of the moon which is as large as an English parish has been mapped, and all the more important objects have been named. PLATE VI. CASSELL & COMPANY, LIM1TED,LITH CHART OF THE MOON'S SURFACE. THE MOON. 61 A general map of the moon is shown in Plate VI. It has been based upon drawings made with small telescopes, and it gives an entire view of that side of our satellite which is pre- sented towards us. We can see on the map some of the cha- racteristic features of lunar scenery. Those dark regions so con- spicuous in the ordinary full moon are easily recognised on the map. They were thought to be seas by astronomers before the day of telescopes, and indeed the name " Mare " is still retained, though it is obvious that they contain no water at present. The map also shows certain ridges or elevated portions, and when we apply measurement to these objects we learn that they must be mighty mountain ranges. But the most striking features on the moon are those ring-like objects which are scattered over the surface in profusion. These are known as the lunar craters. To facilitate reference to the different points of interest we have arranged an index map, which will give a clue to the names of the several objects depicted upon the plate. The so- called seas are represented by capital letters ; so that A is the Mare Crisium, and H the Oceanus Procellarum. The ranges of mountains are indicated by small letters ; thus a on the index is the site of the so-called Caucasus mountains, and similarly the Apennines are denoted by c. The numerous craters are distin- guished by numbers ; for example, the feature on the map cor- responding to 20 on the index is the crater designated Ptolemy. A. Mare Crisium. /. CordillerasandDAlem- 10. Copernicus. B. ,, Fcecunditatis. bert mountains. 11. Kepler. C. ,, Trauquillitatis. 9- Book mountains. 12. Aristarchus. D. ,, Serenitatis. h. Dcerfel 13. Grimaldi. E. ,, Imbrium. i. Leibnitz , , 14. Gassendi. F. , , Siuus Iridium. 15. Schickard. Gr. ,, Vaporum. 16. Wargentin. H. Oceanus Procellarum. 1. Posidonius. 17. Clavius. I. Mare Humorum. 2, Linne. 18. Tycho. J. ,, Nubium. 3. Aristotle. 19. Alphons. K. Nectaris. 4. Great Valley of the 20. Ptolemy. Alps. 21. Catharina. 5. Aristillus. 22. Cyrillus. a. Caucasus mountains. 6. Autolycus. 23. Theophilus. b. Alps ,, 7. Archimedes. 24. Petavius. c. Apennine ,, 8. Plato. 25. Hyginus. d. Carpathian 9. Eratosthenes. 26. Triesnecker. 62 THE STORY OF THE HEAVENS. Lunar objects are well suited for observation when the sun- light falls upon them in such a manner as to exhibit strongly- contrasted lights and shadows. It is impossible to observe the moon satisfactorily when it is full, for then no conspicuous shadows are cast. The most opportune moment for seeing any particular lunar object is when it lies just at the illu- minated side of the boundary between light and shade, for then the features are brought out with exquisite distinctness. Plate VII.* gives an illustration of the lunar scenery, the object represented being known to astronomers by the name of Triesnecker. The district included is only a very small fraction of the entire surface of the moon, yet the actual area is very considerable, embracing as it does many hundreds of square miles. We see in it various ranges of lunar mountains, while the central object in the picture is one of those remarkable lunar craters which we meet with so frequently in every lunar landscape. This crater is about twenty miles in diameter, and it has a lofty mountain in the centre, the peak of which is just illuminated by the rising sun in that phase of our satellite which is illuminated in the picture. A typical view of a lunar crater is shown in Plate VIII. This is no doubt a somewhat imaginary sketch. The point of view from which the artist is supposed to have taken the picture is one quite unattainable by terrestrial astronomers, yet there can be little doubt that it is a fair representation of objects on the moon. We should, however, recollect the scale on which it is drawn. The vast crater must be many miles across, and the mountain at its centre must be thousands of feet high, The telescope will, even at its best, only show the moon as well as we could see it with the unaided eye if it were 250 miles away instead of being 240,000. W^e must not, therefore, expect to see any details on the moon even with the finest telescopes, unless they were coarse enough to be visible at a distance of 250 miles. England from such a point of view would only show * This sketch has been copied by permission from the very beautiful view in Messrs. Xasmyth and Carpenter's book, of which it forms Plate XI. So have also the other illustrations of lunar scenery in Plates VII., VIII., IX. The photographs were obtained by Mr. Nasmyth from models carefully constructed by him to illustrate the features on the moon. THE MOON. 63 London as a coloured spot, in contrast with the general surface of the country. We return, however, from a somewhat fancy sketch to a more prosaic examination of what the telescope does actually reveal. Plate IX. represents the large crater Plato, so well known to every one who uses a telescope. The floor of this remarkable object is flat, and the central mountain, so often seen in other craters, is entirely wanting. We describe it more fully in the general list of lunar objects. The mountain peaks on the moon throw long, well-defined shadows, characterised by a sharpness which we do not find in the shadows of terrestrial objects. The difference between the two cases arises from the absence of air from the moon. Our atmosphere diffuses a certain amount of light, which mitigates the blackness of terrestrial shadows, and tends to soften their outline. No such influences are at work on the moon, and the sharpness of the shadows is taken advantage of in our attempts to measure the heights of the lunar mountains. It is often easy to compute the altitude of a church steeple, a lofty chimney, or any similar object, from the length of its shadow. The simplest and the most accurate process is to measure at noon the number of feet from the base of the object to the end of the shadow. The elevation of the sun on the day in question can be obtained from the almanac, and then the height of the object follows by a simple calculation. Indeed, if the observations can be made either on the 6th of April or the 6th of September, at or near the latitude of London, then cal- culations would be unnecessary. The noonday length of the shadow on either of the dates named is equal to the altitude of the object. In summer the length of the noontide shadow is less than the altitude ; in winter the length of the shadow exceeds the altitude. At sunrise or sunset the shadows are, of course, much longer than at noon, and it is shadows of this kind that we observe on the moon. The necessary measure- ments are made by that indispensable adjunct to the equatorial telescope known as the micrometer. This word denotes an instrument for measuring small dis- tances. In one sense the term is not a happy one. The objects 64 THE STORY OF THE HEAVENS. to which the astronomer applies the micrometer are usually anything but small. They are generally of the most trans- cendent dimensions, far exceeding the moon or the sun, or even our whole system. Still, the name is not altogether in- appropriate, for vast though the objects may be, they generally seem minute, even in the telescope, on account of their great distance. We require for such measurements an instrument capable of the greatest nicety. Here, again, we invoke the aid of the spider, to whose assistance in another department we have already referred. In the filar micrometer two spider lines are parallel, and one intersects them at right angles. One or both of the parallel lines can be moved by means of screws, the threads of which have been shaped by consummate workmanship. The distance through which the line has been moved is accu- rately indicated by noting the number of revolutions and parts of a revolution of the screw. Suppose the two lines be first brought into coincidence, and then separated until the apparent length of the shadow of the mountain on the moon is equal to the distance between the lines : we then know the number of revolutions of the micrometer screw which is equivalent to the lenofth of the shadow. The number of miles on the moon which correspond to one revolution of the screw has been previously ascertained by other observations, and hence the length of the shadow can be determined. The elevation of the sun, as it would have appeared to an observer at this point of the moon, at the moment when the measures were being made, is also obtainable, and hence the actual elevation of the mountain can be calculated. By measurements of this kind some other lunar altitudes, such for example as the height of the rampart sur- rounding a circular-walled plane, can be determined. The beauty and interest of the moon as a telescopic object induces us to give to the student a somewhat detailed account of the more remarkable features which it presents. Most of the objects we are to describe can be effectively exhibited with very moderate telescopic power. It is, however, to be remembered that all of them cannot be well seen at one time. The region most distinctly shown is the boundary between light and dark- ness. The student will, therefore, select for observation such THE MOON. 65 objects as may happen to lie near that boundary at the time when he is observing. 1. Posidonius. — The diameter of this large crater is nearly 60 miles. Although its surrounding wall is comparatively slender, it is so distinctly marked as to make the object very conspicuous. As so frequently happens in lunar volcanoes, the bottom of the crater is below the level of the surrounding plain, in the present instance to the extent of nearly 2,500 feet. Towards the close of the last century, Schroeter, the industrious Hanoverian observer, fancied he saw traces of activity in the little crater on the floor of Posidonius. It is remarkable that he described it as being only grey in the interior, at a time when it ought to have been full of black shadow. This is particularly interesting in connection with what has been observed in the next object. 2. Linne. — This small crater lies in the Mare Serenitatis. Fifty or sixty years ago it was described as being about 6h miles in diameter, and was sufficiently conspicuous to be used by astronomers as a fundamental point in the survey of the moon. In 1866 Schmidt, of Athens, announced that the crater was obscured, apparently by cloud. Since then an ex- ceedingly small crater has been visible, but the whole object is now so inconsiderable that it would hardly be chosen as a landmark. This seems to be the most clearly attested case of change in a lunar object. To give some idea of Schmidt's amazing industry in lunar researches, it may be mentioned that in six years he made nearly 57,000 individual settings of his micrometer in the measurement of lunar altitudes. His great chart of the mountains in the moon is based on no less than 2,731 drawings and sketches, if those are counted twice that may have been used for two divisions of the map. 3. Aristotle. — This great philosopher's name has been at- tached to a grand crater 50 miles in diameter, the interior of which, although very hilly, shows no decidedly marked central cone. But the lofty wall of the crater, exceeding 10,500 feet in height, overshadows the floor so continuously that its features are never seen to advantage. 4. The Great Valley of the Alps. — A wonderfully straight 5 6G THE STORY OF THE HEAVENS. valley, with a width ranging from 3 \ to 6 miles, runs right through the lunar Alps. It is, according to Madler, at least 11,500 feet deep, and over 80 miles in length. A few low ridges which are parallel to the sides of the valley may possibly be the result of landslips. 5. Aristillus. — Under favourable conditions Lord Rosse's great telescope has shown the exterior of this magnificent crater to be scored with deep gullies, radiating from its centre. Aris- tillus is about 34 miles wide, and 10,000 feet in depth. 6. Autolycus is somewhat smaller than the foregoing, to which it forms a companion in accordance with what Madler thought a well-defined relation amongst lunar craters, by which they frequently occurred in pairs, with the smaller one more usually to the south. Towards the edge this arrangement is generally rather apparent than real, and is merely a result of foreshortening. 7. Archimedes. — This large plain, about 50 miles in diameter, has its vast smooth interior divided into seven distinct zones running east and west. There is no central mountain or other obvious internal sign of former activity, but its irregular wall rises into abrupt towers, and is marked outside by decided terraces. 8. Plato. — We have already referred to this extensive circular plain, which is noticeable with the smallest telescope. The average height of the rampart is about 3,800 feet on the eastern side ; the western side is somewhat lower, but there is one peak rising to the height of nearly 7,300 feet. The plain girdled by this vast rampart is of ample proportions. It is a somewhat irregular circle, about sixty miles in diameter, and containing an area of 2,700 square miles. On its floor the shadows of the western wall are shown in Plate IX., as are also three of the small craters, of wmich a large number have been detected by persevering observers. The narrow sharp line leading from the crater to the left is one of those remarkable "clefts" which traverse the moon in so many directions. Another may be seen further to the left. Above Plato are several detached mountains, the loftiest of which is Pico, about 8,000 feet in height. Its long and pointed shadow would at first sight lead one to suppose that it must be very steep ; but Schmidt, who specially studied THE MOON. 67 the inclinations of the lunar slopes, is of opinion that it cannot be nearly so steep as many of the Swiss mountains that are frequently ascended. It must not be inferred that a specially pointed shadow necessarily betokens a slender mountain peak. Under certain aspects even a hemisphere may have as its shadow a long thin cone. 9. Eratosthenes. — This profound crater, upwards of 37 miles in diameter, lies at the end of the gigantic range of the Apennines. Not improbably, Eratosthenes once formed the volcanic vent for the stupendous forces that elevated the comparatively craterless peaks of these great mountains. 10. Copernicus. — Of all the lunar craters this is one of the grandest and best known. The region to the west is dotted over with innumerable minute crateiiets. It has a central many- peaked mountain about 2,400 feet in height. There is good reason to believe that the terracing shown in its interior is mainly due to the repeated alternate rise, partial congelation, and subsequent retreat of a vast sea of lava. At full moon the crater of Copernicus is seen to be surrounded by radiating streaks. 11. Kepler. — Although the internal depth of this crater is scarcely less than 10,000 feet, it has but a very low surround- ing wall, which is remarkable for being covered with the same glistening substance that also forms a system of bright rays, not unlike those surrounding the last object. 12. Aristarchus is the most brilliant of the lunar craters, being specially vivid with a low power in a large telescope. So bright is it, indeed, that it has often been seen on the dark side just after new moon, and has thus given rise to marvellous stories of active lunar volcanoes. To the south-east lies another smaller crater, Herodotus, north of which is a narrow, deep valley, nowhere more than 2 J miles broad, which makes a remarkable zigzag. It is one of the largest of the lunar " clefts." 13. Grimaldi calls for notice as the darkest object of its size in the moon. Under very exceptional circumstances it has been seen with the naked eye, and as its area has been estimated at nearly 14,000 square miles, it gives an idea of how little unaided vision can discern in the moon ; it must, however, be added that we always see Grimaldi considerably foreshortened. 68 THE STORY OF THE HEAVENS. 14. The great crater Gassendi has been very frequently mapped on account of its elaborate system of " clefts." At its northern end it communicates with a smaller but much deeper crater, that is often filled with black shadow after the whole floor of Gassendi has been illuminated. 15. Schickard is another large crater, very little smaller in area than the foregoing. Within its vast expanse Madler detected 23 minor craters. With„ regard to this object, Cha- cornac pointed out that, owing to the curvature of the surface of the moon, a spectator at the centre of the floor " would think himself in a boundless desert," because the surrounding wall, although in part over 10,000 feet high, would lie entirely be- neath his horizon. 16. Close to the foregoing is Wargentin. There can be little doubt that this is really a huge crater almost filled with congealed lava. 17. Clavius. — Near the 60th parallel of lunar south* latitude lies this enormous enclosure, the area of which is not less than 16,500 square miles. Both in its interior and on its walls , are many peaks and secondary craters. The telescopic view of a sunrise upon the surface of Clavius is truly said by Madler to be indescribably magnificent. One of the peaks rises not less than 24,000 feet above the bottom of one of the included craters. Madler even expressed the opinion that in this wild neighbour- hood there are craters so profound that no ray of sunlight ever penetrated their lowest depths, while, as if in compensation, there are peaks whose summits enjoy a mean day almost twice as long as their night. 18. If the full moon be viewed through an opera-glass or any small hand-telescope, one crater is immediately seen to be conspicuous beyond all others, by reason of the brilliant rays or streaks that radiate from it. This is the majestic Tycho, 17,000 feet in depth and 50 miles in diameter (Plate X.). A peak 6,000 feet in height rises in the centre of its floor, while a series of terraces diversify its interior slopes; but it is the mysterious bright rays that chiefly surprise us. When the sun rises on Tycho, these streaks are utterly invisible, indeed the whole object is then so obscure that it requires a practised eye to PLATE VII. CAS SELL 8c COMPANY. LIMITED,UTH LONDON. TRIESNECKER. (AFTER NASMYTH.) THE MOON. 69 recognise Tycho amidst its mountainous surroundings. But as soon as the sun has attained a height of about 25° to 30° above its horizon, the rays emerge from their obscurity, and gradually increase in brightness until the moon becomes full, when they are the most conspicuous objects on her surface. As yet no satis- factory explanation has been given of the origin of these objects. They vary in length, from a few hundred miles to two, or in one instance nearly three thousand miles. They extend indifferently across vast plains, into the deepest craters, or over the loftiest elevations. We know of nothing on our earth to which they can be compared. Near the centre of the moon's disc is a fine range of craters, full open to our view under all illuminations. Of these two may be mentioned — Alphons (19), the floor of which is strangely characterised by two bright and several dark markings which cannot be explained by irregularities in the surface. — Ptolemy (20). Besides several small enclosed craters, its floor is crossed by numerous low ridges, visible when the sun is rising or setting. 21, 22, 23. — When the moon is five or six days old this beautiful group of three craters will be favourably placed for observation. They are named Catharina, Cyrillus, and Theo- philus. Catharina, the most southerly of the group, is more than 16,000 feet deep, and connected with Cyrillus by a wide valley ; but between Cyrillus and Theophilus there is no such connection. Indeed Cyrillus looks as if its huge surrounding ramparts, as high as Mont Blanc, had been completely finished before the volcanic forces commenced the formation of Theophilus, the rampart of which encroaches considerably on its older neigh- bour. Theophilus stands as a well-defined circular crater about 64 miles in diameter, with an internal depth of 14,000 to 18,000 feet, and a beautiful central group of mountains, one-third of that height, on its floor. Although Theophilus is the deepest- crater we can see in the moon, it has suffered little or no deformation from secondary eruptions, while the floor and wall of Catharina show complete sequences of lesser craters of various sizes that have broken in upon and partly destroyed each other. In the spring of the year, when the moon is some- what before the first quarter, this instructive group of extinct 70 THE STORY OF THE HEAVENS. volcanoes can be seen to great advantage at a convenient hour in the evening. 24. Petavius is remarkable not only for its great size, but also for the rare feature of having a double rampart. It is a beautiful object soon after new moon, or just after full moon, but disappears absolutely when the sun is more than 45° above its horizon. The crater floor is remarkably convex, culminating in a central group of hills intersected by a deep cleft. 25. Hyginus is a small crater in the neighbourhood just mentioned. One of the largest of the lunar chasms passes right through it, making an abrupt turn as it does so. 26. Triesnecker. — This fine crater has been already described, but is again alluded to in order to draw attention to the elaborate system of chasms so conspicuously shown in Plate VII. That these chasms are depressions is abundantly evident by the shadows inside. Very often their margins are appreciably raised. They seem to be fractures in the moon's surface. Of the various mountains that are occasionally seen as pro- jections on the actual edge of the moon, those called after Leibnitz (i) seem to be the highest. Schmidt found the highest peak to be upwards of 41,900 feet above a neighbouring valley. In comparing these altitudes with those of mountains on our earth, we must for the latter add the depth of the sea to the height of the land. Reckoned in this way, our highest moun- tains are still higher than any we know of in the moon. We must now discuss the important question as to the origin of these remarkable features on the surface of the moon. We shall admit at the outset that our evidence on this subject is only indirect. To establish by unimpeachable evidence the volcanic origin of the remarkable lunar craters, it would seem almost necessary that volcanic outbursts should have been wit- nessed on the moon, and that such outbursts should have been seen to result in the formation of the well-known ring, with or without the mountain rising from the centre. To say that nothing of the kind has ever been witnessed would be rather too emphatic a statement. On certain occasions careful observers have reported the occurrence of minute local changes on the moon. As we have already remarked, a crater named Linne, of THE MOON. 71 dimensions respectable, no doubt, to a lunar inhabitant, but forming a very inconsiderable telescopic object, was thought to have undergone some change. On another occasion a minute crater was thought to have arisen near the well-known object named Hyginus. The mere enumeration of such instances gives real emphasis to the statement that there is at the present time no appreciable source of disturbance of the moon's surface. Even were these trifling cases of suspected change really established — and this is perhaps rather farther than many astronomers would be willing to go — they are still insignificant when compared with the mighty phenomena that gave rise to the host of great craters which cover so large a portion of the moon's surface. We are led inevitably to the conclusion that our satellite must have once possessed much greater activity than it now displays. We can also give a reasonable, or, at all events, a plausible, explanation of the cessation of that activity in recent times. Let us glance at two other bodies of our system, the earth and the sun, and compare them with the moon. Of the three bodies, the sun is enormously the largest, while the moon is much less than the earth. We have also seen that though the sun must have a very high temperature, there can be no doubt that it is gradually parting with its heat. The surface of the earth, formed as it is of solid rocks and clay, or covered in great part by the vast expanse of ocean, bears but few obvious traces of a high temperature. Nevertheless, it is highly probable from ordinary volcanic phenomena that the interior of the earth still possesses a temperature of incandescence. The argument is, then, as follows. A large body takes a longer time to cool than a small body. A large iron casting will take days to cool ; a small casting will become cold in a few hours. Whatever may have been the original source of heat in our system — a question which need not now be discussed — it seems demonstrable that the different bodies were all originally heated, and have now for ages been gradually cooling. The sun is so vast that he has not yet had time to cool ; the earth, of intermediate bulk, has become cold on the outside, while still retaining vast stores of internal heat ; while the moon, the smallest body 72 THE STORY OF THE HEAVENS. of all, has lost its heat to such an extent that changes of importance on its surface can no longer be originated by internal fires. We are thus led to refer the origin of the lunar craters to some ancient epoch in the moon's history. We have no means of knowing the remoteness of that epoch, but it is reasonable to surmise that the antiquity of the lunar volcanoes must have been extremely great. At the time when the moon was suffi- ciently heated to originate those convulsions, of which the mighty craters are the survivals, the earth must also have been much hotter than it is at present. When the moon possessed sufficient heat for its volcanoes to be active the earth was proba- bly so hot that life was impossible on its surface. This supposi- tion would point to an antiquity for the lunar craters far too great to be estimated by the centuries and the thousands of years which are adequate for such periods as those with which the history of human events are concerned. It seems not unlikely that millions of years may have elapsed since the mighty craters of Plato or of Copernicus consolidated into their present form. We shall now attempt to account for the formation of the lunar craters. The most probable views on the subject seem to be those which have been set forth by Mr. Nasmyth, though it must be admitted that his doctrines are by no means free from difficulty. According to his theory we can explain how the ram- part around the lunar crater has been formed, and how the great mountain arose which so often adorns the centre of the plain. The view in Fig. 26 contains an imaginary sketch of a volcanic vent on the moon in the days when the craters were active. The eruption is here in the fulness of its energy, when the internal forces are hurling forth a fountain of ashes or stones, which fall at a considerable distance from the vent. The ma- terials thus accumulated constitute the rampart surrounding the crater. The second picture depicts the crater in a later stage of its history. The prodigious explosive power has now been exhausted, and has perhaps been intermitted for some time. Again, the volcano bursts into activity, but this time with only a THE MOON. 73 small part of its original energy. A comparatively feeble erup- tion now issues from the same vent, deposits materials close Fig. 26.— Volcano in Activity. Fig. 27.— Subsequent Feeble Activity. around the orifice, and raises a mountain in the centre. Finally, when the activity has subsided, and the volcano is silent and still, we find the evidence of the early energy testified 74 THE STORY OF THE HEAVENS. to by the rampart which surrounds the ancient crater, and by the mountain which adorns the interior. The fiat floor which is found in some of the craters may not improbably have arisen from an outflow of lava which has afterwards consolidated. A sketch of this is shown in Fig. 28. One of the principal difficulties attending this method of accounting for the structure of a crater arises from, the great size which some of these objects attain. There are ancient volcanoes on the moon forty or fifty miles in diameter ; indeed, there is one well-formed ring, with a mountain rising in the centre, the Fig. 28. — Formation of the Level Floor by Lava. diameter of which is no less than seventy-eight miles (Petavius). It seems difficult to conceive how a blowing cone at the centre could convey the materials to such a distance as the thirty-nine miles between the centre of Petavius and the rampart. The explanation is, however, facilitated when it is borne in mind that the force of gravitation is much less on the moon than on the earth. Have we not already seen that our satellite is so much smaller than the earth that eighty moons rolled into one would not weigh as much as the earth ? On the earth an ounce weighs an ounce and a pound weighs a pound ; but a" weight of six ounces here would only weigh one ounce on the moon, and a weight of six pounds here would only weigh one pound on the moon. A labourer who can carry one sack of corn on the earth could, with the same exertion, carry six sacks of corn on the moon. A cricketer who can throw a ball 100 yards on the earth could with precisely the same exertion throw the same ball 600 yards on the moon. Hiawatha could shoot ten arrows into the THE MOON. 75 air one after the other before the first reached the ground ; on the moon he might have emptied his whole quiver. The volcano, which on the moon drove projectiles to the distance of thirty- nine miles, need only possess the same explosive power as would have been sufficient to drive the missiles six or seven miles on the earth. A modern cannon properly elevated would easily achieve this feat. It must also be borne in mind that there are innumerable craters on the moon of the same general type but of the most varied dimensions ; from a tiny telescopic object two or three miles in diameter, we can point out gradually ascending stages until we reach the mighty Petavius just considered. With regard to the smaller craters, there is obviously little or no difficulty in attributing to them a volcanic origin, and as the continuity from the smallest to the largest craters is unbroken, it seems quite reasonable to suppose that even the greatest has arisen in the same way. But we must not linger any longer over these interest- ing features of the moon. Attractive and beautiful as is the spectacle of our nearest neighbour in the heavens, we have yet so long a task to accomplish that we must dismiss any further discussion upon the subject. Even a single spot on the moon is capable of affording a diligent observer materials for many a night's study. All such details we must pass over, and we can only devote the brief space which remains to a few features of more general interest. The lunar landscapes are excessively weird and rugged. They always remind us of sterile deserts, and we cannot fail to notice the absence of grassy plains or green forests such as we are familiar with on our globe. In some respects the moon is not very differently circumstanced from the earth. Like it, the moon has the pleasing alternations of day and night, though the day in the moon is as long as twenty-seven of our days, and the night of the moon is as long as twenty-seven of our nights. We are warmed by the rays of the sun, so too, is the moon ; but, whatever may be the temperature during the long day on the moon, it seems certain that the cold of the lunar night would transcend that known in the bleakest regions of our earth. 