Ctemton |jr*gg Stunt* DESCRIPTIVE ASTRONOMY. CHAMBERS. Honfcon MACMILLAJST AND CO. PUBLISHERS TO THE UNIVERSITY OF LIB UNI \ .L1F Fiy, i. Plate I. COGGIA'S COMET, 1874: on July 13. (Drawn Inj Brodic.) HANDBOOK OF DESCRIPTIVE ASTRONOMY. BY GEORGE F. CHAMBERS, F.R.A.S. OP THE INlfEB TEMPLE, BARBISTEB- AT -LAW : Author of "A Digest of the Law relating to Public Health;" "A Popular Summary of Public Health Law;" "A Digest of the Law relating to Public Libraries and Museum*;" and other Works. 'The heavens declare the glory of God; and the firmament sheweth his handywork." Psalm xix. i. THIRD EDITION. UNIVERSITY OF N T IA.^ AT THE CLARENDON PRESS, M.DCCC.LXXVII. [All rights reserved.] PREFACE TO THE THIRD EDITION. A DVANTAGE has been taken of the call for a new edition -LjL of this work to subject the whole, from the first page to the last, to a searching revision. This has proved to be a task of unusual difficulty and labour, in consequence of the astonishing developement which has taken place in the science of Astronomy during the last ten years. I might, however, have had a better chance of grappling with this had I not laboured for a long time past under serious disadvantages arising out of the demands on my time made by professional work, which have of late been such as to render it very difficult for me to give to Astronomical Studies that close attention which is indispensable if the author of an Astronomical Book would keep his pages up to date and so do justice alike to himself and his readers. It is not open to doubt that this is a matter which sits very lightly upon the consciences of some writers of Text-books. I do not know that it is worth while to attempt to specify at any great length what has been done in preparing this Edition. It may be stated, however, that there is scarcely a single page which has not been, to a greater or less extent, dressed up, or in some way amended, with the object of making its statements more accurate in substance or intelligible in diction. The most important changes will be found in the chapters dealing with the Sun, Sidereal Astronomy, and Astro- nomical Instruments. The descriptions of Clusters and Nebulae have been made more numerous, and the lists of objects critically revised one by one actually at the telescope, so as to make that portion of the work more completely than formerly a vade mecum for the mere star-gazer who is an Astronomer simply in the respect that he is the owner of a telescope. Indeed, it has been chiefly with this idea in view that so much additional matter vi PREFACE TO THE THIRD EDITION. has been introduced into the chapters relating to Astronomical Instruments. The " Practical Hints " and suggestions have been gathered from so many sources, and embody the collective wisdom and experience of so many men, that they cannot fail to deserve attention. I believe also that this volume now stands alone in its full description, so far as regards the wants of amateur observers, of the mounting and use of Reflecting Telescopes. The present edition extends to 961 pages as against 856 in the case of the last edition, but these figures quite fail to convey an idea of the amount of the new matter now given, because large type has been in many places replaced by small type and blank backs of Plates occupied by type, so that the net increase in the contents of the volume is not less than 200 pages. I have to acknowledge a great amount of very useful advice and assistance from observers in all parts of the world, most of them total strangers to me, many of them being persons I had never heard of until the receipt of their letters. Indeed, the letters that I have received, especially from the United States of America, have been a very gratifying encouragement to me to persevere in improving this work in every possible way. At the risk of being invidious, I must tender my particular thanks to the Rev. E. Main, of the Radcliffe Observatory, Oxford ; Prof. A. 8. Hersckel, of Newcastle; Mr. W. T. Lynn, F.R.A.S., and Mr. R W. Maunder, F.R.A.S., both of the Royal Observatory, Greenwich; Capt. Nolle, F.R.A.S., Mr. G. Knott, F.R.A.S., and