76 THE STORY OF THE HEAVENS. Even our largest telescopes can tell nothing directly as to whether life can exist on the moon. The mammoth trees of California might be growing on the lunar mountains, and ele- phants might be walking about on the plains, but our telescopes could not show them. The smallest object that we can see on the moon must be about as large as a good-sized cathedral, so that organised beings, if they existed, could not make themselves visible as telescopic objects. We are therefore compelled to resort to indirect evidence as to whether life would be possible on the moon. We may say at once that astronomers believe that life, as we know it, could not exist. Among the necessary conditions of life, water is one of the first. Take every form of vegetable life, from the lichen which grows on the rock to the giant tree of the forest, and we find the substance of every plant contains water, and could not exist without it. Nor is water less necessary to the existence of animal life. Deprived of this element, all organic life, the life of man himself, would be inconceivable. Unless, therefore, water be present in the moon we shall be bound to conclude that life, as we know it, is impossible. If any one stationed on the moon were to look at the earth through a telescope, would he be able to see any water here ? Most undoubtedly he would. He would see the clouds and he would notice their incessant changes, and the clouds alone would be almost conclusive evidence of the existence of water. An astronomer on the moon would also see our oceans as smooth, coloured surfaces, remarkably contrasted with the land. In fact, considering that much more than half of our globe is covered with oceans, and that most of the remainder is liable to be obscured by clouds, the lunar astronomer in looking at our earth would often see hardly anything but water in one form or other. Very likely he would come to the conclusion that our globe was only fitted to be a residence for amphibious animals. But when we look at the moon with our telescopes we see no direct evidence of water. Close inspection shows that the so-called lunar seas are deserts, often marked with small craters and rocks. The telescope reveals no seas and no oceans, no lakes and no rivers. Nor is the grandeur of the moon's scenery ever THE MOON. 77 impaired by clouds over her surface. Whenever the moon is above our horizon, and terrestrial clouds are out of the way, we can see the features of our satellite's surface with distinctness. There are no clouds in the moon ; there are not even the mists or the vapours which invariably arise wherever water is present, and therefore astronomers have been led to the conclusion that the surface of the globe which attends the earth is a sterile and a waterless desert. Another essential element of organic life is also absent from the moon. Our globe is surrounded with a deep clothing of air resting on the surface, and extending above our heads to the height of about 200 or 300 miles. We need hardly say how necessary air is to life, and therefore we turn with interest to the question as to whether the moon can be surrounded with an atmo- sphere. Let us clearly understand the problem we are about to consider. Imagine that a traveller started from the earth on a journey to the moon ; as he proceeded the air would gradually become more and more rarefied, until at length, when he was a few hundred miles above the earth's surface, he would have left the last perceptible traces of the earth's envelope behind him. By the time he had passed completely through the atmosphere he would have advanced only a very small fraction of the whole journey of 240,000 miles, and there would still remain a vast void to be traversed before the moon would be reached. If the moon were enveloped in the same way as the earth, then, as the traveller approached the end of his journey, and came within a few hundred miles of the moon's surface, he would meet again with traces of an atmosphere, which would gradually increase in density until he arrived at the moon's surface. The traveller would thus have passed through one stratum of air at the beginning of his journey, and through another at the end, while the main portion of the voyage would have been through space more void than that to be found in the exhausted receiver of an air-pump. Such would be the case if the moon were coated with an atmosphere like that surrounding our earth. But what are the facts ? The traveller as he drew near the moon would seek in vain for air to breathe at all resembling ours. It is possible — 78 THE STORY OF THE HEAVENS. indeed it is probable— that when he was close to the surface he might meet with faint traces of some gaseous material surround- ing the moon, but it would not be equal to a fractional part of the ample clothing which the earth now enjoys. For all purposes of respiration, as we understand the term, we may say that there is no air on the moon, and an inhabitant of our earth transferred thereto would be as certainly suffocated as he would be in the middle of space. It may, however, be asked how we learn this. Is not air transparent, and how, therefore, could our telescopes be expected to show whether the moon really possessed such an envelope ? It is by indirect, but thoroughly reliable, methods of observation that we learn the destitute condition of our satellite. There are various arguments to be adduced; but the most conclusive is that obtained on the occurrence of what is called an " occupation." It sometimes happens that the moon comes directly between the earth and a star, and the temporary extinction of the latter is an " occupation." We can observe the moment when the phenomenon takes place, and the suddenness of the disappearance of the star is generally remarked. If the moon were enveloped in a copious atmosphere, the interpo- sition of this gaseous mass by the movement of the moon would produce a gradual evanescence of the star wholly wanting the abruptness which generally marks the obscuration. The absence of air and of water from the moon explains the sublime ruggedness of the lunar scenery. We know that on the earth the action of wind and of rain, of frost and of snow, is constantly tending to wear down our mountains and reduce their asperities. No such agents are at work on the moon. Volcanoes sculptured the surface into its present condi- tion, and, though they have ceased to operate for ages, the traces of their handiwork seem nearly as fresh to-day as they were when the mighty fires were extinguished. " The cloud-capped towers, the gorgeous palaces, the solemn temples" have but a brief career on earth. It is chiefly the incessant action of water and of air that makes them vanish like the " baseless fabric of a vision." On the moon these causes of disintegration and of decay are all absent, though perhaps the THE MOON. 79 changes of temperature in the transition from lunar day to lunar night would be attended with expansions and contractions that might compensate in some degree for the absence of more potent agents of dissolution. It seems probable that a building on the moon would remain for century after century just as it was left by the builders. There need be no glass in the windows, for there is no wind and no rain to keep out. There need not be fireplaces in the rooms, for fuel cannot burn without air. Dwellers in a lunar city would find that no dust could rise, no odours be perceived, no sounds be heard. Man is a creature adapted for life under circumstances which are very narrowly limited. A few degrees of temperature more or less, a slight variation in the composition of air, the precise suitability of food, make all the difference between health and sickness, between life and death. Looking beyond the moon, into the length and breadth of the universe, we find countless celestial globes with every conceivable variety of temperature and of constitution. Amid this vast number of worlds with which space is tenanted, are there any inhabited by living beings ? To this great question science can make no response : we cannot tell. Yet it is impossible to resist a conjecture. We find our earth teeming with life in every part. We find life under the most varied conditions that can be conceived. It is met with under the burning heat of the tropics and in the everlasting frost at the poles. We find life in caves where not a ray of light ever penetrates. Nor is it wanting in the depths of the ocean, at the pressure of tons on the square inch. Whatever may be the external circumstances, nature generally provides some form of life to which those circumstances are congenial. It is not at all probable that among the million spheres of the universe there is a single one exactly like our earth — like it in the possession of air and of water, like it in size and in compo- sition. It does not seem probable that a man could live for one hour on any body in the universe except the earth, or that an oak-tree could live in any other sphere for a single season. Men can dwell on the earth, and oak-trees can thrive therein, because 80 THE STORY OF THE HEAVENS. the constitutions of the man and of the oak are specially adapted to the particular circumstances of the earth. Could we obtain a closer view of some of the celestial bodies, we should probably find that they, too, teem with life, but with life specially adapted to the environment — life in forms strange and weird ; life far stranger to us than Columbus found it to be in the New World when he first landed there. Life, it may be, stranger than ever Dante described or Dore sketched. Intelli- gence may also have a home among those spheres no less than on the earth. There are globes greater and globes less— atmo- spheres greater and atmospheres less. The truest philosophy on this subject is crystallised in the language of Tennyson : — " This truth within thy mind rehearse, That in a boundless universe Is boundless better, boundless worse. Think yon this mould of hopes and fears Could find no statelier than his peers In yonder hundred million spheres 1 " CHAPTER IV. THE SOLAR SYSTEM. Exceptional Importance of the Sun and Moon— The Course to be pursued— The Order of Distance— The Neighbouring Orbs— How are they to be discrimi- nated 1 — The Planets Venus and Jupiter attract notice by their brilliancy— Sirius not a neighbour — The Planets Saturn and Mercury — Telescopic Planets — The Criterion as to whether a Body is to be ranked as a neighbour — Meaning of the word Planet — Uranus and Neptune — Comets — The Planets are Illumi- nated by the Sun— The Stars are not — The Earth is really a Planet — The four Inner Planets, Mercury, Venus, the Earth, and Mars — Velocity of the Earth— The Outer Planets, Jupiter, Saturn, Uranus, Neptune — Light and Heat received by the Planets from the Sun — Comparative Sizes of the Planets — The Minor Planets — The Planets all Revolve in the same direction — The Solar System — An Island Group in Space. In the two preceding chapters of this work we have endeavoured to describe the heavenly bodies in the order of their relative importance to mankind. Could we hesitate for a moment as to which of the many orbs in the universe should be the first to receive our attention ? We do not now allude to the intrinsic significance of the sun when compared with other bodies or groups of bodies scattered through space. It may be that nu- merous globes rival the sun in real splendour, in bulk, and in mass. We shall, in fact, show later on in this volume that this is the case ; and we shall then be in a position to indicate the true rank of the sun amid the countless host of heaven. But what- ever may be the importance of the sun, viewed merely as one of the bodies which teem through space, there can be no hesitation in asserting how immeasurably his influence on the earth sur- passes that of all other bodies in the universe together. It was therefore natural — indeed inevitable — that our first examination of the orbs of heaven should be directed to that mighty body which is the source of our life itself. Nor could there be much hesitation as to the second step which ought to be taken. The intrinsic importance of the 6 82 THE STORY OF TEE HEAVENS. moon, when compared with other celestial bodies, may be small ; it is, indeed, as we shall afterwards see, almost infinitesimal. But in the economy of our earth the moon has played, and still plays, a part second only in importance to that of the sun himself. The moon is so close to us that her brilliant rays pale to invisibility countless orbs of a size and an intrinsic splendour incomparably greater than her own. The moon also occupies an exceptional position in the history of astronomy ; for the law of gravitation, the greatest discovery that science has yet witnessed, was chiefly accomplished by observations of the moon. It was therefore natural that an early chapter in our Story of the Heavens should be devoted to a body the interest of which approximated so closely to that of the sun himself. But the sun and the moon having been partly described (we shall afterwards have to refer to them again), some hesitation is natural in the choice of the next step. The two great luminaries being abstracted from our view, there remains no other celestial body of such exceptional interest and significance as to make it quite clear what course to pursue ; we desire to unfold the story of the heavens in the most natural manner. If we made the attempt to describe the celestial bodies in the order of their actual magnitude, our ignorance must at once pronounce the task to be impossible. We cannot even make a conjecture as to which body in the heavens is to stand first on the list. Even if that mightiest body be within reach of our telescopes (in itself a highly improbable supposition), we have not the least idea in what part of the heavens it is to be sought. And even if this were possible — if we were able to arrange all the visible bodies rank by rank in the order of their magnitude and their splendour — still the scheme would be impracticable, for of most of them we know little or nothing. We are therefore compelled to adopt a different method of procedure, and the simplest, as well as the most natural, will be to follow as far as possible the order of distance of the different bodies. We have already spoken of the moon as the nearest neighbour to the earth, we shall next consider some of the other celestial bodies which are comparatively near to us ; then, as the subject unfolds, we shall discuss the objects further and further THE SOLAR SYSTEM. S3 away, until towards the close of the volume we shall be engaged in considering the most distant bodies in the universe which the telescope has yet revealed to us. Even when we have decided on this principle, our course is still not free from ambiguity. Many of the bodies in the heavens are in motion, so that their relative distances from the earth are in continual change ; this is, however, a difficulty which need not long detain us. We shall make no attempt to adhere closely to the principle in all details. It will be sufficient if we first describe those great bodies — not a very numerous class — which are, comparatively speaking, in our vicinity, though still at varied distances ; and then we shall pass on to the uncounted bodies which are separated from us by distances so vast that the imagi- nation is baffled in the attempt to realise them. Let us, then, scan the heavens, to discover those orbs which lie in our neighbourhood. The sun has set, the moon has not risen ; a cloudless sky discloses a heaven glittering with countless gems of light. Some are grouped together into well-marked constellations ; others seem scattered promiscuously, with every degree of lustre, from the very brightest down to the faintest point that the eye can just glimpse. Amid all this host of ob- jects, how are we to identify those which lie nearest to the earth ? Look to the west : and there, over the spot where the departing sunbeams still linger, we see the lovely evening star shining forth. This is the planet Venus — a beauteous orb, twin-sister to the earth. The brilliancy of this planet, its rapid changes both in position and in lustre, would suggest at once that it was much nearer to the earth than other star-like objects. This pre- sumption has been amply confirmed by careful measurements, and therefore Venus is to be included in the list of the orbs which constitute our neighbours. Another conspicuous planet — almost rivalling Venus in lustre, and vastly surpassing Venus in the magnificence of its propor- tions and its retinue — has borne from all antiquity the majestic name of J upiter. No doubt J upiter is much more distant from us than Venus. Indeed, he is always at least twice as far, and sometimes as much as ten times. But still we must include Jupiter among our neighbours. Compared with the host of 84 THE STORY OF THE HEAVENS. stars which glitter on the heavens, Jupiter must be regarded as quite contiguous. The distance of the great planet requires, it is true, hundreds of millions of miles for its expression ; yet, vast as is that distance, it would have to be multiplied by tens of thousands, or hundreds of thousands, before it would be long enough to span the abyss which intervenes between the earth and the nearest of the stars. Yenus and Jupiter have invited our attention by their ex- ceptional brilliancy. We should, however, fall into error if we assumed generally that the brightest objects were those nearest to the earth. An observer unacquainted with astronomy might not improbably point to the Dog Star — or Sirius, as astronomers more generally know it — as an object whose exceptional lustre showed it to be one of our neighbours. This, however, would be a mistake. We shall afterwards have occasion to refer more particularly to this gem of our southern skies, and then it will appear that Sirius is a mighty globe far transcending our own sun in size as well as in splendour, but plunged into the depths of space to such an appalling distance that his enfeebled rays, when they reach the earth, give us the impression, not of a mighty sun, but only of a brilliant star. The principle of selection, by which the earth's neighbours can be discriminated will be explained presently ; in the mean- time, it will be sufficient to observe that our list is to be aug- mented first, by the addition of the unique object known as Saturn, though its brightness is far surpassed by that of Sirius, as well as by a few other stars. Then we add Mars, an object which occasionally approaches so close to the earth, that it shines with a fiery radiance which would hardly prepare us for the truth that this planet is intrinsically one of the smallest of the celestial bodies. Besides the objects we have mentioned, the ancient astronomers had detected a fifth, known as Mercury — a planet which is usually invisible amid the light surrounding the sun. Mercury, however, occasionally wanders far enough from our luminary to be seen before sunrise or after sunset. These five — Mercury, Venus, Mars, Jupiter, and Saturn — comprised the planets known from remote antiquity. We can, however, now extend the list somewhat further, by THE SOLAR SYSTEM. 85 adding to it the telescopic objects which have in modern times been found to be among our neighbours. Here we must no longer postpone the introduction of the criterion by which we can detect whether a body is near the earth or not. The brighter planets can be recognised by the steady radiance of their light as contrasted with the incessant twinkling of the stars. A little attention devoted to any of the bodies we have named will, however, point out a more definite contrast between the planets and the stars. Observe, for instance, Jupiter, on any clear night when the heavens can be well seen, and note his position with regard to the constellations in his neighbour- hood — how he is to the right of this star, or to the left of that ; directly between this pair, or directly pointed to by that. We then mark down the place of Jupiter on a celestial map, or we make a sketch of the stars in the neighbourhood showing the position of the planet. After a month or two, when the observations are repeated, the place of Jupiter is to be again compared with those stars by which it was defined. It will be found that, while the stars have preserved their mutual positions, the place of Jupiter has changed. Hence this body is with propriety called a planet, or a wanderer, because it is incessantly moving from one part of the starry heavens to another. By similar comparisons it can be shown that the other bodies we have mentioned — Venus and Mercury, Saturn and Mars — are also wanderers, and belong to that group of heavenly bodies known as planets. Here, then, we have the simple criterion by which the earth's neighbours are to be readily discriminated from the stars. Each of the bodies near the earth is a planet, or a wanderer, and the mere fact that a body is a wanderer is alone sufficient to prove it to be one of the class which we are now studying. Provided with this test, we can at once make an addition to our list of neighbours. Amid the myriad orbs which the tele- scope reveals, we occasionally find one which is a wanderer. Two other mighty planets, known as Uranus and as Neptune, must thus be added to the five already mentioned, making in all a group of seven great planets. A vastly greater number may also be reckoned when we admit to our view bodies which not only 86 THE STORY OF THE HEAVENS. seem to be minute telescopic objects, but really are small globes when compared with the mighty bulk of our earth. These lesser planets, to the number of between two and three hundred, are also among the earth's neighbours. We should remark that another class of heavenly bodies widely differing from the planets must also be included in our system. These are the comets, and, indeed, it may happen that one of these erratic bodies will sometimes draw nearer to the earth than even the closest approach ever made by a planet. These mysterious visitors will necessarily engage a good deal of our attention later on. For the present we confine our attention to those more substantial globes, whether large or small, which are always termed planets. Imagine for a moment that some opaque covering could be clasped around our sun so that all his beams were ex- tinguished. That our earth would be plunged into the darkness of midnight is of course an obvious consequence. A moment's reflection will show that the moon, shining as it does by the reflected rays of the sun, would become totally invisible. But would this extinction of the sunlight have any other effect ? Would it influence the countless brilliant points that stud the heavens at midnight ? Such an obscuration of the sun would indeed produce a remarkable effect on the sky at night, which a little attention would disclose. The stars, no doubt, would not exhibit the slightest change in brilliancy. Each star shines by its own light and is not indebted to the sun. The con- stellations would thus twinkle on as before, but a wonderful change would come over the planets. Were the sun to be obscured, the planets would instantly disappear from view. The midnight sky would thus experience the sudden effacement of the planets, while the stars would remain unaltered. It may seem difficult to realise how the brilliancy of Venus, or the lustre of Jupiter have their origin solely in the beams which fall upon these bodies from the distant sun. The evidence is, however, conclusive on the question ; and it will be placed before the reader more fully when we come to discuss the several planets in detail. Suppose that we are looking at Jupiter high in mid- THE SOLAR SYSTEM. 87 heavens on a winter's night, it might be contended that, as the earth lies between Jupiter and the sun, it must be impossible for the rays of the sun to fall upon the planet. This is, perhaps, not an unnatural view for an inhabitant of this earth to adopt until he has become acquainted with the relative sizes of the various bodies concerned, and with the distances by which those bodies are separated. But the question would appear in a widely different form to an inhabitant of the planet Jupiter. If such a being were asked whether he suffered much in- convenience by the intrusion of the earth between himself and the sun, his answer would be something of this kind : — " No doubt such an event as the passage of the earth between me and the sun is possible, and has occurred on rare occasions separated by long intervals ; but so far from the transit being the cause of any inconvenience, the whole earth, of which you think so much, is really so minute, that when it did come in front of the sun it was merely seen as a small tele- scopic point, and the amount of sunlight which it intercepted was quite inappreciable." The fact that the planets shine by the sun's light, points at once to the similarity between them and our earth. We are thus led to regard our sun as a central fervid globe associated with a number of much smaller bodies, each of which, beino* dark itself, is indebted to the sun both for light and for heat. That was, indeed, a grand step in astronomy which demon- strated the nature of the solar system. The discovery that our earth must be a globe isolated in space, Was in itself a mighty exertion of human intellect ; but when it came to be recognised that this globe was but one of a whole group of similar objects, some smaller, no doubt, but others very much larger, and when it was further ascertained that these bodies were subordinated to the supreme control of the sun, we have a chain of discoveries that wrought a fundamental transformation in human knowledge. We thus see that the sun presides over a numerous family. The members of that family are dependent upon the sun, and their dimensions are suitably proportioned to their subordinate position. Even Jupiter, the largest member of that family, 88 THE STORY OF THE HEAVENS. does not contain one-thousandth part of the material which forms the vast bulk of the sun. Yet the bulk of Jupiter alone would exceed that of the rest of the planets were they all rolled together. Around the central luminary in Fig. 29, we have drawn four circles in dotted lines which sufficiently illustrate the orbits Mars /Venus irf/ ^Earth \ Mercury Sun i G8daYS 225 day's 365 days 687 days Fig. 29.— The Orbits of the four interior Planets. in which the different bodies move. The innermost of these four paths represents the orbit of the planet Mercury. The planet moves around the sun in this path, and regains the place from which it started in eighty- eight days. The next orbit, proceeding outwards from the sun, is that of the planet Venus, which we have already referred to as the well- known Evening Star. Venus completes the circuit of its path in 225 days. One step further from the sun and we come to the orbit of another planet. This body is almost the same size as Venus, and is therefore much larger than Mercury. The planet PLATE X. 'OM PAN Y. LJMLTED.LITH LONDC TYCHO AND ITS SURROUNDINGS. (AFTER N A SMYTH.) THE SOLAR SYSTEM. 89 now under consideration accomplishes each revolution in 365 days. This period sounds familiar to our ears. It is the length of the year ; and the planet is the earth on which we stand. There is an impressive way in which to realise the length of the road along which the earth has to travel in each annual journey. The circumference of a circle is about three and one-seventh times the diameter of the same figure ; so that, taking the distance from the earth to the sun as 92,700,000 miles, the diameter of the circle which the earth describes around the sun will be 185,400,000 miles, and consequently the circumference of the mighty circle in which the earth moves round the sun is about 583,000,000 miles. The earth has to travel this distance every year. It is merely a sum in division to find how far we have to move each second in order to accomplish this long journey in a twelvemonth. It will appear that the earth must actually complete eighteen miles every second, as otherwise it would not finish its journey within the allotted time. Pause for a moment to think what a velocity of eighteen miles a second really implies. Can we realise a speed so tre- mendous ? Let us compare it with our ordinary types of rapid movement. Look at that express train how it crashes under the bridge, how, in another moment, it is lost to view ! Can any velocity be greater than that ? Let us try it by figures. The train moves a mile a minute ; multiply that velocity by eighteen and it becomes eighteen miles a minute, but we must further multiply it by sixty to make it eighteen miles a second. The velocity of the express train is not even the thousandth part of the velocity of the earth. Let us take another illustration. We stand at the rifle ranges to see a rifle fired at a target 1,000 feet away, and we find that a second or two is sufficient to carry the bullet over that distance. The earth moves nearly one hundred times as fast as the rifle-bullet. Viewed in another way, the stupendous speed of the earth does not seem immoderate. The earth is a mighty globe, so great indeed that even when moving at this speed it takes almost eight minutes to pass over its own diameter. If a steamer required eight minutes to traverse a distance equal to its own length, its pace would not be a mile an hour. To illustrate this method of considering the subject, 90 THE STORY OF THE HEAVENS. we show here a view of the progress made by the earth (Fig. 30). The distance between the centres of these circles is about six times the diameter ; and, accordingly, if they be taken to represent the earth, the time required to pass from one position to the other is about forty-eight minutes. Outside the path of the earth, we come to the orbit of the fourth planet, Mars, which requires 687 days, or nearly two years, to complete its circuit round the sun. With our arrival at Mars we have gained the limit to the inner portion of the solar system. The four planets we have mentioned form a group in them- selves, distinguished by their comparative nearness to the sun. They are all bodies of moderate dimensions. Venus and the Fig. 30.