Mr. /. Browning, F.R.A.S., in the United Kingdom ; and abroad to MM. E. Schonfeld, of Bonn; R. Wolf, of Zurich; Mr. /. N. Lewis, of Mount Vernon, Ohio, U.S. ; and Mr. N. Pogson, Government Astronomer at Madras. Much material literary assistance of a non-astronomical character has been afforded me by Mr. //. /. Hood, of the Equity Bar, whilst for some of the illustrations I am indebted to the Secretaries of the Royal Astro- nomical Society, Mr. J. N. Locfyer, and Macmillan and Co. '. JF. <. EAST BOUENE, SUSSEX, December, 1876. PREFACE, A STRONOMY is not cultivated in this country, either as a -L\- study or as a recreation, to the extent that it is on the Continent of Europe and in America. I am not exactly prepared to say why this should be so, but perhaps a clue can be found. There is a lack of works in the English language which are at one and the same time attractive to the general reader, serviceable to the student, and handy, for purposes of reference, to the pro- fessional Astronomer ; , in fact, of works which are popular without being vapid, and scientific without being unduly technical. The foregoing observations will serve to indicate why this book has been written. Its aim, curtly expressed, is, general usefulness. Preferring facts to fancies, I have confined within very moderate limits Theoretical considerations, and especially have I avoided chronicling any of those mischievous speculations on matters belonging to the domain of Recondite Wisdom, which have within the last few years borne such pernicious yet natural fruits. Great pains have been taken to present the latest information on all branches of the Science. Astronomical progress is usually so rapid, that unless an author exercises constant vigilance, a book will soon fall behind the times. In regard to this matter of bring- ing up to date, it is believed that the present volume will compare favourably with any of its contemporaries. To Mr. /. Fletcher, M.P., Mr. W. De La Rue, Mr. /. Namyth, Mr. J. Murray, Dr. C. A. F. Peters, the Secretaries of the Royal Astronomical Society, and the Editors of the Popular Science Review and Astronomical Register, I am indebted for divers facilities in regard to the illustrations. viii PREFACE. The Woodcuts have mostly been executed under my own personal superintendence, but those of the Clusters and Nebulae are not all to my satisfaction. It has been very difficult to prevent the Engraver from unduly exaggerating the brilliancy of the stellar details. Perhaps this is a fitting place to draw attention to a matter of some importance in connection with Sir John Hersckel's very numerous drawings of Clusters and Nebulae. It does not appear to be generally known that they are all reversed right and left, and the consequence is., that unless allowance is made for the reversal, when comparisons are instituted, either with the objects themselves or with the figures of other observers (who, so far as I have noticed, invariably represent the objects as they are seen through an inverting telescope), the reader will find it impossible to reconcile the discrepancies which manifest themselves. Every care has been taken to secure accuracy in the printing, and I trust that the errors which may have escaped notice are neither numerous nor important. Finally, I hope that this book may be the means of inducing some, at least, to interest themselves in the study of that noble, but by no means abstruse, Science which in so conclusive a manner shews forth the wonderful Wisdom, Power, and Beneficence of the Great Creator and Omnipotent Ruler of the Universe. #* jr. or, March, 1867. CONTENTS. BOOK I. THE SUN AND PLANETS. CHAPTER I. THE SUN. Astronomical importance of the Sun. Solar parallax.