— The Earth's movement. Earth are globes of about the same size. Mercury and Mars are both smaller objects which lie, so far as bulk is concerned, between the earth and the moon. The four planets which come nearest to the sun are vastly surpassed in bulk and weight by the giant bodies of our system — the stately group of Jupiter and Saturn, Uranus and Neptune. These giant planets enjoy the sun's guidance equally with their weaker brethren. In the diagram on page 91 parts of the orbits of the great outer planets are represented. The sun, as before, presides at the centre, but the inner planets would on this scale be so close to the sun that it is only possible to re- present the orbit of Mars. After the orbit of Mars comes a considerable interval, not, however, devoid of planetary activity, and then follow the orbits of Jupiter and Saturn ; further still, we have Uranus, a great globe on the verge of unassisted vision ; and, lastly, the whole system is bounded by the grand orbit of Neptune — a planet of which we shall have a marvellous story to narrate. The various circles in Fig. 32 show the apparent sizes of the sun as seen from the different planets. Taking the circle THE SOLAR SYSTEM. 91 corresponding to the earth to represent the amount of heat and light which the earth derives from the sun, then the other circles'indicate the heat and the light enjoyed by the correspond- ing planets. The next outer planet to the earth is Mars, whose share of solar blessings is not so very inferior to that of the earth; but we fail to see how bodies so remote as Jupiter or ,.--'°'Neptune ^Uranus ♦Saturn \ Jupiter.* (<> e / ^\nor* \ /C* Mars \ USn) Fig. 31.— The Orbits of the four giant Planets. Saturn can enjoy climates at all comparable with those of the planets which are more favourably situated. Fig. 33 shows a picture of the whole family of planets sur- rounding the sun — represented on the same scale, so as to exhibit their comparative sizes. Measured by bulk, Jupiter is more than 1,200 times as great as the earth, so that it would take at least 1,200 earths rolled into one to form a globe equal to the globe of Jupiter. Measured by weight, the disparity between the earth and Jupiter, though still enormous, is not quite so great ; but this is a matter to be discussed more fully in a later chapter. 92 THE STORY OF THE HEAVENS. Even in this preliminary survey of the solar system we must not omit to refer to the planets which attract our attention, not by their bulk, but by their multitude. In the ample M ER C U RY. Fig. 32. — Comparative apparent size of the Sun as seen from the various Planets. zone bounded on the inside by the orbit of Mars, and on the outside by the orbit of Jupiter, it was thought at one time that no planet revolved. Modern research has shown that this region is tenanted, not by one planet, but by hundreds. The discovery of these planets is a charge which has been under- taken by many diligent astronomers of the present day, while THE SOLAR SYSTEM. 93 the discussion of their movements affords labour to other men of science. We shall find something to learn from the study of these tiny bodies, to which a chapter will be devoted further on in the course of this work. Fig. 33. — Comparative sizes of the Planets. But we do not propose to enter deeply into the mere statistics of the planetary system at present. Were such our intention, the tables at the end of the volume would show that ample materials are available. Astronomers have taken an inventory of each of the planets. They have measured their distances, 94 THE STORY OF THE HEAVENS. the shapes of their orbits and the positions of those orbits 5 their times of revolution, and, in the case of all the larger planets, their sizes and their weights. Such results are of interest for many purposes. It is, however, the more general features of the science which at present claim our attention. Let us, in conclusion, note one or two important truths with reference to our planetary system. We have seen that the planets all revolve in nearly circular paths around the sun. We have now to add another fact possessing much significance. Each of the planets pursues its path in the same direction. It thus happens that one such body may overtake another, but it can never happen that two planets pass by each other as do the trains on adjacent lines of railway. We shall sub- sequently find that the whole welfare of our system, nay, its continuous existence, is dependent upon this remarkable uniformity. Such is our solar system ; a mighty organised group of planets circulating under the control of the sun, and com- pletely isolated from all external interference. No star, no constellation, has any appreciable influence on our solar system We constitute a little island group, separated from the nearest stars by the most amazing distances. It may be that as the other stars are suns, so they too may have systems of planets circulating around them ; but of this we know nothing. Of the stars we can only say that they are points of light, and any planets they may possess must for ever remain invisible to us, even if they were many times larger than Jupiter„ We need not repine at this limitation to our possible know- ledge, for just as we find in the solar system all that is neces- sary for our daily bodily wants, so shall we find ample occupa- tion for whatever faculties we may possess, in endeavouring to understand those mysteries of the heavens which lie within our reach. CHAPTER V. THE LAW OF GRAVITATION. Gravitation — The Falling of a Stone to the Ground— All Bodies fall Equally, Sixteen Feet in a Second— Is this True at Great Heights ?— Fall of a Body at a height of a Quarter of a Million Miles— How Newton obtained an Answer from the Moon— His Great Discovery— Statement of the Law of Gravitation- Illustrations of the Law— How is it that all the Bodies in the Universe do not rush together ?— The Effect of Motion— How a Circular Path can be pro- duced by Attraction— General Account of the Moon's Motion— Is Gravitation a Force of Great Intensity ?— Two Weights of 50 lbs.— Two Iron Globes, 53 yards in diameter, and a mile apart, attract with a force of 1 lb.— Charac- teristics of Gravitation— Orbits of the Planets not strictly Circles— The Discoveries of Kepler — Construction of an Ellipse — Kepler's First Law— Does a Planet move uniformly ?— Law of the Changes of Velocity— Kepler's Second Law — The Relation between the Distances and the Periodic Times — Kepler's Third Law— Kepler's Laws and the Law of Gravitation— Movement in a straight line— A Body unacted on by disturbing Forces would move in a straight line with constant Velocity — Application to the Earth and the Planets — The Law of Gravitation deduced from Kepler's Laws — Universal Gravitation. Our description of the heavenly bodies must undergo a slight interruption, while we illustrate with appropriate detail an important principle, known as the law of gravitation, which underlies the whole of astronomy. By this law we can explain the movements of the moon around the earth, and of the planets around the sun. It is accordingly incumbent upon us to discuss this subject before we proceed to the more particular account of the separate planets. We shall find, too, that the law of gravitation sheds some much-needed light on the nature of the stars situated at the remotest distances in space. It also enables us to cast a glance through the vistas of time past, and to trace with plausibility, if not with certainty, certain early phases in the history of our system. The sun and the moon, the planets and the comets, the stars and the nebulae, all alike are subject to this universal law, which is now to engage our attention. 96 THE STORY OF THE HEAVENS. What is more familiar than the fact that when a stone is dropped, it will fall to the ground? No one at first thinks the matter even worthy of remark. People are often surprised at seeing a piece of iron drawn to a magnet. Yet the fall of a stone to the ground is the manifestation of a force quite as inter- esting as the force of magnetism. It is the earth which draws the stone, just as the magnet draws the iron. In each case the force is one of attraction ; but while the magnetic attraction is confined to a few substances, and is of comparatively limited importance, the attraction of gravitation is significant through- out the universe. Let us commence with a few very simple experiments upon the force of gravitation. Hold in the hand a small piece of lead, and then allow it to drop upon a cushion. The lead requires a certain time to move from the fingers to the cushion, but that time is always the same when the height is the same. Take now a larger piece of lead, and hold one piece in each hand at the same height. If both are released at the same moment, they will both reach the cushion simultaneously. It might have been thought that the heavy body would fall more quickly than the light body ; but when the experiment is tried, it is seen that this is not the case. Repeat the experiment with various other substances. An ordinary marble will be found to fall in the same time as the piece of lead. With a piece of cork we again try the experiment, and again obtain the same result. At first it seems to fail when we compare a feather with the piece of lead ; but that is solely on account of the air, which resists the feather more than it resists the lead. If, however, the feather be placed upon the top of a penny, and the penny be horizontal when dropped, it will clear the air out of the way of the feather in its descent, and then the feather will fall as quickly as the penny, as quickly as the marble, or as quickly as the lead. If the observer were in a gallery when trying these experi- ments, and if the cushion were sixteen feet below his hands, then the time the marble would take to fall through the sixteen feet would be one second. The time occupied by the cork or by the lead would be the same ; and even the feather itself would fall through sixteen feet in one second, if it could be screened from THE LAW OF GRAVITATION. 97 the interference of the air. Try this experiment where we like, in London, or in any other city, in any island or continent, on board a ship at sea, at the North Pole, or the South Pole, or the equator, it will always be found that any body of any size, or any material, will fall about sixteen feet in one second of time. Lest any erroneous impression should arise, we may just mention that the distance traversed in one second does vary slightly at different parts of the earth, but from causes which need not at this moment detain us. We shall for the present regard sixteen feet as the distance through which any body, free from interference, would fall in one second at any part of the earth's surface. But now let us extend our view above the earth's surface, and inquire how far this law of sixteen feet in a second may find obedience elsewhere. Let us, for instance, ascend to the top of a mountain and try the experiment there. It would be found that at the top of the mountain a marble would take a little longer to fall through sixteen feet than the same marble would if let fall at its base. The difference would be very small ; but yet it would be measurable, and would suffice to show that the power of the earth to pull the marble to the ground be- comes somewhat weakened at a point high above the earth's surface. Whatever be the elevation to which we ascend, be it either the top of a high mountain, or the still greater altitudes that have been reached in balloon ascents, we shall never find that the tendency of bodies to fall to the ground ceases, though no doubt the higher we go the more is that tendency weakened. It would be of great interest to find how far this power of the earth to draw bodies towards it can really extend. We cannot attain more than about five or six miles above the earth's surface in a balloon ; yet we want to know what would happen if we could ascend 500 miles, or 5,000 miles, or still further, into the regions of space. Conceive that a traveller were endowed with some means of soaring aloft for miles and thousands of miles, still up and up, until at length he had attained the awful height of nearly a quarter of a million of miles above the ground. Glancing down at the surface of that earth, which is at such a stupendous depth beneath, he would be able to see a wonderful bird's-eye 98 THE STORY OF THE HEAVENS. view. He would lose, no doubt, the details of towns and villages ; the features in such a landscape would be whole con- tinents and whole oceans, in so far as the openings between the clouds would permit the earth's surface to be exposed. At this stupendous elevation he could try one of the most interesting experiments that was ever in the power of a philo- sopher. He could test whether the earth's attraction was felt at such a height, and he could measure the amount of that attrac- tion. Take for the experiment a cork, a marble, or any other object, large or small ; hold it between the fingers, and let it go. Every one knows what would happen in such a case down here ; but it required Sir Isaac Newton to tell what would happen in such a case up there. Newton asserts that the power of the earth to attract bodies extends even to this great height, and that the marble will fall. This is the doctrine that we can now test. We are ready for the experiment. The marble is released, and, lo, our first exclamation is one of wonder. Instead of dropping instantly, the little object appears to remain sus- pended. We are on the point of exclaiming that we must have gone beyond the earth's attraction, and that Newton is wrong, when our attention is arrested ; the marble is beginning to move, so slowly that at first we have to watch it carefully. But the pace gradually improves, so that the attraction is beyond all doubt, until, gradually acquiring more and more velocity, the marble speeds on its long journey of a quarter of a million of miles to the earth. But surely, it will be said, such an experiment must be entirely impossible ; and no doubt it cannot be performed in the way described. The bold idea occurred to Newton of making use of the moon itself, which is almost a quarter of a million of miles above the earth, for the purpose of answering the ques- tion. Never was our satellite put to such noble use before. It is actually at each moment falling in towards the earth. We can calculate how much it is deflected towards the earth in each second, and thus obtain a measure of the earth's attractive power. From such inquiries Newton was able to learn that a body released at the distance of 240,000 miles above the surface of the earth would still be attracted by the earth, that in THE LAW OF GRAVITATION. 99 virtue of the attraction the body would commence to move off towards the earth — not, indeed, with the velocity with which a body falls in experiments on the surface, but with a very much lesser speed. A body dropped down from the distance of the moon would commence its long journey so slowly that a minute, instead of a second, would have elapsed before the distance of sixteen feet had been accomplished.* It was by pondering on information thus won from the moon that Newton made his immortal discovery. The gravitation of the earth is a force which extends far and wide through space. The more distant the body, the weaker the gravitation becomes; here Newton found the means of determining the great problem as to the law according to which the intensity of the gravitation decreased. The information derived from the moon, that a body 240,000 miles away requires a minute to fall through a space equal to that through which it Avould fall in a second down here, was of paramount importance. In the first place, it shows that the attractive power of the earth, by which it draws all bodies earthwards, becomes weaker at a distance. This might, indeed, have been anticipated. It is as reasonable to suppose that as we retreated further and further into the depths of space the power of attraction should diminish, as that the lustre of light should diminish as we recede from it ; and it is remarkable that the law according to which the attraction of gravitation decreases with the increase of distance, is precisely the same as the law according to which the brilliancy of a light decreases as its distance increases. The law of nature, stated in its simplest form, asserts that the intensity of gravitation varies inversely as the square of the distance. Let me endeavour to elucidate this somewhat abstract statement by one or two simple illustrations. Suppose a body were raised above the surface of the earth to a height of nearly 4,000 miles, so as to be at an altitude equal to the radius of the earth. In other words, a body so situated would * The space described by a falling body is proportional to the product of the force and the square of the time. The force varies inversely as the square of the distance from the earth, so that the space will vary as the square of the time, and inversely as the square of the distance. If, therefore, the distance be increased sixty-fold, the time must also be increased sixty -fold, if the space fallen through is to remain the same. 100 THE STORY OF THE HEAVENS. be twice as far from the centre of the earth as a body which lay on the surface. The law of gravitation says that the intensity of the attraction is then to be decreased to one-fourth part, so that the pull of the earth on a body 4,000 miles high is only one quarter of the pull of the earth on that body so long as it lies on the ground. We may imagine the effect of this pull to be shown in different ways. Allow the body to fall, and in the interval of one second it will only drop through four feet, a mere quarter of the distance that gravity would cause near the earth's surface. To put the matter in another way ; suppose that the attraction of the earth is measured by one of those little weighing machines known as a spring balance. If a weight of four pounds be hung on such a contrivance, at the earth's surface, the index of course shows a weight of four pounds ; but conceive this balance, still bearing the weight appended thereto, were to be carried up and up, the indicated strain would become less and less, until by the time the balance reached 4,000 miles high, where it was twice as far away from the earth's centre as at first, the indicated strain would be reduced to the fourth part, and the balance would only show one pound. If we could imagine the instrument to be carried still further into the depths of space, the indication of the scale would steadily continue to decrease. By the time the apparatus had reached a distance of 8,000 miles high, being then three times as far from the earth's centre as at first, the law of gravitation tells us that the attraction must have decreased to one-ninth part. The strain thus shown on the balance would be only the ninth part of four pounds, or less than half a pound. But let the voyage be once again resumed, and let not a halt be made this time until the balance and its four-pound weight have retreated to that orbit which the moon traverses in its monthly course around the earth. The distance thus attained is about sixty times the radius of the earth, and consequently the attraction of gravitation is diminished in the proportion of one to the square of sixty ; the spring will then only be strained by the inappreciable fraction of 1-3,600 part of four pounds. It therefore appears that a weight which on the earth weighed a ton and a half would, if raised 240,000 miles, weigh less than a pound. But even at this vast distance THE LAW OF GRAVITATION. 101 we are not to halt; imagine that we retreat still further and further ; the strain shown by the balance will ever decrease, but it will still exist, no matter how far we go. Astronomy appears to teach us that the attraction of gravitation can extend, with suitably enfeebled intensity, across the most profound gulfs of space. The principle of gravitation is of far wider scope than we have yet indicated. We have spoken merely of the attraction of the earth, and we have stated that this force extends throughout space. But the law of gravitation is not so limited. Not only does the earth attract every other body, and every other body attract the earth, but each of these bodies attracts the other ; so that in its more complete shape the law of gravitation announces that " every body in the universe attracts every other body with a force which varies inversely as the square of the distance." It is impossible for us to over-estimate the importance of this law. It supplies the clue by which we can unravel the compli- cated movements of the planets. It has led to marvellous discoveries, in which the law of gravitation has enabled us to anticipate the telescope, and to feel the existence of bodies before those bodies have even been seen. An objection which may be raised at this point must first be dealt with. It seems to be, indeed, a plausible one. If the earth attracts the moon, why does not the moon tumble down on the earth ? If the earth is attracted by the sun, why does it not tumble into the sun ? If the sun is attracted by other stars, why do they not rush together with a frightful collision ? It may not unreasonably be urged that if all these bodies in the heavens are attracting each other, it would seem that they must all rush together in consequence of that attraction, and thus weld the whole material universe into a single mighty mass. We know, as a matter of fact, that these collisions do not often happen, and that there is extremely little likelihood of their taking place. We see that although our earth is said to have been attracted by the sun for countless ages, yet that the earth is just as far from the sun as ever it was. Is not this in conflict with the doctrine of universal gravitation ? In the early days of astronomy such 102 THE STORY OF THE HEAVENS. objections would be regarded, and doubtless were regarded, as wellnigh insuperable ; even still we occasionally hear thern raised, and it is therefore the more incumbent on us to explain how it happens that the solar system has been able to escape from the catastrophe by which it seems to be threatened. There can be no doubt that if the moon and the earth had been initially placed at rest, they would have been drawn together by their mutual attraction. So, too, if the system of planets surrounding the sun had been left initially at rest they would have dashed into the sun, and the system would have been annihilated. It is the fact that the planets are moving, and that the moon is moving, which has enabled these bodies successfully to resist the attraction in so far, at least, as that they are not drawn thereby to total destruction. It is so desirable that the student should understand clearly how a central attraction is compatible with revolution in a nearly circular path, that we give an illustration to show how the moon pursues its monthly orbit under the guidance and the control oi the attracting earth. The imaginary sketch in Fig. 34 denotes a section of the earth with a high mountain thereon.* If a cannon were stationed on the top of the mountain at C, and if the cannon-ball were fired off in the direction C E with a moderate charge of powder, the ball would move down along the first curved path. If it be fired a second time with a heavier charge, the path will be along the second curved line, and the ball would again fall into the sea. But let us try next time with a charge still further increased, and, indeed, with a far stronger cannon than any piece of ordnance ever yet made. The velocity of the pro- jectile must now be assumed to be some miles per second, but we can conceive that the speed shall be so adjusted that the ball shall move along the path C D, always at the same height above the sea, though still curving, as every projectile must curve, from the horizontal line in which it moved at the first moment. Arrived at D, the ball will still be at the same height above the surface, and its velocity must be unabated. It will therefore continue in its path and move round another quadrant of * See Newcomb's " Popular Astronomy," p. 78. THE LAW OF GRAVITATION. 103 the circle without getting nearer to the surface. In this manner the projectile will travel completely round the whole globe, coming back again to C and then taking another start in the same path. If we could abolish the mountain and the cannon at the top, we should have a body revolving for ever around the earth in consequence of the attraction of gravitation. Make now a bold stretch of the imagination. Conceive a terrific cannon capable of receiving a round bullet not less than e ^_ c Fig. 34.— Illustration of the Moon's motion. 2,000 miles in diameter. Discharge this enormous bullet with a velocity of about 3,000 feet per second, which is two or three times as great as the velocity actually attainable in modern artillery. Let this notable bullet be fired horizontally from some station nearly a quarter of a million miles above the surface of the earth. That fearful missile would sweep right round the earth in a nearly circular orbit, and return to where it started in about four weeks. It would then commence another revolution, four weeks more would find it again at the starting point, and this motion would go on for ages. Do not suppose that we are entirely romancing. We cannot indeed show the cannon, but we can point to a great projectile. 104 THE STORY OF THE HEAVENS. We see it every month ; it is the beautiful moon herself. No one asserts that the moon was ever shot from such a cannon ; but it must be admitted that she moves as if she had been. In a later chapter we shall inquire into the history of the moon, and show how she came to revolve in this wonderful manner. As Avith the moon around the earth, so with the earth around the sun. The illustration shows that a circular or nearly circular motion harmonises with the conception of the law of universal gravitation. We are accustomed to regard gravitation as a force of stupendous magnitude. Does not gravitation control the moon in its revolution around the earth ? Is not even the mighty earth itself retained in its path around the sun by the surpassing power of the sun's attraction ? No doubt the actual force which keeps the earth in its path, as well as that which retains the moon in our neighbourhood, is of vast intensity, but that is because gravitation is in such cases associated with bodies of enormous mass. No one can deny that all bodies accessible to our observation appear to attract each other in accordance with the law of gravitation ; but it must be confessed that, unless one or both of the attracting bodies is of gigantic dimensions, the intensity is almost immeasurably small. Let us attempt to illustrate how feeble is the gravitation between masses of easily manageable dimensions. Take for instance two iron weights, each weighing about 50lb., and separated by a distance of one foot from centre to centre. There is a certain attraction of gravitation between these weights. The two weights are drawn together, yet they do not move. The attraction between them, though it certainly exists, is an extremely minute force, not at ail comparable as to intensity with magnetic attraction. Every one knows that a magnet will draw a piece of iron with considerable vigour, but the intensity of gravitation is very much, less on masses of equal amount. The attraction between these two 50lb. weights is less than the tenth-millionth part of a single pound. Such a force is utterly infinitesimal in comparison with the friction between the weights and the table on which they stand, and hence there is no response to the attraction by even the slightest movement. Yet, if we can conceive each of THE LAW OF GRAVITATION. 3 05 these weights mounted on wheels absolutely devoid of friction, and running on absolutely perfect horizontal rails, then there is no doubt that the bodies would slowly commence to draw together, and in the course of time would arrive in actual contact. If we desire to conceive gravitation as a force of measureable intensity, we must employ masses immensely more ponderous than those 50lb. weights. Imagine a pair of globes, each composed of 417,000 tons of cast iron, and each, if solid, being about 53 yards in diameter. Imagine these globes placed at a distance of one mile apart. Each globe attracts the other by the force of gravitation. It does not matter that buildings and obstacles of every description intervene ; gravitation will pass through such impediments as easily as light passes through glass. No screen can be devised dense enough to intercept the passage of this force. Each of these iron globes will therefore under all circumstances attract the other ; but, notwithstanding their ample proportions, the intensity of that attraction is still very small, though appreciable. The attraction between these two globes is a force no greater than the pressure exerted by a single pound weight. A child could hold back one of these massive globes from its attraction by the other. Suppose that all was clear, and that friction could be so neutralised as to permit the globes to follow the impulse of their mutual attractions. The two globes will then commence to approach, but the masses are so large, while the attraction is so small, that the speed will be accelerated very slowly. A microscope would be necessary to show when the motion has actually commenced. An hour and a half must elapse before the distance is diminished by a single foot ; and although the pace improves subsequently, yet three or four days must elapse before the two globes will come together. The most remarkable characteristic of the force of gravitation must be here specially alluded to. The intensity appears to depend only on the quantity of matter in the bodies, and not at all on the nature of the substances of which these bodies are composed. We have described the two globes as made of cast iron, but if either or both were composed of lead or copper, of wood or stone, of air or water, the attractive power would still be 106 THE STORY OF THE HEAVENS. the same, provided only that the masses remained unaltered. In this we observe a profound difference between the attraction of gravitation and magnetic attraction. In the latter case the attraction is not perceptible at all in the great majority of sub- stances, and is only considerable in the case of iron. In our account of the solar system we have represented the moon as revolving around the earth in a nearly circular path, and the planets as revolving around the sun in orbits which are also approximately circular. It is now our duty to give a more minute description of these remarkable paths ; and, instead of dismissing them as being nearly circles, we must ascertain pre- cisely in what respects they differ therefrom. If a planet revolved around the sun in a truly circular path, of which the sun was> always at the centre, it would then be obvious that the distance from the sun to the planet, being always equal to the radius of the circle, must be of constant magnitude. Now, there can be no doubt that the distance from the sun to each planet is approximately constant ; but when accurate observations are made, it becomes clear that the dis- tance is not absolutely constant. The variations in distance may amount to many millions of miles, but, even in extreme cases, the variation in the distance of the planet is only a small fraction — usually a very small fraction — of the total amount of that distance. The circumstances vary in the case of each of the planets. The orbit of the earth itself is such that the distance from the earth to the sun departs but little from its mean value. Venus makes even a closer approach to perfectly circular movement ; while, on the other hand, the path of Mars, and much more the path of Mercury, show considerable relative fluctuations in the distance from the planet to the sun. It has often been noticed that many of the great discoveries in science have their origin in the nice observation and explana- tion of minute departures from some law approximately true. We have in this department of astronomy an excellent illustra- tion of this principle. The orbits of the planets are nearly circles, but they are not exactly circles. Now, why is this? There must be some natural reason. That reason has been ascertained, and it has led to several of the grandest dis- THE LAW OF GRAVITATION. 