-The means of determining it. By observations of Mars. By Transits of Venus. Numerical data. Light and Heat of the Sun. Gravity on the Sun. Spots. Description of their appear- ance. How distributed. Their duration. Effect of the varying position of the Earth with respect to the Sun. Their size. Instances of large Spots visible to the naked eye. The Great Spot of October 1865. Their periodicity. Dis- covered by Schwabe. Table of his results. Curious connexion between their periodicity and that of other physical phenomena. Singular occurrence in September 1859. Wolf's researches. Spots and Terrestrial Temperatures. Their Physical Nature. The Wilson-Herschel Theory. Historical Notices. Schemer. Faculae. Luculi. Nasmyth's observations on the character of the Sun's Surface. Huggins's ditto. Ballot's inquiry into Terrestrial Tempera- tures. .. .. .. .. .. v .. .. .. .. Pages 1-37 CHAPTER II. THE PLANETS. Epitome of the motions of the Planets. Characteristics common to them all. Kepler's laws. Elements of a Planet^ orbit. -Curious relation between the distances and the periods of the Planets. The Ellipse. Popular illustration of the extent of the Solar system. Bode's law. Miscellaneous characteristics of the Planets. Curious coincidences. Conjunctions of the Planets. Conjunctions recorded in History. Statistical Tables of the Major Planets. Different sys- tems. The Ptolemaic system. The Egyptian system. The Copernican system. The Tychonic system. 38-52 CHAPTER III. VULCAN. (?) Le Verrier's investigation of the orbit of Mercury. Narrative of the discovery of Vulcan. Le Verrier's interview with M. Lescarbault. Approximate elements of Vulcan. Concluding note. 5 3-58 x Contents. CHAPTER IV. MERCURY. $ Period, &c. Phases. Physical observations by Schrb'ter and Sir W. Herschel. Determination of its Mass. When best seen. Acquaintance of the Ancients with Mercury. Copernicus and Mercury. Tables of Mercury. . . 59~^3 \ CHAPTER V. VENUS. 2 Period, &c. Phases resemble those of Mercury. Most favourably placed for obser- vations once in eight years. Daylight observations. Its brilliancy. Its Spots and Axial Rotation. Suspected mountains and atmosphere. Its "ashy light." Phase irregularities. Suspected Satellite. Observations on it. The Mass of Venus. Ancient observations. Galileo's anagram. Venus useful for nautical observations. Tables of Venus. .. .. 64-71 CHAPTER VI. THE EARTH. Period, &c. Figure of the Earth. The Ecliptic. The Equinoxes. The Solstices. Diminution of the obliquity of the Ecliptic. The eccentricity of the Earth's orbit. Motion of the Line of Apsides. Familiar proofs and illustrations of the i sphericity of the Earth. Madler's tables of the duration of day and night on the Earth. Opinion of ancient philosophers. English mediaeval synonyms. The Zodiac. Mass of the Earth. .. .. 7 2 ~77 CHAPTER VII. THE MOON. <[ Period, &c. Its Phases. Its motions and their complexity. Libration. Evection. Variation. Parallactic inequality. Annual equation. Secular acceleration. Diversified character of the Moon's surface. Lunar mountains. Seas. Craters. Volcanic character of the Moon. Lunar atmosphere. Researches of Schrb'ter, &c. Hansen's curious speculation. The Earth-shine. The Harvest Moon. Astronomy to an observer on the Moon. Luminosity and calorific rays. His- torical notices as to the progress of Lunar Chartography. . . .. 7 8 -9 J CHAPTER VIII. THE ZODIACAL LIGHT. General description of it. When and where visible. Sir J. Herschel's theory. His- torical notices. Modern observations of it. .. .. .. .. 92-6 Contents. xi CHAPTER IX. MAES. $ Period, &c. Phases. Apparent motions. Its brilliancy. Telescopic appearance. Its ruddy hue. Polar snow. Axial rotation. The seasons of Mars. Its atmosphere. Has Mars a Satellite ? Ancient observation of Mars. Tables of Mars. .. .. .. 97- I0 3 CHAPTER X. THE MINOR PLANETS. Sometimes called Ultra-Zodiacal Planets. Summary of facts. Notes on Ceres. Pallas. Juno. Vesta. Olbers's theory. History of the search made for them. Independent discoveries. Progressive diminution in their size. .. 104-9 CHAPTER XI. JUPITER. 