107 coveries that the mind of man has ever achieved in the realms of Nature. In the first place, let us see the inferences to be drawn from the fact that the distance of a planet from the sun is not constant The motion in a circle is one of such beauty and simplicity, that - we are reluctant to abandon it, unless the necessity for doing so be made clearly apparent. Can we not devise any way by which the circular motion might be preserved, and yet be compatible with the fluctuations in the distance from the planet to the sun? This is clearly impossible with the sun at the centre of the circle. But suppose the sun did not occupy the centre, while the planet, as before, revolved around the sun. The dis- tance between the two bodies would then necessarily fluctuate. The more eccentric the position of the sun, the larger would be the proportionate variation in the distance of the planet when at the different parts of its orbit. It might further be supposed that by placing a series of circles around the sun the various planetary orbits could be accounted for. The centre of the circle belonging to Yenus is to coincide very nearly with the centre of the sun, and the centres of the orbits of all the other planets are to be placed at such suitable distances from the sun as will render a satisfactory explanation of the gra- dual increase and decrease of the distance between the two bodies. There can be no doubt that the movements of the moon and of the planets would be, to a large extent, explained by such a system of circular orbits ; but the spirit or astronomical inquiry is not satisfied with approximate results. Again and again the planets are observed, and again and again the observations are compared with the places which the planets would occupy if they moved in accordance with the system here indicated. The centres of the circles are moved hither and thither, their radii are adjusted with greater care ; but it is all of no avail. The observations of the planets are minutely examined to see if they can be in error ; but of errors there are none at all sufficient to account for the discrepancies. The conclusion was thus in- evitable — astronomers were forced to abandon the circular motion, which was thought to possess such unrivalled symmetry 108 THE STORY OF THE HEAVENS. and beauty, and were compelled to admit that the orbits of the planets were not circular. Then if these orbits be not circles, Avhat are they ? Such was the great problem which Kepler proposed to solve, and which, to his immortal glory, he succeeded in solving and in proving to demonstration. The great discovery of the true shape of the planetary orbits stands out as one of the most con- spicuous events in the history of astronomy. It may, in fact, be doubted whether any other discovery in the whole range of science has led to results of such far-reaching interest. We must here adventure for a while into the field of science known as geometry, and study therein the nature of that curve which the discovery of Kepler has raised to such unparalleled importance. The subject, no doubt, is a difficult one, and to pursue it with any detail would involve us in many abstruse calculations which would be out of place in this volume ; but a general sketch of the subject is indispensable, and we must attempt to render it such justice as may be compatible with our limits. The curve which represents with perfect fidelity the move- ments of a planet in its revolution around the sun, belongs to that well-known group of curves which mathematicians describe as the conic sections. The particular form of conic section which denotes the orbit of a planet is known by the name of the ellipse : it is spoken of somewhat less accurately as an oval. The ellipse is a curve which can be readily constructed. There is no simpler method of doing so than that which is familiar to draughtsmen, and which we shall here briefly describe. We represent on the next page (Fig. 35) two pins passing through a sheet of paper. A loop of twine passes over the two pins in the manner here indicated, and is stretched by the point of a pencil. With a little care the pencil can be guided so as to keep the string stretched, and its point will then describe a curve completely round the pins, returning to the point from which it started. We thus produce that celebrated geometrical figure which is called an ellipse. It will be very instructive to draw a number of ellipses, varying in each case the circumstances under which they are formed. If, for instance, the pins remain THE LAW OF GRAVITATION. 109 placed as before, while the length of the loop is increased, so that the pencil is farther away from the pins, then it will be observed that the ellipse has lost some of its elongation, and approaches more closely to a circle. On the other hand, if the length of the cord in the loop be lessened, while the pins remain as before, the ellipse will be found more oval, or, as a mathe- matician would say, its eccentricity is increased. It is also useful to study the changes which the form of the ellipse under- goes when one of the pins is altered, while the length of the loop remains unchanged. If the two pins be brought nearer Fig. 35.— Drawing an Ellipse. together the eccentricity will decrease, and the ellipse will approximate more closely to the shape of a circle. If the pins be separated more widely the eccentricity of the ellipse will be increased. That the circle is an extreme form of ellipse will be evident, if we suppose the two pins to draw in so close together that they become coincident ; the point will then simply trace out a circle, as the pencil moves round the figure. It will be obvious that the points marked by the pins possess very remarkable relations with respect to the curve. Each one is called a focus, and an ellipse can only have one pair of foci. In other words there is but a single pair of positions possible for the two pins, when an ellipse of specified size and shape is to be constructed. 110 THE STORY OF THE HEAVENS. The ellipse differs principally from a circle in the circum- stance that it possesses variety of form. We can have large and small ellipses just as we can have large and small circles, but we can also have ellipses of greater or less eccentricity. If the ellipse has not the perfect simplicity of the circle, it has at least the charm of variety which the circle has not. The oval curve has also the beauty derived from an outline of perfect grace, and an association with ennobling conceptions. The ancient geometricians had studied the ellipse ; they had noticed its foci ; they were acquainted with its geometrical relatus ; and thus Kepler was familiar with the ellipse at the time when he undertook his celebrated researches on the movements of the planets. He had found, as we have already indicated, that the movements of the planets could not be reconciled with circular orbits. What shape of orbit should next be tried ? The ellipse was ready to hand, its properties were known, and the comparison could be made ; memorable, indeed, was the con- sequence of this comparison. Kepler found that the movement of the planets could be explained, by supposing that the path in which each one revolved was an ellipse. This in itself was a discovery of the most commanding importance. On the one hand it reduced to order the movements of the great globes which circulate round the sun ; while on the other, it took that beautiful class of curves which had exercised the geometrical talents of the ancients, and assigned to them the dignity of defining the highways of the universe. But we have as yet only partly enunciated the first discovery of Kepler. We have seen that a planet revolves in an ellipse around the sun, and that the sun is therefore at some point in the interior of the ellipse — but at what point ? Interesting indeed is the answer to this question. We have pointed out how the foci possess a geometrical significance which no other points enjoy ? Kepler showed that the sun must be situated in one of the foci of the ellipse in which each planet revolves. We thus enunciate the first law of planetary motion in the following words : — Each 'planet revolves around the sun in an ellipt ic path, having the sun at one of the foci. THE LAW OF GRAVITATION. Ill We are now enabled to form a clear picture of the orbits of the planets, be they ever so numerous, as they revolve around the sun. In the first place we observe that the ellipse is eminently a plane curve ; that is to say, each planet must, in the course of its long journey, confine its movements to one plane. Each planet has thus a certain plane appropriated to it. It is true that all these planes are very nearly coincident, at least in so far as the great planets are concerned ; but still they are distinct, and the only feature in which they all agree is that each one of them passes through the sun. All the elliptic orbits of the planets have one focus in common, and that focus lies at the centre of the sun. It is well to illustrate this remarkable law by considering the circumstances of two or three different planets. Take first the case of the earth, the path of which, though really an ellipse, is very nearly circular. In fact, if it were drawn accurately to scale on a sheet of paper, the difference between the elliptic orbit and the circle would hardly be detected without careful measurement. In the case of Venus the ellipse is still more nearly a circle, and the two foci of the ellipse are very nearly coincident with the centre of the circle. On the other hand, in the case of Mercury, we have an ellipse which departs from the circle to a very marked extent, while in the orbits of some of the lesser planets the eccentricity is still greater. It is extremely remarkable that every planet, no matter how far from the sun, should be found to move in an ellipse of some shape or other. We shall presently show that necessity compels each planet to pursue an elliptic path, and that no other form of path is possible. Started on its elliptic path, the planet pursues its stately course, and after a certain duration, known as the periodic time, regains the position from which its departure was taken. Again the planet traces out anew the same elliptic path, and thus, revolution after revolution, an identical track is traversed around the sun. Let us now attempt to follow the body in its course, and observe the history of its motion during the time requisite for the completion of one of its circuits. The dimensions of a planetary orbit are so stupendous that the planet must run its course very rapidly in order to finish the journey within the 112 THE STORY OF THE HEAVENS. allotted time. The earth, as we have already seen, has to move eighteen miles a second to accomplish one of its voyages round the sun in the lapse of days. The question then arises as to whether the rate at which a planet moves is uniform or not. Does the earth, for instance, actually move at all times with the velocity of eighteen miles a second, or does our planet sometimes move more rapidly and sometimes more slowly, so that the average of eighteen miles a second is still maintained ? This is a question of very great importance, and we are able to Mean Velo city Mean Veloc'itV Fig. 36. — Varying velocity of elliptic motion. answer it in the clearest and most emphatic manner. The velocity of a planet is not uniform, and the variations of that velocity can be explained by the adjoining figure. Let us first of all imagine the planet to be situated at that part of its path most distant from the sun towards the right of the figure. In this position the body's velocity is at its lowest ; as the planet begins to approach the sun the speed gradually improves until it attains its mean value. After this point has been passed, and the planet is now rapidly hurrying on towards the sun, the velocity with which it moves becomes gradually greater and greater, until at length, as it dashes round the sun, its speed attains a maximum. After passing the sun, the distance of the planet from the luminary increases, and the velocity of the motion begins to abate; gradually it declines until the mean THE LAW OF GRAVITATION. 113 value is again reached, and then it falls still lower, until the body recedes to its greatest distance from the sun, by which time the velocity has abated to the value from which we supposed it to commence. We thus observe that the nearer the planet is to the sun the quicker it moves. We can, however, give numerical definiteness to the principle according to which the velocity of the planet varies. The adjoining figure shows a planetary orbit, with, of course, the sun at the focus S. We have taken two portions, A B and C D, round the ellipse, and joined their extremities to the focus. Kepler's second law may be stated in these words : — A Fig. 37. — Equal areas in equal times. " Every planet moves round the sun with such a velocity at every point, that a straight line drawn from it to the sun passes over equal areas in equal times." For example, if the two shaded portions, A B S and DCS, are equal in area, then the times occupied by the planet in tra- velling over the portions of the ellipse, A B and C I), are equal. If the one area be greater than the other, then the times required are in the proportion of the areas. This law being admitted, the reason of the increase in the planet's velocity when it approaches the sun is at once apparent. To accomplish a definite area when near the sun, a larger arc is obviously necessary than at other parts of the path. At the opposite extremity, a small arc suffices for a large area, and the velocity is accordingly less. These two laws completely prescribe the motion of a planet 8 114 THE STORY OF THE HEAVENS. round the sun. The first defines the path which the planet pursues ; the second describes how the velocity of the body varies at different points along its path. But Kepler added to these a third law, which enables us to compare the movements of two different planets revolving round the same sun. Before stating this law, it is necessary to explain exactly what is meant by the mean distance of a planet. In its elliptic path the distance from the sun to the planet is constantly changing ; but it is nevertheless easy to attach a distinct meaning to that distance which is an average of all the distances. This average is called the mean distance. The simplest way of finding the mean distance is to add the greatest of these quantities to the least, and take half the sum. We have already defined the periodic time of the planet ; it is the number of days which the planet requires for the completion of a journey round its path. Kepler's third law establishes a relation between the mean distance and the periodic time. That relation is stated in the following words : — " The squares of the periodic times are "proportional to the cubes of the mean distances." Kepler knew that the different planets had different periodic times ; he also saw that the greater the mean distance of the planet the greater was its periodic time, and he was determined to find out the connection between the two. It was easily found that it would not be true to say that the periodic time is merely proportional to the mean distance. Were this the case, if one planet had a distance twice as great as another, the periodic time of the former would have been double that of the latter; ob- servation showed, however, that the periodic time of the more distant planet exceeded twice, and was indeed nearly three times, that of the other. By repeated trials, which would have exhausted the patience of one less confident in his own sagacity, and less assured of the accuracy of the observations which he sought to interpret, Kepler at length discovered the true law, and expressed it in the form we have stated. To illustrate the nature of this law, we shall take for com- parison the earth and the planet Venus. If we denote the mean distance of the earth from the sun by unity, then the mean THE LAW OF GRAVITATION. 115 distance of Venus from the sun is 0-7233. Omitting decimals beyond the first place, we can represent the periodic time of the earth as 365*3 days, and the periodic time of Venus as 224 7 days. Now the law which Kepler asserts is that the square of 365 3 is to the square of 224*7 in the same proportion as unity is to the cube of 0*7233. The reader can easily verify the truth of this identity by actual multiplication. It is, however, to be remembered that, as only four figures have been retained in the expressions of the periodic times, so only four figures are to be considered significant in making the calculations. Perhaps, however, the most striking manner of making the verification will be to regard the time of the revolution of Venus as an unknown quantity, and deduce it from the known revolution of the earth and the mean distance of Venus. In this way, by assuming Kepler's law, we deduce the cube of the periodic time by a simple proportion, and the resulting value of 224*7 days can then be obtained. As a matter of fact, in the calculations of astronomy, the distances of the planets are usually ascertained from Kepler's law. The periodic time of the planet is an element which can be measured with great accuracy ; and once it is known, then the square of the mean distance, and consequently the mean distance itself, is determined. Such are the three celebrated laws of Planetary Motion, which have always been associated with the name of their discoverer. The profound skill by which these laws were elicited from the mass of observations, the intrinsic beauty of the laws themselves and their absolute truthfulness, their wide- spread generality, and the bond of union which they have established between the various members of the solar system, have given them quite an exceptional position in astronomy. As established by Kepler, these planetary laws were merely the results of observation. It was found as a matter of fact, that the planets did move in ellipses, but Kepler assigned no reason why they should adopt this curve rather than any other. Still less was he able to offer a reason why these bodies should sweep over equal areas in equal times, or why that third law was in- variably obeyed. The laws as they came from Kepler's hands stood out as three independent truths ; thoroughly established, 116 THE STORY OF THE HEAVENS. no doubt, but unsupported by any arguments as to why these movements rather than any others should be appropriate for the revolutions of the planets. It was the crowning triumph of the great law of universal gravitation to remove this empirical character from Kepler's laws. Newton's grand discovery bound together the three isolated laws of Kepler into one beautiful doctrine. He showed not only that those laws are true, but he showed why they must be true, and why no other laws could have been true. He proved to demon- stration in his immortal work, the " Principia," that the ex- planation of the famous planetary laws was to be sought in the attraction of gravitation. Newton set forth that a power of attraction resided in the sun, and as a necessary consequence of that attraction every planet must revolve in an elliptic orbit round the sun, having the sun as one focus ; the radius of the planet's orbit must sweep over equal areas in equal times ; and in comparing the movements of two planets, it was necessary to have the squares of the periodic times pro- portional to the cubes of the mean distances. As this is not a mathematical treatise, it will be impossible for us to discuss the proofs which Newton has given, and which have commanded the immediate and universal acquies- cence of all who have taken the trouble to understand them. We must here only confine ourselves to a very brief and general survey of the subject, which will indicate the character of the reasoning employed, without introducing details of a technical character. Let us, in the first place, endeavour to think of a globe freely poised in space, and completely isolated from the influence of every other body in the universe. Let us imagine that this globe is set in motion by some impulse which starts it forward on a rapid voyage through the realms of space. When the impulse ceases the globe is in motion, and continues to move onwards. But what will be the path which it pursues? We are so ac- customed to see a stone thrown into the air moving in a curved path, that we might naturally think a body projected into free space will also move in a curve. A little consideration will, how- ever, show that the cases are very different. In the realms of free THE LAW OF GRAVITATION. 117 space we find no conception of upwards or downwards ; all paths are alike ; there is no reason why the body should swerve to the right or to the left ; and hence we are led to surmise that under these circumstances a body, once started and freed from all inter- ference, would move in a straight line. It is true that this statement is one which can never be submitted to the test of direct experiment. Circumstanced as we are on the surface of the earth, we have no means of isolating a body from external forces. The resistance of the air, as well as friction in various other forms, no less than the gravitation towards the earth itself, interfere with our experiments. A stone thrown along a sheet of ice will be exposed to but little resistance, and in this case we see that the stone will take a straight course along the frozen surface. A stone similarly cast into empty space would pursue a course absolutely rectilinear. This we demonstrate, not by any attempts at an experiment which would necessarily be futile, but by indirect reasoning. The truth of this principle can never for a moment be doubted by one who has duly weighed the argu- ments which have been produced in its behalf. Admitting, then, the rectilinear path of the body, the next question which arises relates to the velocity with which that movement is performed. The stone gliding over the smooth ice on a frozen lake will, as every one has observed, travel a long dis- tance before it comes to rest. There is but little friction between the ice and the stone, but still there is some ; and as that friction always tends to stop the motion, it will at length happen that the stone is brought to rest. In a voyage through the solitudes of space, a body experiences no friction ; there is no tendency for the velocity to be reduced, and consequently we believe that the body could journey on for ever with unabated speed. No doubt such a statement seems at variance with our ordinary experience. A sailing ship makes no progress on the sea when the wind dies away. A train will gradually lose its velocity when the steam has been turned off. A humming-top will slowly expend its rotation and come to rest. From such instances it might be plausibly argued that when the force has ceased to act, the motion that the force generated gradually wanes, and ultimately vanishes. But in all these cases it will be 118 THE STORY OF THE HEAVE XS. found, on reflection, that the decline of the motion is to be at- tributed to the action of resisting forces. The sailing ship is retarded by the rubbing of the water on its sides ; the train is checked by the friction of the wheels, and by the fact that it has to force its way through the air ; and the atmospheric resistance is mainly the cause of the stopping of the humming-top, for if the air be withdrawn, by making the experiment in a vacuum, the toy will continue to spin for a greatly lengthened period. We are thus led to admit that a body, once projected freely in space and acted upon by no external resistance, will continue to move on for ever in a straight line, and will preserve unabated to the end of time the velocity with which it originally started. This principle is known as the first law of motion. Let us apply this principle to the important question of the movement of the planets. Take, for instance, the case of our earth, and discuss the consequences of the first law of motion. We know it is moving each moment with a velocity of about eighteen miles a second, and the first law of motion assures us that if this odobe were submitted to no external force, it would for ever pursue a straight track through the universe, nor would it depart from the precise velocity which it possesses at the present moment. But is the earth moving in this manner ? Obviously not. We have already found that our globe is moving round the sun, and the comprehensive laws of Kepler have given to that motion the most perfect distinctness and precision. The consequence is irresistible. The earth cannot be free from ex- ternal force. Some potent influence on our globe must be in ceaseless action. That influence, whatever it may be, constantly deflects the earth from the rectilinear path which it tends to pursue, and constrains it to trace out an ellipse instead of a straight line. The great problem to be solved is now easily stated. There must be some external agent constantly influencing the earth. What is that agent, from whence does it proceed, and to what laws is it submitted ? Nor is the question confined to the earth. Mercury and Venus, Mars, Jupiter, and Saturn, unmistakably show that, as they are not moving in rectilinear paths, they must be exposed to some force. What is this force which guides THE LAW OF GRAVITATION. 119 the planets in their paths ? Before the time of Newton this question might have been asked in vain. It was the splendid genius of Newton which supplied the answer, and thus revolu- tionised the whole of modern science. The data from which the question is to be answered must be obtained from observation. We have here no problem which can be solved by mere mathematical meditation. Mathematics is no doubt a useful, indeed an indispensable instrument in the inquiry ; but we must not attribute to mathematics a potency which it does not possess. In a case of this kind, all that mathematics can do is to interpret the results obtained by observation. The data from which Newton proceeded were the observed phenomena in the movement of the earth and the other planets. Those facts had found a succinct expression by the aid of Kepler's laws. It was, accordingly, the laws of Kepler which Newton took as the basis of his labours, and it was for the interpretation of Kepler's laws that Newton invoked the aid of that celebrated mathematical reasoning which he created. The question is then to be approached in this way : A planet being subject to some external influence, we have to determine what that influence is, from our knowledge that the path of each planet is an ellipse, and that each planet sweeps round the sun over equal areas in equal times. The influence on each planet is what a mathematician would call a force, and a force must have a line of direction. The most simple conception of a force is that of a pull communicated along a rope, and the direction of the rope is in this case the direction of the force. Let us imagine that the force exerted on each planet is imparted by an invisible rope. Kepler's laws will inform us with regard to the direction of this rope and the intensity of the strain transmitted through it. The mathematical analysis of Kepler's laws would be beyond the scope of this volume. We must, therefore, confine ourselves to the results to which they lead, and omit the details of the reasoning. Newton first took the law which asserted that the planet moved over equal areas in equal times, and he showed by unimpeachable logic that this at once gave the direction in 120 THE STORY OF THE HEAVENS. which the force acted on the planet. He showed that the imaginary rope by which the planet is controlled must be in- variably directed towards the sun. In other words, the force exerted on each planet was at all times pointed from the planet towards the sun. It still remained to explain the intensity of the force, and to show how the intensity of that force varied when the planet was at different points of its path. Kepler's first law enables this question to be answered. If the planet's path be elliptic, and if the force be always directed towards the sun at one focus of that ellipse, then mathematical analysis obliges us to say that the intensity of the force must vary inversely as the square of the distance from the planet to the sun. The movements of the planets, in conformity with Kepler's laws, would thus be accounted for even in their minutest details, if we admit that an attractive power draws the planet towards the sun, and that the intensity of this attraction varies inversely as the square of the distance. Can we hesitate to say that such an attraction does exist ? We have seen how the earth attracts a falling body ; we have seen how the earth's attraction extends to the moon, and explains the revolution of the moon around the earth. We have now learned that the movement of the planets round the sun can also be explained as a conse- quence of this law of attraction. But the evidence in support of the law of universal gravitation is, in truth, much stronger than any we have yet presented. We shall have occasion to dwell on this matter further on. We shall show not only how the sun attracts the planets, but how the planets attract each other ; and we shall find how this mutual attraction of the planets has led to remarkable discoveries which have elevated the law of gravitation beyond the possibility of doubt. Admitting the existence of this law, we can show that the planets must revolve around the sun in elliptic paths with the sun in the common focus. We can show that they must sweep over equal areas in equal times. We can prove that the squares of the periodic times must be proportional to the cubes of their mean distances. Still further, Ave can show how the mysterious movements of comets can be accounted for. By the same great THE LAW OF GRAVITATION. 121 law we can explain the revolutions of the satellites. We can account for the tides, and for other phenomena throughout the Solar System. Finally, we shall show that when we extend our view beyond the limits of our Solar System to the beautiful starry systems scattered through space we find even there evidence of the great law of universal gravitation. CHAPTER VI. THE PLANET OF ROMANCE. Outline of the Subject— Is Mercury the Planet nearest the Sun? — Transit of an Interior Planet across the Sun — Has a Transit of Vulcan ever been seen ? — Visibility of Planets during a Total Eclipse of the Sun — Professor Watson's Researches in 1878. Provided with a general survey of the Solar System, and with such an outline of the law of universal gravitation as the last chapter has afforded us, we commence the more detailed exami- nation of the planets and their satellites. We shall begin with the planets nearest to the sun, and then we shall gradually proceed outwards to one planet after another, until we reach the confines of the system. We shall find much to occupy our attention. Each planet is itself a globe, and it will be for us to describe as much as is known of it. The satellites by which so many of the planets are accompanied possess many points of interest. The circumstances of their discovery, their sizes, their movements, and their distances, must all be duly considered. Then, too, it will be found that the movements of the planets present much matter for reflection and examination. We shall have occasion to show how the planets mutually disturb each other, and what remarkable consequences have arisen from these influences. We must also occasionally refer to the important problems of celestial measuring and celestial weighing. We must show how the sizes, the weights, and the distances of the various members of our system are to be discovered. A great part of our task will lead us over ground which is thoroughly certain, and where the results have been confirmed by frequent observation. It happens, however, that at the very outset of our course we are obliged to deal with observations which are far from certain. The existence of a planet much closer to the sun than those hitherto known has been asserted by competent THE PLANET OF ROMANCE. 123 authority. The question is still unsettled, and the planet cannot with certainty be pointed out. Hence it is that Ave have called the subject of this chapter, The Planet of Romance. It had often been thought that Mercury, long supposed to be the nearest planet to the sun, was perhaps not really the body entitled to that distinction. Mercury revolves round the sun at an average distance of about 36,000,000 miles. In the interval between it and the sun there might have been one or many other planets. There might have been one revolving at ten million miles, another at fifteen, and so on. But did such, planets exist ? Did even one planet revolve inside the orbit of Mercury ? There were certain reasons for believing in such a planet. In the movements of Mercury indications were perceptible of an in- fluence that could have been accounted for by the supposition of an interior planet. But there was necessarily a great difficulty about seeing this object. It must always be close to the sun, and even in the best telescope it is generally impossible to see a starlike point in that position. Nor could such a planet be seen after sunset, for under the most favourable conditions it would set almost immediately after trie sun, and a like difficulty would make it invisible at sunrise. Our ordinary means of observing a planet have therefore completely failed. We are compelled to resort to extraordinary methods, if we would seek to settle the great question as to the existence of the intra-Mercurial planets. There are at least two lines of observation which might be expected to answer our purpose. An opportunity for the first would arise when it happened that the unknown planet came directly between the earth and the sun. In the adjoining diagram we show the sun at the centre ; the internal orbit denotes that of the unknown planet, which has received the name of Vulcan before even its very existence has been at all satisfactorily established. The outer orbit denotes that of the earth. As Vulcan moves more rapidly than the earth, it will frequently happen that the planet will overtake the earth, so that the three bodies will have the positions represented in the diagram. It would not, however, necessarily follow that Vulcan was exactly between the earth and 124 THE STORY OF THE HEAVENS. the luminary. The path of the planet may be tilted, so that, as seen from the earth, Vulcan would be over or under the sun, according to circumstances. If, however, Vulcan really does exist, we can be assured that sometimes the three bodies will be directly in line, and this would then give the desired opportunity of making the telescopic discovery of the planet. We should expect on such an occasion to observe the planet as a dark spot, moving slowly across the face of the sun. The two other planets interior to the earth, namely, Mercury and Venus, are occasionally seen 9* \\\ of the. E a , / Earth A-—- Fig. 38.— The Transit of the Planet of Romance. in the act of transit ; and there cannot be a doubt that if Vulcan exist, its transits across the sun must be more numerous than those of Mercury, and far more numerous than those of Venus. On the other hand, it may reasonably be anticipated that Vulcan is a small globe, and as it will be much more distant from us than Mercury at the time of its transit, we could not expect that the transit of the planet of romance would be at all com- parable as a spectacle with those of either of the two other bodies Ave have named. The question arises as to whether telescopic research has ever disclosed anything which can be regarded as a transit of Vulcan. On this point it is not possible to speak with any THE PLANET OF ROMANCE. 125 degree of certainty. It has, on more than one occasion, been asserted by observers that a spot has been seen rapidly traversing the sun, and from its shape and general appearance they have presumed it to have been an intra-Mercurial planet. But a close examination of the circumstances under which such observations have been made has not tended to increase confidence in this presumption. Such discoveries have usually been made by persons little familiar with telescopic observations. It is certainly a significant fact that, notwithstanding the diligent scrutiny to which the sun has been exposed during the past century by astronomers who have specially devoted themselves to this branch of research, no telescopic discovery of Vulcan on the sun has been announced by any really experienced astronomer. From an examination of the whole subject, we are inclined to believe that there is not at this moment any reliable telescopic evidence of the transit of an intra-Mercurial planet over the face of the central luminary. But there is still another method by which we might reason- ably hope to detect new planets in the vicinity of the sun. This method is one of great rarity, and requires observations possess- ing the opportunities for putting it in practice. Its application is only possible when the sun is obscured by a total eclipse. When the moon is interposed directly between the earth and the sun, the brightness of day is temporarily exchanged for the gloom of night. If the sky be free from clouds, the stars spring forth, and can be seen around the obscured sun. Even if a planet were quite close to the luminary, it would be visible on such an occasion if its magnitude were comparable with that of Mercury. Careful preparation is necessary when it is proposed to make a trial of this kind. The danger to be specially avoided is the risk of confounding the planet with the ordinary stars, which it will probably resemble. The late distinguished American astronomer, Professor Watson, specially prepared to devote himself to this research during the notable total eclipse in 1878. The duration of this total eclipse was very brief; it lasted only two or three minutes, and all the work had to be compressed into this very short interval. Professor Watson had previously carefully studied the stars 126 THE STORY OF THE HEAVENS. in the neighbourhood of the sun. When the eclipse oc- curred, the light of the sun vanished, and the stars burst forth. The practised eye of Professor Watson at once identified the stars, of which he had formed a map, not merely drawn on paper, but also engraved on his memory ; and among them he saw an object, which certainly seemed to be the long-sought intra-Mercurial planet. We should add, that this zealous ob- server seems to have also detected a second planet on the same occasion. To a certain extent Mr. Watson's observation was confirmed by another astronomer — Mr. Swift. It is necessary to record that Vulcan has not been observed, though specially looked for, during the eclipses which have occurred since 1878, and it has been suggested that the object seen by Watson was a comet. I cannot, however, believe it possible that so ex- perienced an astronomer as Mr. Watson was entirely mistaken. He has been one of the • most successful discoverers of minor planets, so that perhaps it would be the wisest course to post- pone for the present any definite assertion on the matter. CHAPTER VII. MERCURY. The Ancient Astronomical Discoveries— How Mercury was first found?— Not easily seen— Mercury was known in 265 B.C.— Skill necessary in the Dis- covery — The Distinction of Mercury from a Star — Mercury in the East and in the West— The Prediction— How to Observe Mercury— Its Telescopic Appearance— Difficulty of Observing its Appearance— Orbit of Mercury- Velocity of the Planet— Can there be Life on the Planet ?— Changes in its Temperature — Transit of Mercury over the Sun — Gassendi's Observations — Atmosphere around Mercury— The Weight of Mercury. Long and glorious is the record of astronomical discovery. The discoveries of modern days have succeeded each other with such rapidity, they have so often dazzled our imaginations with their brilliancy, that we are sometimes apt to think that astronomical discovery is a purely modern product. But no idea could be more fundamentally wrong. While we appreciate to the utmost the achievements of modern times, let us endeavour to do justice to the labours of the astronomers of antiquity. And when we speak of the astronomers of antiquity, let us understand clearly what is meant. The science is now growing so rapidly that each century witnesses a surprising advance ; each generation, each decade, each year, has its own rewards for those diligent astronomers by whom the heavens are so care- fully scanned. We must, however, project our glance to a remote epoch in time past, if we would view the memorable discovery of Mercury. Compared with it, the discoveries of Newton are to be regarded as very modern achievements ; even the announcement of the Copernican system of the heavens is itself a recent event in comparison with the detection of this planet now to be discussed. By whom was this great discovery made ? Let us see if the question can be answered by the examination of astronomical records. At the close of his memorable life Copernicus was 128 THE STORY OF THE HEAVERS. heard to express his sincere regret that he never enjoyed an op- portunity of beholding the planet Mercury. He had specially longed to see this body, the movements of which were to such a marked extent illustrative of the theory of the celestial motions which it was his immortal glory to have established, but he had never been successful Mercury is not to be seen so easily as are some of the other planets, and it may well have been that the vapours from the Vistula obscured the horizon at Frauenburg, where Copernicus dwelt, and that thus his oppor- tunities of viewing Mercury were probably even rarer than they are at other places. But one circumstance is plain. The existence of the planet was quite familiar to Copernicus, and therefore we must look to some earlier epoch for its discovery. In the scanty astronomical literature of the Middle Ages we find occasional references to the existence of this object. We can trace observations of Mercury through remote centuries to the commencement of our era. Records from dates still earlier are not wanting, until at length we come on an observation which has descended to us for more than 2,000 years, having been made in the year 265 before the Christian era. It is not pre- tended, however, that this observation records the discovery of the planet. Earlier still we find the chief of the astronomers at Nineveh alluding to Mercury in a report which he made to Assurbanipal, the King of Assyria. It does not appear in the least degree likely that the discovery was even then a recent one. It may have been that the planet was independently discovered in two or more localities, but all records of such discoveries are totally wanting ; and we are ignorant alike of the name of the discoverer, of the nation to which he belonged, and of the epoch at which he lived. Although this discovery is of such vast antiquity, although it was made before correct notions were entertained as to the true system of the universe, and, it is needless to add, long before the invention of the telescope, yet it must not be assumed that the detection of Mercury was by any means a simple or obvious matter. This will be manifest when we try to conceive the manner in which the discovery must probably have been made. MERCURY. 129 Some primaeval astronomer, long familiar with the heavens, had learned to recognise the various stars and constellations. Experience had impressed upon him the permanence of these objects ; he had seen that Sirius invariably appeared at the same seasons of the year, and he had noticed how it was placed with regard to Orion and the other neighbouring constellations. In the same manner each of the other bright stars was to him a familiar object always to be found in a particular region of the heavens. He saw how the heavens, as a whole, rose and set in such a way, that though each star appeared to move, yet the relative positions of the stars were incapable of alteration. No doubt this ancient astronomer was acquainted with Venus ; he knew it as the evening star ; he knew it as the morning star ; and he was accustomed to regard Venus as a body which oscillated from one side of the sun to the other. We can easily imagine how the discovery of Mercury was made in the clear skies over an Eastern desert. The sun has set, the brief twilight has almost ceased, when lo, near that part of the horizon where the glow of the setting sun still illuminates the sky, a bright star is seen. Bat surely, the careless observer might say, there is nothing wonderful here. Is not the whole heaven spangled with stars ? why should there not be a star in this locality also ? The primaeval astronomer will not accept this explanation. He knows that there is no bright star at this place in the heavens. If the object of his attention be not a star, what then can it be ? Eager to examine this question, the heavens are watched next night, and there again, higher above the horizon, and more brilliant still, is the object seen the night before. Each successive night the body gains more and more lustre, until at length it becomes a conspicuous gem. Perhaps it will rise still higher and higher ; perhaps it will increase till it attains the brilliancy of Venus itself. Such were the surmises not improbably made by those who first watched this object ; but they were not realised. After a few nights of exceptional splendour the lustre of this mysterious orb declines. The planet again draws near the horizon at sunset, until at length it sets so soon after the sun that it has become invisible. Is it lost for ever ? Years may elapse before another opportunity 9 130 THE STORY OF THE HEAVENS. of observing the object after sunset may be available ; but then again it will be seen to run through the same series of changes, though, perhaps, under very different circumstances. The greatest height above the horizon, and the greatest brightness both vary considerably. It was not until after long and careful observations had been made, that the primeval astronomer could assure himself that the various appearances were to be all attri- buted to a single body. In the Eastern deserts the phenomena of sunrise must have been nearly as familiar as those of sunset, and in the clear skies, at the point where the sunbeams were commencing to dawn above the horizon, a bright starlike point might sometimes be perceived. Each successive day this object rose higher and higher above the horizon before the moment of sunrise, and its lustre increased with the distance ; then again it would draw in towards the sun, and return for a while to invisibility. Such were the data which were presented to the mind of the primitive astronomer. One body was seen after sunset, another body was seen before sunrise. To us it may seem an obvious inference from the observed facts, that the two bodies were identical. The inference is a correct one, but it is in no sense an obvious one. Long and patient observation estab- lished the remarkable law that one of these bodies was never seen until the other had disappeared. Hence it was inferred that the phenomena, both at sunrise and at sunset, were due to the same body, which oscillated to and fro about the sun. We can easily imagine that the announcement of the identity of these two objects was one which would have to be carefully tested before it could be accepted. How are the tests to be applied in a case of this kind ? There can hardly be a doubt that the most complete and convincing demonstration of scientific truth is found in the fulfilment of prediction. When Mercury had been observed for years, a certain regularity in the recurrence of its visibility was noticed. Once a periodicity had been fully established, prediction became possible. The time when Mercury would be seen after sunset, the time when it would be seen before sunrise, could be foretold with accuracy ! When it was found that these predictions were obeyed to the letter — that the planet was always seen when looked for in accordance with the MERCURY. 131 predictions — it was impossible to refuse assent to the hypothesis on which these predictions were based. Underlying that hypo- thesis was the assumption that all the various appearances arose from the oscillations of a single body, and hence the discovery of Mercury was established on a basis as firm as the discovery of Jupiter or of Venus. In the latitudes of the British Islands it is generally possible to see Mercury some time during the course of the year. It is not practicable to lay down, within reasonable limits, any general rule for finding the dates at which the search should be made ; but the student who is determined to see the planet will generally succeed with a little patience. He must first consult an almanac which gives the positions of the body, and select an occasion when Mercury is stated to be an evening or a morning star. Such an occasion during the spring months is especially suitable, as the elevation of Mercury above the horizon is usually greater then than at other seasons ; and in the evening twilight, about three- quarters of an hour after sunset, a view of this shy but beautiful object will reward the observer's attention. To those astronomers who are provided with equatorial telescopes such instructions are unnecessary. To enjoy a tele- scopic view of Mercury, we first turn to the Nautical Almanac, and find the position in which the planet lies. If it happen to be above the horizon, we can at once direct the telescope to the place, and even in broad daylight the planet will very often be seen. The telescopic appearance of Mercury is, however, not un- frequently disappointing. Though it is much larger than the moon, yet it is sufficiently far off to render its telescopic appear- ance insignificant. There is, however, one feature in such a spectacle, which would immediately attract attention. Mercury is not usually observed to be a circular object, but more or less crescent-shaped, like a miniature moon. The phases of the planet are also to be accounted for on exactly the same principles as the phases of the moon. Mercury is a globe composed, like our earth, of materials possessing in themselves no source of illumi- nation, one hemisphere of the planet must necessarily be turned towards the sun, and this side is accordingly lighted up brilliantly by the solar rays. When we look at Mercury we see nothing 132 THE STORY OF THE HEAVENS. of the non-illuminated side, and the crescent is due to the fore-shortened view which we obtain of the illuminated part. The planet is such a small object that, in the glitter of the naked- eye view, the shape of the luminous body cannot be defined. Indeed, even in the much larger crescent of Venus, the aid of the telescope has to be invoked before the characteristic form can be observed. Beyond, however, the fact that Mercury is a crescent, and that it undergoes varying phases in correspondence with the changes in its relative position to the earth and the sun, we cannot see much of the planet. It is too small and too bright to admit of the delineation of details on its surface. No doubt c Fig. 39. — The Movement of Mercury, showing the variations in Phase and in apparent size. attempts have been made, and observations have been recorded, as to certain physical features on the planet. It has been sup- posed that glimpses of lofty mountains have been seen. But such statements must be received with great hesitation, if not with actual discredit, and beyond this mere allusion they need not further engage our attention. The facts which have been thoroughly established with regard to Mercury are mainly numerical statements as to the path it describes around the sun. The time taken by the planet to complete one of its revolutions is eighty-eight days nearly. The average distance from the sun is about 36,000,000 miles, and the mean velocity with which the body moves is over twenty- nine miles a second. We have already alluded to the most MERCURY. 133 characteristic and remarkable feature of the orbit of Mercury. That orbit differs from the paths of all the other large planets by its much greater departure from the circular form. In the majority of cases the planetary orbits are so little elliptic that a diagram of the orbit drawn accurately to scale would not be per- ceived to differ from a circle unless careful measurements were made. In the case of Mercury the circumstances are different. The elliptic form of the path Avould be quite unmistakable by the most casual observer. The distance from the sun to the planet fluctuates between very considerable limits. The lowest value it Fig. 40. — Mercury as a Crescent. can attain is about 30,000,000 miles; the highest value is about 43,000,000 miles. In accordance with Kepler's second law, the velocity of the planet must exhibit corresponding changes. It must sweep rapidly around that part of his path near the sun, and more slowly round the remote parts of his path. The greatest velocity is about thirty-five miles a second, and the least is twenty-three miles a second. For an adequate conception of the movements of Mercury we ought not to dissociate the velocity from the true dimensions of the body by which it is performed. No doubt a speed of twenty-nine miles a second is enormous when compared with the velocities with which daily life makes us familiar. The 134 THE STORY OF THE HEAVENS. speed of the planet is not less than a hundred times as great as the velocity of the rifle-bullet. But when we compare the sizes of the bodies with their velocities, the velocity of Mercury seems relatively much less than that of the bullet. A rifle-bullet traverses a distance equal to its own diameter many thousands of times in a second. But even though Mercury is moving so much faster, yet the dimensions of the planet are so considerable that a period of two minutes will be required for it to move through a distance equal to its diameter. Viewing the globe of the planet as a whole, the velocity of its movement is but a stately and dignified progress appropriate to its dimensions. As we can learn little or nothing of the true surface of Mercury, it is utterly impossible for us to say whether life can exist on the surface of that planet. We may, however, reasonably conclude that there -cannot be life on Mercury in any respect analogous to the life which we know on the earth. The heat of the sun and the light of the sun beat down on Mercury with an intensity many-fold greater than we experience. When this planet is at its utmost distance from the sun the intensity of solar radiation is even then more than four times greater than the greatest heat which ever reaches the earth. But when Mercury, in the course of its remarkable changes of distance, draws in to the warmest part of its orbit, it is exposed to an appalling scorching. The intensity of the sun's heat must then be not less than nine times as great as the greatest radiation to which we are exposed. These changes succeed each other much more rapidly than the variations of our seasons. On Mercury the interval between midsummer to midwinter is only forty-four days, while the whole year is only eighty-eight days. Such rapid and tremendous variations in solar heat must in themselves exercise a profound effect on the habitability of Mercury. Mr. Ledger well remarks, in his interesting work,* that if there be inhabitants on Mercury the notions of "perihelion" and "aphelion," which are here often regarded as expressing ideas of an intricate or recondite character, must on the surface of that planet be familiar to everybody. The words imply " near the sun," and " away from the sun ; " but we do not associate these * "The Sun : its Planets, and their Satellites." London : 1882 (page 147). MERCURY. 135 expressions with any obvious phenomena, because the changes in the distance of the earth from the sun are inconsiderable. But on Mercury, where in six weeks the sun rises to more than double his apparent size, and gives more than double the quantity of light and of heat, such changes as are signified by perihelion and aphelion embody ideas obviously and intimately connected with the whole economy of the planet. It is nevertheless rash to found any inferences as to climate merely upon the proximity or the remoteness of the sun. Climate depends upon other matters besides the sun's distance. The atmosphere surrounding the earth has a profound influence on heat and cold, and ' if Mercury have an atmosphere — as has often been supposed — its climate may be thereby modified to any necessary extent. It seems, however, hardly possible to suppose that any atmosphere could form an adequate protection for the inhabitants from the violent and rapid fluctuations of solar radiation. All Ave can say is, that the problem of life in Mercury belongs to the class of unsolved, and perhaps unsolvable, mysteries. It was in the year 1627 that Kepler made an important an- nouncement as to impending astronomical events. He had been studying profoundly the movements of the planets. He had ex- amined the former observations which had been recorded ; and from his study of the past he had ventured to predict the future. Kepler announced that in the year 1631 the planets Venus and Mercury would both make a transit across the sun, and he assigned the dates to be November 7th for Mercury, and Decem- ber 6th for Venus. This was at the time a very remarkable prediction. We are so accustomed to turn to our almanacs, and learn from thence all the astronomical phenomena which are anticipated during the year, that we are apt to forget how in early times this was impossible. It has only been by slow degrees that astronomy has been rendered so perfect as to enable us to foretell, with accuracy, the occurrence of the more delicate phe- nomena. The prediction of those transits by Kepler, some years before they occurred, was justly regarded at the time as a most remarkable achievement. The illustrious Gassendi prepared to apply the test of actual observation to the announcements of 136 THE STORY OF THE HEAVENS. Kepler. We can now assign the time of the transit accurately to within a few minutes, but in those early attempts equal pre- cision was not practicable. Gassendi considered it necessary to commence watching for the transit of Mercury two whole days before the time indicated by Kepler, and he had arranged an in- genious plan for making his observations. The light of the sun was admitted into a darkened room through a hole in the shutter, and an image of the sun was formed on a white screen by a lens. This is, indeed, an admirable and a very pleasing way of studying the surface of the sun, and even at the present day, with our best telescopes, one of the methods of viewing our luminary is founded on the same principle. Gassendi commenced his watch on the 5th of November, and carefully studied the sun's image at every available opportunity. It was not, however, until five hours after the time assigned by Kepler that the transit of Mercury actually commenced. Gassendi's preparations had been made with all the resources which he could command, but these resources seem very imperfect when compared with the appli- ances of our modern observatories. He was anxious to note the * time when the planet appeared, and for this purpose he had stationed an assistant in the room beneath, who was to observe the altitude of the sun at the moment indicated by Gassendi. The signal to the assistant was to be conveyed by a very primitive apparatus. Gassendi was to stamp on the floor when the critical moment had arrived. In spite of the long delay, which exhausted the patience of the assistant, some valuable observations were obtained, and thus the first passage of a planet across the sun was observed. The transits of Mercury are chiefly of importance on account of the accuracy which their observation infuses into our calcula- tions of the movements of the planet. It has often been hoped that the opportunities afforded by a transit would he available for procuring information as to the physical character of the globe of Mercury. To some extent — but not, unhappily, to any large extent — these hopes have been realised. Skilful observers have described the appearance of the planet in transit as that of a round, dark spot, surrounded by a luminous margin of a depth variously estimated on different occasions at from one-third to MERCURY. 137 two-thirds of trie planet's diameter. There can be hardly a doubt that a dense atmosphere surrounding Mercury would be capable of producing an appearance resembling that which has been described, and, therefore, the probability that such an atmo- sphere really exists must be admitted. It has even been shown that the vapour of water is a probable constituent in the atmo- sphere of Mercury as it is in the atmosphere surrounding our own earth. A distinguished Italian astronomer, Professor Schiaparelli, has recently announced a remarkable discovery with respect to the rotation of the planet Mercury. He has found that the planet rotates on its axis in the same period as its revolution around the sun. The practical consequence of the identity between these two periods is that Mercury always turns the same face to the sun. If our earth were to rotate in a similar fashion, then the hemisphere directed to the sun would enjoy eternal day, while the opposite hemisphere would be rele- gated to perpetual night. According to this discovery Mercury revolves around the sun in the same way as the moon revolves around the earth. Perhaps it would be prudent to add that so remarkable an announcement will demand a little more confir- mation by additional observation. I must, however, say that there is so strong a probability in favour of its truth that it seems hard even at this stage to withhold a cordial assent. Mercury being one of the planets devoid of a moon, will be solely influ- enced by the sun in so far as tidal phenomena are concerned. Owing, moreover, to the proximity of Mercury to the sun, the solar tides on that planet possess an especial vehemence. As the tendency of tides is to make Mercury present a constant face to the sun, there need be little hesitation in accepting testimony that tides have wrought exactly the result that we know they were competent to perform. Here we take leave of the planet Mercury — an interesting and beautiful object, which stimulates our intellectual curiosity, while at the same time it eludes our attempts to make a closer acquaintance. There is, however, one point Of attainable know- ledge which we must mention in conclusion. It is a difficult, but not by any means an impossible, task to weigh Mercury in 138 THE STORY OF THE HEAVENS. the celestial balance, and determine his mass in comparison with the other globes of our system. This is a delicate operation, but it leads us through some of the most interesting paths of astro- nomical discovery. The weight of the planet, as recently deter- mined by Von As ten, is about one twenty-fourth part of the weight of the earth. CHAPTER VIII. VENUS. Interest attaching to this Planet— The unexpectedness of its appearance— The Evening Star— Visibility in Daylight— Only Lighted by the Sun— The Phases of Venus— Why the Crescent is not Visible to the Unaided Eye— Variations in the Apparent Size of the Planet— Resemblance of Venus to the Earth— The Transit of Venus— Why of such especial Interest— The Scale of the Solar System— Orbits of the Earth and Venus not in the same Plane— Recurrence of the Transits in pairs— Appearance of Venus in Transit — Transits of 1874 and 1882— The early Transits of 1631 and 1639— The observations of Hor- rocks and Crabtree— The Announcement of Halley — How the track of the Planet differs from different places— Illustrations of Parallax — Voyage to Otaheite — The result of Encke — Probable Value of the Sun's Distance — Observations of the recent Transit of Venus at Dunsink— The Question of an Atmosphere to Venus — Dr. Copeland's Observations — Utility of such Researches— Other Determinations of the Sun's Distance — Statistics about Venus. It might, for one reason, have been not inappropriate to com- mence our review of the planetary system by the description of the planet Venus. This body is not especially remarkable for its size, for there are other planets hundreds of times larger. The orbit of Venus is no doubt larger than that of Mercury, but it is much smaller than that of the outer planets. Venus has not even the splendid retinue of minor attendants which gives such dignity and such interest to the mighty planets of our system. Yet the fact still remains that Venus is peerless among the planetary host. We speak not now of celestial bodies only seen in the telescope, we refer to the ordinary observation which detected Venus ages before telescopes were invented. Who has not been delighted with the view of this glorious object ? It is not to be seen at all times. For months together the star of evening is hidden from mortal gaze. Its beauties are even en- hanced by the caprice and the uncertainty which attend its appearance. We do not say that there is any caprice in the movements of Venus, as known to those who diligently consult 140 THE STORY OF THE HEAVENS. their almanacs. The movements of the lovely planet are there prescribed with a prosaic detail hardly in harmony with the usual character of the goddess of love. But to those who do not devote particular attention to the stars, the very unexpectedness of its appearance is one of its greatest charms. Venus has not been noticed, not been thought of, for many months. It is a beautifully clear evening ; the sun has just set. The lover of nature turns to admire the sunset, as every lover of nature will In the golden glory of the west a beauteous gem is seen to glitter ; it is the evening star — the planet Venus. A few weeks later another beautiful sunset is seen, and now the planet is no longer a point low down in the western glow ; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. Again, a little later, and Venus has gained its full brilliancy and splendour. All the heavenly host — even Sirius and even Jupiter — must pale before the splendid lustre of Venus, the unrivalled queen of the fir- mament. After weeks of splendour, the height of Venus at sunset diminishes, and its lustre begins gradually to decline. It sinks to invisibility, and is forgotten by the great majority of mankind ; but the capricious goddess has only moved from one side of the sky to the other. Ere the sun rises the morning star will be seen in the east. Its splendour gradually augments until it rivals the beauty of the evening star. Then again the planet draws near to the sun, and remains lost to view for many months, until the same cycle of changes recommences, after an interval of a year and seven months. When Venus is at its brightest it can be easily seen in broad daylight with the unaided eye. This striking spectacle proclaims in an unmistakable manner the unrivalled supremacy of this planet as compared with its fellow planets and with the fixed stars. Indeed, at this time Venus is from forty to sixty times more brilliant than any stellar object in the northern heavens. The beautiful evening star is often such a very conspicuous object that it may seem difficult at first to realise that the body is not self-luminous. Yet it is impossible to doubt that the planet is really only a dark globe, and to that extent resembling VENUS. 141 our own earth. The brilliancy of the planet is not so very much greater than that of the earth on a sunshiny day. The splendour of Yenus entirely arises from the reflected light of the sun, in the manner already explained with respect to the moon. We cannot distinguish the characteristic crescent shape of the planet with the unaided eye, which merely shows a brilliant point too small to possess sensible form. This is to be explained on physiological grounds. The optical contrivances in the eye form an image of the planet on the retina, which is necessarily very small. Even when Venus is nearest to the earth, the diameter of the planet subtends an angle not much more than one minute of arc. On the delicate membrane a picture of Venus is thus drawn about one six-thousandth part of an inch in diameter. Great as may be the delicacy of the retina, it is not adequate to the perception of form in a picture so minute. The nervous structure, which has been described as the source of vision, forms too coarse a canvas for the reception of the details of this tiny picture. Hence it is that to the unaided eye the brilliant Venus appears merely as a bright spot. Ordinary vision cannot tell what shape it has ; still less can it reveal the true beauty of the creseent. If the diameter of Venus were several times as great as it actually is ; were this body, for instance, as large as Jupiter or some of the other great planets, then its crescent could be readily discerned by the unaided eye. It is curious to speculate on what might have been the history of astronomy had Venus only been as large as Jupiter. Were every one able to see the crescent form without a telescope, it would then have been an elementary and almost obvious truth that Venus must be a dark body revolving round the sun. The analogy between Venus and our earth would have been at once perceived ; and the doctrine which was left to be discovered by Copernicus in com- paratively modern times might not improbably have been handed down to us with the other discoveries which have come from the ancient nations of the East. In Fig. 41 we show three views of Venus under different aspects. The planet is so much closer to the earth when the crescent is seen, that it appears to be part of a much larger circle than that made by Venus when more nearly full. This 142 THE STORY OF THE HEAVENS. drawing shows the different aspects of the globe in their true relative proportions. It is very difficult to perceive distinctly any markings on the brilliantly lighted surface. Sometimes ob- servers have seen spots or other features, and occasionally the pointed extremities of the horns have been irregular, as if to show that the surface of Venus is not smooth. Attempts have even been made to prove from such observations that there must be lofty mountains in Venus, but we cannot place much con- fidence in the results. It happens that our earth and Venus are very nearly Fig. 41. — Different Aspects of Venus in the Telescope. equal in bulk. The difference is hardly perceptible, but the earth has a diameter a few miles greater than that of Venus. There are also indications of the existence of an atmosphere around Venus, but we have no means of knowing at present what the gases may be of which that atmosphere is composed. If there be oxygen in the atmosphere of Venus, then it would seem possible that there might be life on that globe not differ- ent in character from some forms of life on the earth. No doubt the sun's heat on Venus is greatly in excess of the sun's heat with which we are acquainted, but this is not an insuperable diffi- culty. We see at present on the earth, life in very hot regions and life in very cold regions. Indeed, as we go into the tropics VENUS. 143 we find life more and more exuberant, so that, if water be present on the surface of Venus and if oxygen be a constituent of its atmosphere, we might expect to find in that planet a luxuriant tropical life, of a kind perhaps analogous in some respects to life on the earth. In our account of the planet Mercury, as well as in the brief description of the hypothetical planet Vulcan, it has been neces- sary to allude to the phenomena presented by the transit of a planet over the face of the sun. Such an event is always of interest to astronomers, and especially so in the case of Venus. The transit of that planet rises, in fact, to an importance hardly surpassed by any other phenomenon in our system, and hence it will necessarily engage our attention in the present chapter. We have in recent years had the opportunity of witnessing two of these rare occurrences. It is hardly too much to assert that the recent transit of 1882 and the previous one of 1874 have received a degree of attention never before accorded to any astronomical phenomenon. The transit of Venus cannot be described as a very striking or beautiful spectacle. It is not nearly so fine a sight as a great comet or a shower of shooting stars. Why is it, then, that the transit of Venus is regarded as of so much scientific importance ? It is because the phenomenon enables us to solve one of the greatest problems which has ever engaged the mind of man. It is by the transit of Venus that we may determine the scale on which our solar system is constructed. Truly this is a noble problem. Let us dwell upon it for a moment. In the centre of our system we have the sun — a majestic globe more than a million times as large as the earth. Circling round the sun we have the planets, of which our earth is but one. There are hundreds of small planets. There are a few comparable with our earth ; there are others vastly surpassing the earth. Besides the planets there are other bodies in our system. Many of the planets are accompanied by systems of revolving moons. There are hundreds, perhaps thousands, of comets, while the minor bodies of our system exist in countless millions. Each member of this stupendous host moves in a prescribed orbit around the sun, and collectively they form the solar system. 144 THE STORY OF THE HEAVENS. It is comparatively easy to learn the proportions of this system, to measure the relative distances of the planets from the sun, and even the relative sizes of the planets themselves. Peculiar difficulties are, however, experienced when we seek to ascertain the actual size of the system as well as its shape. It is this latter question which the transit of Venus enables us to solve. Look, for instance, at an ordinary map of Europe. We see the various countries laid down with precision ; we can tell the courses of the rivers ; we can say that France is larger than England, and Russia larger than France ; but no matter how perfectly the map may be constructed, something else is necessary before we can have a complete conception of the dimensions of the country. We must know the scale on which the map is drawn. The map contains a reference line with certain marks upon it. This line is to give the scale of the map. Its duty is to tell us that an inch on the map corresponds with so many miles on the actual surface. Unless it be supplemented by the scale, the map would be quite useless for many purposes. Suppose that we consulted it in order to choose a route from London to Vienna, we can see at once the direction to be taken and the various towns and countries to be traversed ; but unless we refer to the little scale in the corner, the map will not tell how many miles long the journey is to be. A map of the solar system can be readily constructed. We can draw on it the orbits of some of the planets and of their satellites, and we can include many of the comets. We can assign to the stars and to the orbits their proper proportions. But to render the map quite efficient something more is necessary. We must have the scale which is to tell us how many millions of miles on the heavens correspond to one inch of the map. It is at this point we encounter a difficulty. It is comparatively easy to have the relative sizes of the orbits of the different bodies all correct — very simple observations suffice for this purpose, but it is not so easy to assign, with accuracy, the correct scale of the celestial map. There are, however, several ways of solving the problem, though they are all difficult and laborious. The most celebrated method is that presented on an occasion of the transit of Venus. Herein, then, lies the importance of this VENUS. 145 rare event. It is one of the best known means of finding the actual scale on which our system is constructed. Observe the full importance of the problem. Once the transit of Venus has given us the scale, then all is known. We know the size of the sun ; we know his distance ; we know the bulk of J upiter, and the distances at which his satellites revolve ; we know the dimensions of the comets, and the number of miles to which they recede in their wanderings ; we know . the velocity of the shooting stars ; and we learn the important lesson that our earth is but one of the minor members of the sun's family. As the path of Venus lies inside that of the earth, and as Venus moves more quickly than the earth, it follows that the earth is frequently passed by the planet, and just at the critical moment it will sometimes happen that the earth, the planet, and the sun lie in the same straight line. We can then see Venus on the face of the sun, and this is the phenomenon which we call the transit of Venus. It is, indeed, quite plain that if the three bodies were exactly in a line, an observer on the earth, looking at the planet, would see it brought out vividly against the brilliant background of the sun. Considering that the earth is overtaken by Venus once every nineteen months, it might seem that the transits of the planet should occur with corresponding frequency. This is not the case ; the transit of Venus is an exceedingly rare occurrence, and a hundred years or more will often elapse without a single one taking place. The rarity of these phenomena arises from the fact that the path of the planet is inclined to the plane of the earth's orbit ; so that for half of its path Venus is above the plane of the earth's orbit, and in the other half it is below. When Venus overtakes the earth, the line from the earth to Venus will there- fore usually pass over or under the sun. If, however, it should happen that Venus overtakes the earth at or near either of the points in which the plane of the orbit of Venus passes through that of the earth, then the three bodies will be in line, and a transit of Venus will be the consequence. The rarity of the occurrence of a transit need no longer be a mystery. The earth passes through one of the critical parts every December, and through the other every June. If it happens that the conjunction 10 146 THE STORY OF THE HEAVENS. of Venus occurs on, or close to, June 6th or December 7th, then a transit of Venus will occur at that conjunction, but under no other circumstances. The most remarkable law with reference to the repetition of the phenomenon is the well-known eight-year interval. The transits may be all grouped together into pairs ; the two transits of any single pair being separated by an interval of eight years. For instance, a transit of Venus took place in 1761, and again in 1769. No further transits occurred until those recently wit- nessed in 1874 and in 1882. Then, again, comes a long interval, for another transit will not occur until 2004, but it will be fol- lowed by another in 2012. This arrangement of the transits in pairs admits of a very simple explanation. It happens that the periodic time of Venus bears a remarkable relation to the periodic time of the earth. The planet accomplishes thirteen revolutions around the sun in veiy nearly the same time that the earth requires for eight revolutions. If, therefore, Venus and the earth were in line with the sun in 1874, then in eight years more the earth will again be found in the same place ; and so will Venus, for it has just been able to accomplish thirteen revolutions. A transit of Venus having occurred on the first occasion, a transit must also occur on the second. It is not, however, to be supposed that every eight years the planets will again resume the same position with sufficient precision for a regular eight-year transit interval. It is only approximately true that thirteen revolutions of Venus are coincident with eight revolutions of the earth. Each conjunction after an interval of eight years takes place at a slightly different position of the planets, so that when the two planets came together again in the year 1890 the point of conjunction was so far removed from the critical point that the line from the earth to Venus did not intersect the sun, and thus, although Venus passed very near the sun, yet no transit took place. Fig. 42 represents the transit of Venus in 1874. It is taken from a photograph obtained, during the occurrence, by M. Janssen. His telescope was directed towards the sun during the eventful minutes while it lasted, and thus an image of the sun was VENUS. 147 depicted on the photographic plate placed in the telescope. The circular margin represents the disc of the sun. On that disc we see the round, sharp image of Venus, showing the characteristic appearance of the planet during the progress of the transit. The only other features to be noticed are a few of the solar spots, Fig. 42.— Venus on the Sun at the Transit of 1874. rather dimly shown, and a network of lines which were stretched across the field of view of the telescope to facilitate measure- ments. It might be supposed that the appearance of Venus in front of the sun could be mistaken for one of the ordinary spots, which are often large and round, and have occasionally even simulated the appearance of a planet. But this view will not bear examination. The occurrence of the transit at the predicted 148 THE STORY OF THE HEAVENS. moment, and at the precise point of the sun's margin which the calculations had indicated, the sharpness of the planet's shape, and the circumstances of its motion, all discriminate the planet as something totally distinct from a sun spot. The adjoining sketch exhibits the course which the planet pursued in its course across the sun on the two occasions in 1874 and 1882. Our generation has had the good fortune to witness the two occurrences indicated on this picture. The white circle denotes the disc of the sun ; the planet encroaches on the white surface, and at first is like a bite out of the sun's margin. Gradually the black spot steals in front of the sun, until, after nearly half an hour, the black disc is entirely visible. Slowly the planet wends its way across, followed by hundreds of telescopes from every accessible part of the globe whence the phenomenon is visible, until at length, in the course of a few hours, it emerges from the other side. It will be useful to take a brief retrospect of the different transits of Venus of which there is any historical record. They are not numerous. Hundreds of such phenomena have occurred since man first came on the earth. It was not until the approach of the year 1631 that attention began to be directed to the matter, though the transit which undoubtedly occurred in that year was not, so far as we know, noticed by any one. The success of Gassendi in observing the transit of Mercury, to which we have referred in the last chapter, led him to hope that he would be equally fortunate in observing the transit of Venus, which Kepler had also foretold. Gassendi looked at the sun on the 4th, 5th, and 6th December. He looked at it again on the 7th, but he saw no sign of the planet. We now know the reason. The transit of Venus took place during the night, between the 6th and the 7th, and must therefore have been invisible to European observers. Kepler had supposed that there would not be another transit until 1761. In this instance the usual acumen of the great astronomer seems to have deserted him. He appears not to have fully appreciated the remarkable eight-year period, which necessi- tates that the transit of 1631 would be followed by another in 1639. This latter transit is the one with which the history of VENUS. 149 the subject may be said to commence. It was the first occasion on which the phenomenon was ever actually witnessed ; nor was it then seen by many. So far as is known, it was witnessed by only two persons. A young and ardent English astronomer, named Horrocks, had undertaken some computations about the motions of Venus. Fig. 43.— The path of Venus across the Sun in the Transits of 1874 and 1882. He made the discovery that the transit of Venus would be re- peated in 1639, and he prepared to verify the fact. The sun rose bright on the morning of the day— which happened to be a Sunday. The clerical profession, which Horrocks followed, here came into collision with his desires as an astronomer. He tells us that at nine he was called away by business of the highest importance— referring, no doubt, to his official duties ; but the service was quickly performed, and a little before ten he was again on the watch, only to find the brilliant face of the sun without any unusual feature. It was marked with a spot, but nothing that could be mistaken for a planet. Again, at noon, 150 THE STORY OF THE HEAVENS. came an interruption ; he went to church, but he was back by one. Nor were these the only impediments to his observations. The sun was also more or less clouded over during part of the day. However, at a quarter past three in the afternoon his clerical work was over ; the clouds had dispersed, and he once more resumed his observations. To his intense delight he then saw on the sun the round, dark spot, which was at once identified as the planet Venus. The observations could not last long ; it was the depth of winter, and the sun was rapidly setting. Only half an hour was available, but he had made such careful pre- parations beforehand that it sufficed to enable him to secure some valuable measurements. Horrocks had previously acquainted his friend, William Crab- tree, with the impending occurrence. Crabtree was therefore on the watch, and succeeded in seeing the transit ; a striking pic- ture of Crabtree's famous observation is shown in one of the beautiful frescoes in the Town Hall at Manchester. But to no one else had Horrocks communicated the intelligence ; as he says, " I hope to be excused for not informing other of my friends of the expected phenomenon, but most of them care little for trifles of this kind, rather preferring their hawks and hounds, to say no worse ; and although England is not without votaries of astronomy, with some of whom I am acquainted, I was unable to convey to them the agreeable tidings, having my- self had so little notice." It was not till long afterwards that the full importance of the transit of Venus was appreciated. Nearly a century had rolled away when the great astronomer, Halley (1656 — 1741), drew attention to the subject. The next transit was to occur in 1761, and forty-five years before that event Halley explained his cele- brated method of finding the distance of the sun by means of the transit of Venus. He was then a man sixty years of age ; he could have no expectation that he would live to witness the event ; but in noble language he commends the problem to the notice of the learned, and thus addresses the Royal Society of London : — " And this is what I am now desirous to lay before this illustrious Society, which I foretell will continue for ages, that I may explain beforehand to young astronomers, who may VENUS. 151 perhaps live to observe these things, a method by which the immense distance of the sun may be truly obtained. ... I recommend it, therefore, again and again to those curious astro- nomers who, when I am dead, will have an opportunity of observing these things, that they would remember this my ad- monition, and diligently apply themselves with all their might in making the observations, and I earnestly wish them all ima- ginable success — in the first place, that they may not by the unseasonable obscurity of a cloudy sky be deprived of this most desirable sight, and then that, having ascertained with more exactness the magnitudes of the planetary orbits, it may redound to their immortal fame and glory." Halley lived to a good old age, but he died nineteen years before the transit occurred. The student of astronomy who desires to learn how the tran- sit of Venus will tell the distance from the sun must prepare to encounter a geometrical problem of no little complexity. We cannot give to the subject the detail that would be requisite for a full explanation. All we can attempt is to render a general account of the method, sufficient to enable the reader to see that the transit of Yenus really does contain all the elements necessary for the solution of the problem. We must first explain clearly the conception which is known to astronomers by the name of parallax ; for it is by parallax that the distance of the sun, or, indeed, the distance of any other celestial body, must be determined. Let us take a simple illus- tration. Stand near a window from whence you can look at buildings, or the trees, the clouds, or any distant objects. Place on the glass a thin strip of paper vertically in the middle of one of the panes. Close the right eye, and note with the left eye the position of the strip of paper relatively to the objects in the background. Then, while still remaining in the same position, close the left eye and again observe the position of the strip of paper with the right eye. You will find that the position of the paper on the background has changed. As I sit in my study and look out of the window I see a strip of paper, with my right eye, in front of a certain bough on a tree a couple of hundred yards away ; with my left eye the paper is no longer in front of that bough, it has moved to a position near the edge of the tree. 152 THE STORY OF THE HEAVENS. This apparent displacement of the strip of paper, relatively to the distant background, is what is called parallax. Move closer to the window, and repeat the observation, and yon find that the apparent displacement of the strip increases. Move away from the window, and the displacement decreases. Move to the other side of the room, the displacement is much less, though probably still visible. We thus see that the change in the apparent place of the strip of paper, as viewed with the right eye or the left eye, varies in amount as the distance changes ; but it varies in the opposite way to the distance, for as either becomes greater the other becomes less. We can thus associate with each particular distance a corresponding particular displace- ment. From this it will be easy to infer, that if we have the means of measuring the amount of displacement, then we have the means of calculating the distance from the observer to the window. It is this principle, applied on a gigantic scale, which enables us to measure the distances of the heavenly bodies. Look, for instance, at the planet Venus ; let this correspond to the strip of paper, and let the sun, on which Venus is seen in the act of transit, be the background. Instead of the two eyes of the observer, we now place two observatories in distant regions of the earth ; we look at Venus from one observatory, we look at it from the other ; we measure the amount of the displacement, and from that we calculate the distance of the planet. All depends, then, on the means which we have of measuring the displacement of Venus as viewed from the two different stations. There are various ways of accomplishing this, but the most simple is that originally proposed by Halley. , From the observatory at A Venus seems to pursue the upper of the two tracks shown in the adjoining figure. From the observatory at B it follows the lower track, and it is for us to measure the distance between the two tracks. This can be accomplished in several ways. Suppose the observer at A notes the tie?.e that Venus has occupied in crossing the disc, and that similar observations be made at b. As the track seen from b is the larger, it must follow that the time observed at B will be greater than that at A. When the observers from the different VENUS. 153 hemispheres come together and compare their observations, the times observed will enable the lengths of the tracks to be calculated. The lengths being known, their places on the circular disc of the sun are determined, and hence the amount of displacement of Venus in transit is ascertained. Thus it is that the distance of Venus is measured, and the scale of the solar system is known. The two transits to which Halley's memorable researches Fig. 44. — To illustrate the observation of the Transit of Venus from two localities, A and B, on the Earth. referred occurred in the years 1761 and 1769. The results of the first were not very successful, in spite of the arduous labours of those who undertook the observations. The transit of 1769 is of particular interest, not only for the determination of the sun's distance, but also because it gave rise to the first of the celebrated voyages of Captain Cook. It was to see the transit of Venus that Captain Cook was commissioned to sail to Otaheite, and there, on the 3rd of June, on a splendid day in that most exquisite climate, the phenomenon was carefully 154 THE STORY OF THE HEAVENS. observed and measured by different observers. Simultaneously with these observations others were obtained in Europe, and from the combination of the two an approximate knowledge of the sun's distance was gained. The most complete discussion of these observations did not, however, take place for some time. It was not until the year 1824 that the illustrious Encke computed the distance of the sun, and gave as the definite result 95,000,000 miles. For many years this number was invariably adopted, and many of the present generation will remember how they were taught in their school-days that the sun was 95,000,000 miles away. At length doubts began to be whispered as to the accuracy of this result. The doubts arose in different quarters, and were presented with different degrees of importance ; but they all pointed in one direction, they all indicated that the distance of the sun was not really so great as the result which Encke had obtained. It must be remembered that there are several ways of finding the distance of the sun, and it will be our duty to allude to some other methods later on. It has been ascertained that the result obtained by Encke was too great, and that the distance of the sun may probably be now stated at 92,700,000 miles. I venture to record our personal experience of the last transit of Venus, which we had the good fortune to view from Dunsink Observatory on the afternoon on the 6th of December, 1882. The morning of the eventful day appeared to be about as unfavourable for a grand astronomical spectacle as could well be imagined. Snow, a couple of inches thick, covered the ground, and more was falling, with but little intermission, all the forenoon. It seemed almost hopeless that a view of the phenomenon could be obtained from this observatory ; but it is well in such cases to bear in mind the injunction given to the observers on a cele- brated eclipse expedition. They were instructed, no matter what the day should be like, that they were to make all their prepara- tions precisely as they would have done were the sun shining with undimmed splendour. By this advice no doubt many ob- servers have profited ; and we acted upon it here with very con- siderable success. VENUS. 155 We have at this observatory two equatorials, one of them an old, but tolerably good instrument, of about six inches aperture, the other the great South equatorial of twelve inches aperture already referred to. At eleven o'clock the day looked worse than ever ; but we at once proceeded to make all ready. I stationed Mr. Rambaut at the small equatorial, while I myself took charge of the South instrument. The snow was still falling when the domes were opened ; but, according to our pre-arranged scheme, the telescopes were directed, not indeed upon the sun, but to the place where we knew the sun was, and the clockwork was set in motion which carried round the telescopes, still constantly pointing towards the invisible sun. The predicted time of the transit had not yet arrived. Mr. Hind, the distinguished super- intendent of the " Nautical Almanac," had kindly sent us his computations, showing that, viewed from Dunsink, the transit ought to commence at 1 h. 35 min. 48 sees, mean time at Dublin, and that the point on the sun's disc where the planet would enter was 147° from the north point of the sun round by east. This timely intimation was of twofold advantage. It told us, in the first place, the precise moment when the event was to be expected ; and what was perhaps quite as useful, it told us the exact point of the sun to which the attention was to be directed. This is a matter of very considerable importance, for in a large telescope it is only possible to see a part of the sun at once, and therefore, unless the proper part of the sun be placed in the field of view, the phenomenon may be entirely missed. The eye-piece employed on the South equatorial must also receive a brief notice. It will, of course, be obvious that the full glare of the sun has to be greatly mitigated before the eye can view it with impunity. The light from the sun falls upon a piece of transparent glass inclined at a certain angle, and the chief portion of the sun's heat, as well as a certain amount of its light, pass through the glass and are lost. A certain fraction of the light is, however, reflected from the glass, and enters the eye-piece. This light is already much reduced in intensity, but it undergoes as much further reduction as we please by an ingenious contrivance. The glass which reflects the light does so at what 156 THE STORY OF THE HEAVENS. is called the polarising angle, and between the eye-piece and the eye is a plate of tourmaline. This plate of tourmaline can be turned round by the observer. In one position it hardly inter- feres with the light at all, while in the position at right jtngles thereto it cuts off nearly the whole of the light. By simply adjusting the position of the tourmaline, the observer has it in his power to render the image of any brightness that may be convenient, and thus the observations of the sun can be con- ducted with the appropriate degree of illumination. But such appliances seemed on this occasion to be a mere mockery. The tourmaline was all ready, but up to one o'clock not a trace of the sun could be seen. Shortly after one o'clock, however, we noticed that the day was getting lighter ; and, on looking to the north, from whence the wind and the snow were coming, we saw, to our inexpressible delight, that the clouds were clearing. At length, the sky towards the south began to improve, and at last, as the critical moment approached, we could detect the spot where the sun was becoming visible. But Mr. Hind's predicted moment arrived and passed, and still the sun had not broken through the clouds, though every moment the certainty that it would do so became more apparent. The ex- ternal contact was therefore missed. We tried to console our- selves by the reflection that this was not, after all, a very im- portant phase, and hoped that the internal contact would be more successful. At length the struggling beams pierced the obstruction, and I saw the round, sharp disc of the sun in the finder, and eagerly glanced at the point on which attention was concentrated. Some minutes had now elapsed since Mr. Hind's predicted moment of first contact, and, to my delight, I saw the small notch in the margin of the sun showing that the transit had commenced, and that the planet was then one-third on the sun. But the critical moment had not yet arrived. By the expres- sion " first internal contact " we are to understand the moment when the planet has completely entered on the sun. This first contact was timed to occur twenty-one minutes later than the external contact already referred to. But the clouds again dis- appointed our hope of seeing the internal contact. While VEXUS. 157 steadily looking at the exquisitely beautiful sight of the gradual advance of the planet, I became aware that there were other objects besides Venus between me and the sun. They were the snowflakes, which again began to fall rapidly. I must admit the phenomenon was singularly beautiful. The telescopic effect of a snowstorm with the sun as a background I had never before seen. It reminded me of the golden rain which is sometimes seen falling from a flight of sky-rockets during pyrotechnic dis- plays ; I would gladly have dispensed with the spectacle, for it necessarily followed that the sun and Venus again disappeared from view. The clouds gathered, the snowstorm descended as heavily as ever, and we hardly dared to hope that we should see anything more. 1 h. 57 min. came and passed, the first internal contact was over, and Venus had fully entered on the sun. We had only obtained a brief view, and we had not yet been able to make any measurements or other observations that could be of service. Still, to have seen even a part of a transit of Venus is an event to remember for a lifetime, and we felt more delight than can be easily expressed at even this slight gleam of success. But better things were in store. My assistant came over with the report that he had also been successful in seeing Venus in the same phase as I had. We both resumed our posts, and at half-past two the clouds began to disperse, and the prospect of seeing the sun began to improve. It was now no question of the observations of contact. Venus by this time was well on the sun, and we therefore prepared to make observations with the micrometer attached to the eye-piece. The clouds at length dispersed, and at this time Venus had so completely entered on the sun that the distance from the edge of the planet to the edge of the sun was about twice the diameter of the planet. We measured the distance of the inner edge of Venus from the nearest limb of the sun. These observations were repeated as frequently as possible, but it should be added that they were only made with very considerable difficulty. The sun was now very low, and the edges of the sun and of Venus were by no means of that steady character which is suitable for micro- metrical measurement. The margin of the luminary was quiver- ing, and Venus, though no doubt it was sometimes circular, was 158 THE STORY OF THE HEAVENS. very often distorted to such a degree as to make the measures very uncertain. We succeeded in obtaining sixteen measures altogether ; but the sun was now getting low, the clouds began again to inter- fere, and we saw that the pursuit of the transit must be left to the thousands of astronomers in happier climes who had been eagerly awaiting it. But before the phenomena had ceased I spared a few minutes from the somewhat mechanical work at the micrometer to take a view of the transit in the more pic- turesque form which the large field of the finder presented. The sun was already beginning to put on the ruddy hues of sunset, and there, far in on its face, was the sharp, round, black disc of Venus. It was then easy to sympathise with the supreme joy of Horrocks, when, in 1639, he for the first time witnessed this spectacle. The intrinsic interest of the phenomenon, its rarity, the fulfilment of the prediction, the noble problem which the transit of Venus enables us to solve, are all present to our thoughts when we look at this pleasing picture, a repetition of which will not occur again until the flowers are blooming in the June of a.d. 2004. The occasion of a transit of Venus also affords an opportunity of studying the physical nature of the planet, and we may here briefly indicate the results that have been obtained. In the first place, a transit will throw some light on the question as to whether Venus is accompanied by a satellite. If Venus were attended by a small body in close proximity, it would be con- ceivable that under ordinary circumstances the brilliancy of the planet would obliterate the feeble beam of rays from the minute companion, and thus the satellite would remain undiscovered. It was therefore a matter of great interest to scrutinise the vicinity of the planet while in the act of transit. If a satellite existed— and the existence of one or more of such bodies has often been suspected — then it would be capable of detection against the brilliant background of the sun. Special attention was directed to this point during the recent transits, but no satellite of Venus was to be found. It seems, therefore, to be very unlikely that Venus can be attended by any companion globe of appreciable dimensions. VENUS. 159 The observations directed to the investigation of the atmo- sphere surrounding Venus have been more successful. If the planet were devoid of an atmosphere, then it would be totally invisible just before commencing to enter on the sun, and would relapse into total invisibility as soon as it had left the sun. The observations made during the transits are not in conformity with such suppositions. Special attention has been directed to this point during the recent transits. The result has been very remarkable, and has proved in the most conclusive manner the existence of an atmosphere around Venus. As the planet gra- dually moved off the sun, the circular edge of the planet extend- ing out into the darkness was seen to be bounded by a circular arc of light, and Dr. Copeland, who observed this transit under exceptionally favourable circumstances, was actually able to follow the planet until it had passed entirely away from the sun, at which time the globe, though itself invisible, was distinctly marked by the girdle of light by which it was surrounded. This luminous circle is inexplicable save by the supposition that the globe of Venus is surrounded by an atmospheric shell in the same way as the earth. It may be asked, what is the advantage of devoting so much time and labour to a celestial phenomenon like the transit of Venus which has so little bearing on practical affairs ? What does it matter whether the sun be 95,000,000 miles off, or whether it be only 93,000,000, or any other distance ? We must admit at once that the inquiry has but a slender bearing on matters of practical utility. No doubt a fanciful person might contend, that to compute our nautical almanacs with perfect accuracy we require a precise knowledge of the distance of the sun. Our vast commerce depends on skilful navigation, and one factor necessary for success is the reliability of the "Nautical Almanac." The increased perfection of the almanac must therefore bear some relation to increased perfection in navigation. Now, as good authorities tell us that in running for a harbour on a tempestuous night, or in other critical emergencies, even a yard of sea-room is often of great consequence, so it may conceivably happen that to the infinitesimal influence of the transit of Venus on the " Nautical Almanac " is due the safety of a gallant vessel 160 THE STORY OF THE HEAVENS. But the time, the labour, and the money expended in observing the transit of Yenus are really to be defended on quite different grounds. We see in it a fruitful source of information. It tells us the distance of the sun, which is the foundation of all the great measurements of the universe. It gratifies the intellectual curiosity of man by a view of the true dimensions of the majestic solar system, in which the earth is seen to play a digni- fied, though still subordinate, part ; and it leads us to a con- ception of the stupendous scale on which the universe is con- structed. It is not possible for us, with a due regard to the limits of this volume, to protract any longer our discussion of the transit of Yenus. When we begin to study the details of the observations, we are immediately confronted with a multitude of technical and intricate matters. On the occasion of a transit, it has first to be decided where the observations are to be made — in itself a question that has led to no little discussion. Then the instru- ments that are to be used, and the description of observations to be made, have to be investigated with considerable care. The observers must be specially trained for the work, for even Methuselah himself could hardly have lived long enough to have had much practice in the observations of transits of Yenus. To compensate for the inevitable want of experience, the observers have to be prepared by a special course of instruction, in which a fictitious transit is observed. Then, too, the interpretation of the observations involves many thorny and many controverted questions. To pursue all these matters so as to render them in- telligible would lead us into great detail, and therefore we do not make the attempt. This course is the more advisable when it is remembered that the transit of Yenus is only one of the methods of finding the sun's distance— a celebrated one, no doubt, but not perhaps the most reliable. It seems not un- likely that the final determination of the sun's distance will be obtained in quite a different manner. This will be explained in Chapter XL, and hence we feel the less reluctance in passing away from the consideration of the transit of Yenus as a method of celestial surveying. We must now close our description of this lovely planet ; but VENUS. 161 before doing so, let us add — or in some cases repeat — a few statistical facts as to the size and the dimensions of the planet and its orbit. The diameter of Venus is about 7,660 miles, and the planet shows no measurable departure from the globular form, though we can hardly doubt that its polar diameter must really be somewhat shorter than the equatorial diameter. This diameter is only about 258 miles less than that of the earth. The mass of Venus is about three-quarters of the mass of the earth ; or if, as is more usual, we compare the mass of Venus with the sun, it is to be represented by the fraction 1 divided by 425,000. It is to be observed that the mass of Venus is not quite so great in comparison with its bulk as might have been expected. The density of this planet is about 0*850 of that of the earth. Venus would weigh 4 81 times as much as a globe of water of equal size. The gravitation at its surface will, to a slight extent, be less than the gravitation at the surface of the earth. A body here falls sixteen feet in a second ; a body let fall at the surface of Venus would fall about three feet less. It seems not unlikely that the time of rotation of Venus may be equal to the period of its revolution around the sun. The orbit of Venus is remarkable for the close approach which it makes to a circle. The greatest distance of this planet from the sun does not exceed the least distance by 1 per cent. Its mean distance from the sun is about 67,000,000 miles, and the movement in the orbit has a mean velocity of nearly 22 miles per second, the entire journey being accomplished in 224-70 days. 11 CHAPTER IX. THE EARTH. The Earth is a great Globe — How the Size of the Earth is Measured — The Base Line — The Latitude found by the Elevation of the Pole— A Degree of the Meridian — The Earth not a Sphere — The Pendulum Experiment— Is the Motion of the Earth slow or fast 1 — Coincidence of the Axis of Rotation and the Axis of Figure — The Existence of Heat in the Earth — The Earth once in a Soft Condition — Effects of Centrifugal Force — Comparison with the Sun and Jupiter — The Protuberance of the Equator — The Weighing of the Earth — Comparison between the weight of the Earth and an equal Globe of Water — Comparison of the Earth with a Leaden Globe — The Pendulum— Use of the Pendulum in Measuring the Intensity of Gravitation — The Principle of Isochronism— Shape of the Earth Measured by the Pendulum. That the earth must be a round body is a truth immediately suggested by simple astronomical considerations. The sun is round, the moon is round, and telescopes show that the planets are round. No doubt comets are not round, but then a comet seems to be in no sense a solid body. We can see right through one of these frail objects, and its weight is too small for our methods of measurement to appreciate. If, then, all the solid bodies we can see are round globes, is it not likely that the earth is a globe also ? But we have far more direct information than mere surmise. There is no better way of actually seeing that the surface of the ocean is curved than by watching a distant ship on the open sea. When the ship is a long way off and is still receding, its hull will gradually disappear, while the masts will remain visible. On a fine summer's day we can often see the top of the funnel of a steamer appearing above the sea, while the body of the steamer is below. If the sea were perfectly fiat, there is nothing to obscure the body of the vessel, and it there- fore would be visible as long as the funnel remains visible ; but if the sea be really curved, the protuberant part intercepts the view of the hull, while leaving the funnel visible. THE EARTH. 163 We thus learn how the sea is curved at every part, and there- fore it is natural to suppose that the earth is a sphere. When we make more careful measurements we find that the globe is not perfectly round. It is flattened to some extent at each of the poles. This may be easily illustrated by an indiarubber ball, which can be compressed on two opposite sides so as to bulge out at the centre. The earth is similarly flattened at the poles, and bulged out at the equator. The divergence of the earth from the truly globular form is, however, not very great, and would hardly be noticed without very careful measurements. The determination of the size of the earth involves operations of no little delicacy. Very much skill and very much labour have been devoted to the work, and the dimensions of the earth .are known with a high degree of accuracy, though perhaps not with all the precision that we may ultimately hope to attain. The scientific importance of an accurate measurement of the earth can hardly be over-estimated. The radius of the earth is itself the unit in which astronomical magnitudes are generally expressed. For example, when observations are made with the view of finding the distance of the moon, the observations, when discussed and reduced, tell us that the distance of the moon is equal to fifty-nine times the equatorial radius of the earth. If we want to find the distance of the moon in miles, we require to know the number of miles in the earth's radius. A level part of the earth's surface having been chosen, a line a few miles long is measured. This is called the base, and as all the subsequent measures depend ultimately on it there is necessity that this measurement be made with scrupulous ac- curacy. To measure a line four or five miles long with such precision as to exclude any errors greater than a few inches, demands the most minute precautions. We do not now enter upon a description of the operations that are necessary. It is a most laborious piece of work, and many ponderous volumes have been devoted to the discussion of the results. But when a few base lines have been obtained in different places on the earth's surface, the measuring rods are to be laid aside, and the subse- quent task of the survey of the earth is to be conducted by the measurement of angles and trigonometrical calculations based 164 THE STORY OF THE HEAVENS. thereon. Starting from a base line a few miles long, distances of greater length are measured, until at length stretches of 100 miles long, or even more, can be accomplished. It is thus possible to measure a long line running due north and south. So far the work has been merely that of the terrestrial sur- veyor. The distance thus ascertained is handed over to the astronomer to deduce from it the dimensions of the earth. The astronomer fixes his observatory at the northern end of the long line, and proceeds to determine his latitude by observation. There are various ways by which this can be accomplished. They will be found fully described in works on practical astro- nomy. We shall here only indicate in a very brief manner the principle on which such observations are made. Everyone ought to be familiar with the Pole Star, which, though by no means the most brilliant, is probably the most important star in the whole heavens. In these latitudes we are accustomed to find the Pole Star at a considerable elevation, and there we can invariably find it, always in the same place in the northern sky. But suppose we start on a voyage to the southern hemisphere : as we approach the equator we find, night after night, the Pole Star coming closer to the horizon, till at the equator it is on the horizon ; while if we cross the line, we find on entering the southern hemisphere that this useful celestial body has become invisible. On the other hand, a traveller leaving England for Norway observes that the Pole Star is every night higher in the heavens than he has been accustomed to see it. If he extend his journey farther north, the same object will gradually rise higher and higher, until at length, when approaching the pole of the earth, the Pole Star is high up over his head. We are thus led to perceive that the higher our latitude, the higher, in general, is the elevation of the Pole Star. But we cannot use precise language until we replace the twinkling point by the pole of the heavens itself. The pole of the heavens is near the Pole Star, which must itself revolve around the pole of the heavens, as all the other stars do, once every day. The circle described by the Pole Star is however so small that, unless we pay special atten- tion to it, the motion is not perceived. The true pole is not a THE EARTH. 165 visible point, but it is capable of being accurately denned, and it enables us to state with the utmost precision the relation between the pole and the latitude. The statement is, that the elevation of the pole above the horizon is equal to the latitude of the place. The astronomer stationed at one end of the long line measures the elevation of the pole above the horizon. This is an operation of some delicacy. In the first place, as the pole is invisible, h@ has to measure, instead of it, the height of the Pole Star when that height is greatest, and repeat the operation twelve hours later, when the height of the Pole Star is least ; the mean be- tween the two gives the height of the pole, but this has to be corrected in various ways which it is not necessary for us to dis- cuss here. Suffice it to say that by such operations the latitude of one end of the line is determined. The astronomer then, with all his equipment of instruments, moves to the other end of the line. He there repeats the process, and he finds that the pole has now a different elevation, corresponding to the different latitude. The difference of the two elevations thus gives him an accurate measure of the number of degrees and fractional parts of a degree between the latitudes of the two stations. This can be compared with the actual distance in miles between the two stations, which has been ascertained by the trigonometrical survey. A simple calculation will then show the number of miles and fractional parts of a mile corresponding to one degree of latitude — or, as it is more usually expressed, the length of a degree of the meridian. This operation has to be repeated in different parts of the earth — in the northern hemisphere and in the southern, in high latitudes and in low. If the sea-level over the entire earth were £i perfect sphere, an important consequence would follow — the length of a degree of the meridian would be the same everywhere. It would be the same in Peru as in Sweden, the same in India as in England. But the lengths of the degrees are not all the same, and hence we learn that our earth is not really a sphere. The measured lengths of the degrees enable us to see in what way the shape of the earth departs from a perfect sphere. Near the pole the length of a degree is longer than near the equator. This shows that the earth is flattened at the poles and 166 THE STORY OF THE HEAVENS. protuberant at the equator, and it provides the means by which we may calculate the actual lengths of the polar and the equa- torial axes. The polar axis of the earth may be defined to be the shortest diameter of the earth. This axis intersects the surface at the north and south poles. Around this axis the earth performs one rotation every sidereal day. The sidereal day is a little shorter than the ordinary day, being only 23 hours, 56 minutes, and 4 seconds. The rotation is performed just as if a rigid axis passed through the centre of the earth ; or, to use the old and homely illustration, the earth rotates just as a ball of worsted may be made to rotate around a knitting-needle thrust through its centre. The rotation of the earth upon its axis admits of being demon- strated in a very remarkable manner by actual experiment. If the north pole were an attainable locality, then the experi- ment would have a degree of simplicity that can only be partially realised at other stations. For the purpose of describing the principle of this experiment we may suppose that the observer is actually situated at the pole, and that a lofty dome has been erected over the point on the earth where the polar axis inter- sects the surface. From the summit of the dome a long wire descends nearly to the ground, and to this a ponderous weight is attached. The weight is to be drawn aside and then released. It will swing slowly from side to side, but the plane in which it moves will remain invariable. If the building were also at rest,, then the position of the plane of oscillation would remain in the same position relatively to the surrounding walls. But the building rotates with the earth, and will turn completely round . in the sidereal day. There will be a corresponding change in the place of the plane of oscillation relatively to the building, which will appear to rotate at such a rate that it would complete a revolution in the sidereal day. II the building be situated at a lower latitude the apparent change in the plane of oscillation of the pendulum is not so rapid, yet the movement of the plane is still sufficient to be perfectly observed, and the measured amount of displacement of the plane has been found to agree with the amount calculated on the supposition that the earth rotates. This is known as Foucault's celebrated pendulum experiment. THE EARTH. 167 Kegarding the earth as a spinning body, it might be thought that the rotation was a very slow movement, being only once in a day of twenty-four hours, all but four minutes. A wheel of ordinary dimensions would seem to be turning very slowly at this rate, at about half the speed of the hour-hand of a clock. When, however, the size of the earth is taken into consideration, our conception of the speed will be somewhat modified. The radius of the earth is so great that every point on the equator has to dash along at the rate of a thousand miles per hour in order to complete its circuit in the allotted time. It is a noteworthy circumstance that the axis about which the earth rotates occupies an identical position with the short- est diameter of the earth as found by actual surveying. This is a coincidence which would be utterly inconceivable if the shape of the earth was not in some way physically connected with the fact that the earth is rotating. What connection can then be traced ? Let us inquire into the subject, and we shall find that the shape of the earth is a consequence of its rotation. The earth at the present time is subject, at various localities, to occasional volcanic outbreaks. The phenomena of such erup- tions, the allied occurrence of earthquakes, the well-known fact that the heat increases the deeper we descend into the earth, the existence of hot springs, the gej^sers found in Iceland and else- where, all testify to the fact that heat exists in the interior of the earth. Whether that heat be, as some suppose, universal in the interior of the earth, or whether it be merely local at the several places where its manifestations are felt, is a matter which need not now concern us. All that is necessary for our present purpose is the admission that heat is present to some extent. This internal heat, be it much or little, has obviously a different origin from the heat which we know on the surface. The heat we enjoy is derived from the sun. The internal heat cannot have been derived from the sun ; its intensity is far too great, and there are other insuperable difficulties attending the supposition that it has come from the sun. Where then has this heat come from ? This is a question which at present we can hardly answer — nor, indeed, does it much concern our 168 THE STORY OF THE HEAVENS. argument that we should answer it. The fact that the heat is there being admitted, all that we require is to apply one or two of the well-known thermal laws to the interpretation of the facts. We have first to consider the general principle by which heat tends to diffuse itself and spread away from its original source. The heat, deep-seated in the interior of the earth, is transmitted through the superincumbent rocks, and slowly reaches the surface. It is true that the rocks and materials with which our earth is covered are not good conductors of heat ; most of them are, indeed, extremely inefficient in this way ; but good or bad, they are in some shape conductors, and through them the heat must creep to the surface. It cannot be urged against this conclusion that we do not feel this heat. A few feet of brickwork will so confine the heat of a mighty blast furnace that but little will escape through the bricks ; but some heat does escape, and the bricks have never been made, and, so far as we know, never could be made, which would absolutely intercept all the heat. If a few feet of brickwork can thus nearly mask the heat of a furnace, cannot some scores of miles of rock nearly mask the heat in the depths of the earth, even though that heat were seven times hotter than the mightiest furnace that ever existed ? The heat would escape slowly, and perhaps imper- ceptibly, but unless all our knowledge of nature is a delusion, no rocks, however thick, can prevent, in the course of time, the leakage of the heat to the surface. When this heat arrives at the surface of the earth it must, in virtue of another thermal law, gradually radiate away and be lost. It would lead us too far to discuss fully the objections which may perhaps be raised against what we have here stated. It is often said that the heat in the interior of the earth is being produced by chemical combination or by mechanical process, and thus that the heat may be constantly renewed as fast or even faster than it escapes. This, however, is more a difference in form than in substance. If heat be produced in the way just supposed (and there can be no doubt that there may be such an origin for some of the heat in the interior of the globe) there must be a certain expenditure of chemical or mechanical energies that produces a certain exhaustion. For THE EARTH. 169 every unit of heat which, escapes, there will either be a loss of an unit of heat from the globe, or, what comes nearly to the same thing, a loss of an unit of heat-making power from the chemical or the mechanical energies. The substantial result is the same, the heat of the earth must be decreasing. It should, of course, be observed that a great part of the thermal losses experienced by the earth is of an obvious character, and not dependent upon the slow processes of conduction. Each outburst of a volcano discharges a stupendous quantity of heat, which disappears very speedily from the earth ; while in many places, as in the hot springs, there is a perennial discharge of the same kind which in the course of years attains enormous proportions. The earth is thus losing heat, while it never acquires any fresh supplies of the same kind to replace the losses. The consequence is obvious ; the interior of the earth must be growing colder. No doubt this is an extremely slow process ; the life of an individual, the life of a nation, perhaps the life of the human race itself, has not been long enough to witness any pronounced change in the store of terrestrial heat. But the law is inevitable, and though the decline in heat may be slow, yet it is continuous, and in the lapse of ages must necessarily produce great and important results. It is not our present purpose to offer any forecast as to the consequences which must necessarily arise from this process. We wish at present rather to look back into past time and see to what results we are inevitably led ; we may here at once dismiss as inappreciable such intervals of time as we are familiar with in ordinary life or even in ordinary history. As our earth is daily losing internal heat, or the equivalent of heat, it must have contained more heat yesterday than it does to-day, more last year than this year, more twenty years ago than ten years ago. The effect has not been appreciable in historic time ; but Avhen we rise from hundreds of years to thousands of years, from thousands of years to hundreds of thousands of years, and from hundreds of thousands of years to millions of years, the effect is not only appreciable, but even of startling magnitude. There must have been a time when the earth contained much more heat than at present. There must have been a time when the 170 THE STORY OF THE HEAVENS. surface of the earth was sensibly hot from this source. We cannot pretend to say how many thousands or millions of years ago this epoch was ; but we may be sure that earlier still the earth was even hotter, until at length we seem to see the temperature increase to a red heat, from a red heat we look back to a still earlier age when the earth was white hot, back again till we find the surface of our now solid globe was actually molten. We need not push the retrospect any further at present, still less is it necessary for us to attempt to assign the probable origin of that heat. This, it will be observed, is not required in our argument. We find heat now, and we know that heat is being lost every day. From this the con- clusion that we have already drawn seems inevitable, and thus we are conducted back to some remote epoch in the abyss of time past when our solid earth was a globe molten and soft throughout. A dewdrop on the petal of a flower is nearly globular ; but it is not quite a globe, because the gravitation presses it against the flower and somewhat distorts the shape. A falling drop of rain is a globe ; a drop of oil suspended in a liquid with which it does not mix, forms a globe. Passing from small things to great things, let us endeavour to conceive a stupendous globe of molten matter. Let that globe be as large as the earth, and let its materials be so soft as to obey the forces of attraction exerted by each part of the globe on all the other parts. There can be no doubt as to the effect of these attractions ; they would tend to smooth down any irregularities on the surface just in the same way as the surface of the ocean is smooth when freed from the disturbing influences of the wind. We might therefore expect that our molten globe, isolated from all external interference, would assume the form of a sphere. But now suppose that this great sphere, which we have hitherto assumed to be at rest, is made to rotate round an axis passing through its centre. We need not suppose that this axis is a material object, nor are we concerned with any supposition as to how the velocity of rotation was caused. We can, however, easily see what the consequence of the rotation would be. The sphere would become deformed, the centrifugal THE EARTH. 171 force would make the molten body bulge out at the equator and flatten down at the poles. The greater the velocity of rotation the greater would be the bulging. To each velocity of rotation a certain degree of bulging would be appropriate. The molten earth thus bulged out to an extent which was de- pendent upon the fact that it turned round once a day. Now suppose that the earth, while still rotating, commences to pass from the liquid to the solid state. The form which the earth would assume on consolidation would, no doubt, be very irregular on the surface ; it would be irregular in consequence of the upheavals and the outbursts incident to the transformation of so mighty a mass of matter ; but irregular though it be, we can be sure that, on the whole, the form of the earth's surface would coincide with the shape which it had assumed by the movement of rotation. Hence we can explain the protuberant form of the equator of the earth, and we can appeal to that form in corro- boration of the view that this globe was once in a soft or molten condition. The argument may be supported and illustrated by com- paring the shape of our earth with the shapes of some of the other celestial bodies. The sun, for instance, seems to be almost a perfect globe. No measures that we can make show that the polar diameter of the sun is shorter than the equatorial diameter. But this is what we might have expected. No doubt the sun is rotating on its axis, and, as it is the rotation that causes the protuberance, why should not the rotation have deformed the sun like the earth ? The probability is that a difference really does exist between the two diameters of the sun, but that the difference is too small for us to measure. It is impossible not to connect this with the slowness of the sun's rotation, The sun takes twenty-five days to complete a rotation, and the protuber- ance appropriate to so low a velocity is not appreciable. On the other hand, when we look at one of the quickly- rotating planets, we obtain a very different result. Let us take the most striking instance, which is presented in the great planet Jupiter. Viewed in the telescope, Jupiter is at once seen not to be a globe. The difference is so conspicuous that accurate measures are not necessary to show that the polar diameter of 172 THE STORY OF THE HEAVENS. Jupiter is shorter than the equatorial diameter. The departure of Jupiter from the truly spherical shape is indeed much greater than the departure of the earth. It is impossible not to connect this with the much more rapid rotation of Jupiter. We shall presently have to devote a chapter to the consideration of this splendid orb. We may, however, so far anticipate what we shall then say as to state that the time of Jupiter's rotation is under ten hours, and this notwithstanding the fact that Jupiter is more than one thousand times greater than the earth. His enor- mously rapid rotation has caused him to bulge out at the equator to a remarkable extent. The survey of our earth, and the measurement of its dimen- sions having been accomplished, the next operation for the astronomer is the determination of its weight. Here, indeed, is a problem which taxes the resources of science to the very utter- most. Of the interior of the earth we know little — I might almost say we know nothing. No doubt we sink deep mines into the earth. These mines enable us to penetrate half a mile, or even a Avhole mile, into the depths of the interior. But this is, after all, only a most insignificant attempt to explore the interior of the earth. What is an advance of one mile in com- parison with the distance to the centre of the earth ? It is only about one four- thousandth part of the whole. Our knowledge of the earth merely reaches to an utterly insignificant depth below the surface, and we have not a conception of what may be the nature of our globe only a few miles below where we are standing. Seeing then, our almost complete ignorance of the solid contents of the earth, does it not seem a hopeless task to attempt to weigh the entire globe ? Yet that problem has been solved, and the result is known— not, indeed, with the accuracy attained in other astronomical researches, but still with tolerable approximation. It is needless to enunciate the weight of the earth in our ordinary units. The enumeration of billions of tons does not convey any distinct impression. It is a far more natural course to compare the mass of the earth with that of an equal globe of water. We should be prepared to find that our earth was heavier than a like volume of water. The rocks which form its surface TEE EARTH. 173 are heavier, bulk for bulk, than the oceans which repose on those rocks. The abundance of metals in the earth, the gradual in- crease in the density of the earth, which must arise from the enormous pressure at great depths — all these considerations will prepare us to learn that the earth is very much heavier than a globe of water of equal size. Newton supposed that the earth was between five and six times as heavy as an equal bulk of water. Nor is it hard to see that such a suggestion is plausible. The rocks and materials on the surface are usually about two or three times as heavy as water, but the density of the interior must be much greater. There is good reason to believe that down in the remote depths of the earth there is a very large proportion of iron. An iron earth would weigh about seven times as much as an equal globe of water. We are thus led to see that the earth's weight must be probably more than three, and probably less than seven, times an equal globe of water ; and hence, in fixing the density betAveen five and six, Newton adopted a result plausible at the moment, and since shown to be probably correct. Several methods have been proposed by which this important question can be solved with accuracy. Of all these methods we shall here only describe one, because it illustrates, in a very remarkable manner, the law of universal gravitation. In our chapter on gravitation it was pointed out that the intensity of this force between two masses of moderate dimensions was extremely minute, and the difficulty in weighing the earth arises from this cause. The practical application of the process is encumbered by multitudinous details, which it will be un- necessary for us to consider at present. The principle of the process is simple enough. To give definiteness to our description, let us conceive a large globe about two feet in diameter ; and as it is desirable for this globe to be as heavy as possible, let us suppose it to be made of lead. A small globe brought near the large one is attracted by the force of gravitation. The amount of this attraction is extremely small, but, nevertheless, it can be measured by a refined process which renders extremely small forces sensible. The intensity of the attraction depends both on the masses of the globes and on their distance apart, as well as 174 THE STORY OF THE HEAVENS on the force of gravitation. We can also readily measure the attraction of the earth upon the small globe. This is in fact nothing more nor less than the weight of the small globe in the ordinary acceptation of the word. We can thus compare the attraction exerted by the leaden globe with the attraction exerted by the earth. If the centre of the earth and the centre of the leaden globe were at the same distance from the attracted body, then the intensity of their attractions would give at once the ratio of their masses by simple proportion. In this case, how- ever, matters are not so simple ; the leaden ball is only distant by a few inches from the attracted ball, while the centre of the earth's attraction is nearly 4,000 miles away at the centre of the earth. Allowance has to be made for this difference, and the attraction of the leaden sphere has to be reduced to what it would be were it removed to a distance of 4,000 miles. This can fortunately be effected by a simple calculation depending upon the general law that the intensity of gravitation varies inversely as the square of the distance. We can thus, partly by calculation and partly by experiment, compare the intensity of the attrac- tion of the leaden sphere with the attraction of the earth. It is known that the attractions are proportional to the masses, so that the comparative masses of the earth and of the leaden sphere have been measured ; and it has been ascertained that the earth is about half as heavy as a globe of lead of equal size would be. We may thus state finally that the mass of the earth is a little more than five times as great as the mass of a globe of water equal to it in bulk. In the chapter on Gravitation we have mentioned the fact that a body let fall near the surface of the earth drops through sixteen feet in the first second. This distance varies slightly at different parts of the earth. If the earth were perfectly round, then the attraction would be the same at every part, and the body would fall through the same distance everywhere. The earth is not round, so the distance which the body falls in one second differs slightly at different places. At the pole the radius of the earth is shorter than at the equator, and accordingly the attraction of the earth at the pole is greater than at the equator. Had we accurate measurements of the distance a body would THE EARTH. 175 fall in one second at the pole and at the equator, we would have the means of ascertaining the shape of the earth. It is, however, difficult to measure correctly the distance .a body will fall in one second. We have, therefore, been obliged to resort to other means for determining the force of attraction of the earth at the equator and other accessible parts of its surface. The methods adopted are founded on the pendulum, which is at the same time the simplest and one of the most useful of philosophical instruments. The ideal pendulum is a small and heavy weight suspended from a fixed point by a fine and flexible wire. If we draw the pendulum aside from its vertical position and then release it, the weight will swing to and fro. For its journey to and fro the pendulum requires a certain amount of time, which does not appreciably depend on the length of the circular arc through which the pendulum Swings. To verify this law we suspend another pendulum beside the first, both being of the same length. If we draw both pendulums aside and then release them : they swing together and return together. This might have been expected. But if we draw one pendulum a great deal to one side, and the other only a little, the two pendulums still swing sympathetically. This, perhaps, would not have been expected. Try it again, with even a still greater difference in the arc of vibration, and still we see the two weights occupy the same time for the swing. We can vary the experi- ment in another way. Let us change the weights on the pendulums, so that they are of unequal size, though both of iron. Shall we find any difference in the periods of vibration ? We try again : the period is the same as before ; swing them through different arcs, large or small, the period is still the same. But it may be said that this is due to the fact that both weights are of the same material. Try it again, using a leaden weight instead of one of the iron weights ; the result is identical. Even with a ball of wood the period of oscillation is the same as that of the ball of iron, and this is true no matter what be the arc through which the vibration takes place. If, however, we change the length of the wire by which the weight is supported, then the period will not remain unchanged. This can be very easily illustrated. Take a short pendulum with 176 THE STORY OF THE HEAVENS. a wire only one-fourth of the length of that of the long one; suspend the two close together, and compare the periods of vibration of the short pendulum with that of the long one, and we find that the former has a period only half that of the latter. We may state the result generally, and say that the time of vibration of a pendulum is proportional to the square root of its length. If we quadruple the length of the suspending cord we double the time of its vibration ; if we increase the length of the pendulum ninefold, we increase its period of vibration threefold. It is the gravitation of the earth which makes the pendulum swing. The greater the attraction, the more rapidly will the pendulum oscillate. This may be easily accounted for. If the earth pulls the weight down very vigorously, the time will be short ; if the power of the earth's attraction be lessened, then it cannot pull the weight down so quickly, and the period will be lengthened. It is possible to determine the time of vibration of the pen- dulum with great accuracy. Let it swing for 10,000 oscillations, and measure the time that these oscillations have consumed. The arc through which the pendulum swings may not have remained quite constant, but this does not appreciably affect the time of its oscillation. Suppose that an error of a second is made in the determination of the time of 10,000 oscillations ; this will only entail an error of the ten-thousandth part of the second in the time of a single oscillation, and will afford a correspondingly accurate determination of the gravity. Take a pendulum to the equator. Let it perform 10,000 oscillations, and determine carefully the time that these oscilla- tions have required. Bring the same pendulum to another part of the earth, and repeat the experiment. We have thus a means of comparing the gravitation at the two places. There are no doubt a multitude of precautions to be observed which need not here concern us. It is not necessary to enter into details as to the manner in which the motion of the pendulum is to be sus- tained, nor as to the effect of changes of temperature in the alteration of its length. It will suffice for us to see how the time of the pendulum's swing can be measured accurately, and THE EARTH. 177 how from that measurement the intensity of gravitation can be calculated. The pendulum thus enables us to make a gravitational survey of the surface of the earth with the highest degree of accuracy. We cannot, however, infer that gravity alone affects the oscillations of the pendulum. We have seen how the earth rotates on its axis, and we have attributed the bulging of the earth at the equator to this influence. But the centrifugal force arising from the rotation has the effect of decreasing the ap- parent weight of bodies, and the change is greatest at the equator, and lessens gradually as we approach the poles. From this cause alone the attraction of the pendulum at the equator is less than elsewhere, and therefore the oscillations of the pendulum will take a longer time there than at other localities. A part of the apparent change in gravitation is accordingly due to the centrifugal force ; but there is, in addition, a real alteration. At a hasty glance it might be thought that, as there was a protuber- ance of matter at the equator, there ought to be a greater attrac- tion at the equator than elsewhere. This is not so. The effect of the additional matter is more than compensated by the greater distance of the pendulum from the centre of the earth. Indeed, a moment's reflection will show that the pendulum at the pole is, on the whole, nearer to the mass of the earth generally than when it is at the equator. It illustrates, in a marked way, how the researches in different branches of science are inter- woven when we find that, by observing the swing of a pendulum at different parts of the earth, we are enabled to determine the shape of our globe as accurately as by the elaborate measure- ments of the arcs of the meridian. In a work on astronomy it does not come within our scope to enter into further detail on the subject of our planet. The surface of the earth, its contour and its oceans, its mountain chains and its rivers, are for the physical geographer ; while its rocks and their contents, its volcanoes and its earthquakes, are to be studied by the geologists and the physicists. 12 CHAPTER X. MARS. Our nearer Neighbours in the Heavens— Surface of Mars can be Examined in the Telescope — Remarkable Orbit of Mars — Resemblance of Mars to a Star — Meaning- of Opposition — The Eccentricity of the Orbit of Mars — Different Oppositions of Mars — Apparent Movements of the Planet — Effect of the Earth's Movement — Measurement of the Distance of Mars — Theoretical In- vestigation of the Sun's Distance — Drawings of the Planet — Is there Snow on Mars ? — The Rotation of the Planet — Gravitation on Mars — Has Mars any Satellites? — Mr. Asaph Hall's great Discovery — The Revolutions of the Satellites — Deimos and Phobos — " Gulliver's Travels." The special relation in which we stand to one planet of our system has necessitated a somewhat different treatment of that globe from the treatment appropriate to the others. We dis- cussed Mercury and Venus as distant objects known chiefly by telescopic research, and by calculations of which astronomical observations were the foundation. Our knowledge of the earth is of a different character, and attained in a different Avay. Yet it was necessary for symmetry that we should discuss the earth after the planet Venus, in order to give to the earth its true position in the solar system. But now that the earth has been passed in our outward progress from the sun, we come to the planet Mars ; and here again we resume, though in a some- what modified form, the methods that were appropriate to Venus and to Mercury. Venus and Mars have, from one point of view, quite peculiar claims on our attention. They are our nearest planetary neigh- bours, on either side. We may naturally expect to learn more of them than of the other planets farther off In the case of Venus, however, this anticipation can hardly be realised, for, as we have already pointed out, its dazzling brilliancy prevents us from making a satisfactory telescopic examination. When we turn to our other planetary neighbour, Mars, we are enabled to learn a good deal with regard to his appearance. Indeed, with the MARS. 179 exception of the moon, we are better acquainted with the details of the surface of Mars than with those of any other celestial body. This beautiful planet offers many features for consideration besides those presented by its physical structure. The orbit of Mars is one of remarkable proportions, and it was by the observations of this orbit that the celebrated laws of Kepler were discovered. During the occasional approaches of Mars to the earth, it has been possible to measure its distance with accuracy, and thus another method of finding the sun's distance has arisen, which, to say the least, may compete in precision with that afforded by the transit of Venus. It must also be observed that the greatest achievement in pure telescopic re- search which this century has witnessed was that of the discovery of the satellites of Mars. To the eye this planet generally appears like a star of the first magnitude. It is usually to be distinguished by its ruddy colour, but the beginner in astronomy cannot rely on its colour only for the identification of Mars. There are several stars nearly, if not quite, as ruddy as this globe. The bright star Aldebaran, the brightest star in the constellation of the Bull, has often been mistaken for the planet. It often resembles Betel- geuze, a brilliant point in the constellation of Orion. Mistakes of this kind will be impossible if the learner has first studied the principal constellations and the more brilliant stars. He will then find great interest in tracing out the positions of the planets, and in watching their ceaseless movements. The position of each orb at any time can always be ascertained from the almanac. Sometimes the planet will be too near the sun to be visible. It will rise with the sun and set with the sun, and consequently will not be above the horizon during the night. The best time for seeing one of the planets situated like Mars will be during what is called its opposition. This state of things occurs when the earth intervenes directly between the planet and the sun. In this case, the distance from Mars to the earth is less than at any other time. There is also another advantage in viewing Mars during opposition. The planet is then at one side of the earth and the sun at the opposite side, so that when 180 THE STORY OF THE HEAVENS. Mars is high in the heavens the sun is directly beneatn the earth ; in other words, the planet is then at its greatest elevation above the horizon at midnight. Some oppositions of Mars are, however, much more favourable than others. This is distinctly Orb't of Wars • : ■ X. . ;, ; ' •- \ \: ■■'■■< ■"■ s ■ - ■ ' '■ ■ t : ■ : - - ■ -- . ■ : - •.,/ "; ■ / i \ ■ „ ^„ ■ „ / / ^ I / ' ' ■ > • • ' . : • -. ; v. ■ '': ■ * ■■' - • - ,. • -. \ • .. •: -. -. • . .. "• • \ ' • '-■ . ;'A*;' \v ■ \ ■ ■ ■' . ■ ; ' >U ■■■ ■ V .;. 'V v ■ • " '■' x > . • • • ; \ . ■ \ • .... v . ■ K s '' ' \ ' ■■ \ :> \ . '• V t ' ■ ,' " - \ ■ ■' ':. ' v.. ? \ < "a Sun \ \ V \ \ \ '. . . \ V \ V \, ■ : ■■ •« \ .: - V V . \ X \ / -%< / £5 Earth ^ . x ^ 0Mars v ^ • Mth.August