11 Period, &c. Jupiter subject to a slight phase. Its Belts. Their physical nature. First observed by Zucchi. Dark Spots. Luminous Spots. Alleged Connection between Spots on Jupiter and Spots on the Sun. Axial rotation of Jupiter. Centrifugal force at its Equator. Its Apparent Motions. Astrological influ- ences. Attended by 4 Satellites. Are they visible to the Naked Eye ? Table of them. Eclipses of the Satellites. Occultations. Transits. Peculiar aspects of the Satellites when in transit. Singular circumstance connected with the interior ones. Instances of all being invisible. Variations in their brilliancy. Observations of Eclipses for determining the longitude. Practical difficulties. Romer's discovery of the progressive transmission of light. Mass of Jupiter. Tables of Jupiter 110-30 CHAPTER XII. SATURN. TZ Period, &c. Figure and Colour of Saturn. Belts and Spots. Probable atmosphere. Observations of Galileo, and the perplexity they caused. Logogriph sent by him to Kepler. Huyghens's discovery of the Ring. His logogriph. The bisec- tion of the Ring discovered by the brothers Ball. Sir W. Herschel's Doubts. Historical epitome of the progress of discovery. The "Dusky" Ring. Facts relating to the Rings. Appearances presented by them under different circum- stances. Rotation of the Ring. Secchi's inquiries into this. The Ring not concentric with the Ball. Measurements by W. Struve. Other measurements. Miscellaneous particulars. Ring probably fluid. 0. Struve's surmise about its contraction. Irregularities in the appearances of the ansse. Rings not bounded by plane surfaces. Mountains suspected on them. An atmosphere suspected. Saturn attended by 8 Satellites. Table of them. Physical data relating to each. Elements by Jacob. Transits of Titan. Peculiarity relative to the illumination of lapetus. Mass of Saturn. Ancient observations. Saturnian astronomy. .. .. .. .. .. .. .. 131-56 xii Contents. CHAPTER XIII. UKANUS. # Circumstances connected with its discovery by Sir W. Herschel. Names proposed for it. Early observations. Period, &c. Physical appearance. Position of its axis. Attended by 4 Satellites. Table of them. Miscellaneous information concerning them. Mass of Uranus. .. .. .. .. .. 157-63 CHAPTER XIV. NEPTUNE. f Circumstances which led to its discovery. Summary of the investigations of Adams and Le Verrier. Telescopic labours of Challis and Galle. The perturbations ot Uranus by Neptune. Period, &c. Attended by I Satellite. Elements of its orbit. Mass of Neptune. Observations by Lalande in 1795. .. 164-70 BOOK II. ECLIPSES AND ASSOCIATED PHENOMENA. CHAPTER I. GENERAL OUTLINES. Definitions. Position of the Moon's orbit as regards the Earth's. Consequences re- sulting from their being inclined. Retrograde motion of the nodes of the Moon's orbit. Coincidence of 223 sy nodical periods with 19 synodical revolutions of the node. Known as the "Saros." Statement of Diogenes Laertius. Illustration of the use of the Saros. Number of Eclipses which can occur. Solar Eclipses more frequent than Lunar ones. Duration of Annular and Total Eclipses of the Sun .. .. 171-8 CHAPTER II. ECLIPSES OF THE SUN. Grandeur of a Total Eclipse of the Sun. How regarded in ancient times. Effects of the progress of Science. Chief phenomena seen in connexion with Total Eclipses. Change in the colour of the sky. The obscurity which prevails. Effect noticed by Piola. Physical explanation. Baily's Beads. Extract from Baily's original memoir. Probably due to irradiation. Supposed to have been first noticed by Halley in 1715. His description. The Corona. Hypothesis advanced to explain its origin. Probably caused by an atmosphere around the Sun. Remarks by Grant. First alluded to by Philostratus. Then by Plutarch. Corona visible during Annular Eclipses. The Red Flames. Re- marks by Dawes. Physical cause unknown. First mentioned by Stannyan. Note by Flamsteed. Observations of Vassenius. Aspect presented by the Moon. Remarks by A rago. J 79-9 Contents. xiii CHAPTER III. THE TOTAL ECLIPSE OF THE SUN OF JULY 28, 1851. Observations by Airy. By Hind. By Lassell. .. I9 X ~5 CHAPTER IV. THE ANNULAK ECLIPSE OF THE SUN OF MARCH 14-15, 1858. Summary of observations in England 196-9 CHAPTER V. THE TOTAL ECLIPSE OF THE SUN OF JULY 18, 1860. Extracts from the observations of the Astronomer Royal. Observations of the Red Flames by Bruhns. Meteorological observations by Lowe 200-5 CHAPTER VI. RECENT TOTAL ECLIPSES OF THE SUN. Eclipse of August 18, 1868. Observations by Col. Tennant and M. Janssen at Guntoor. Summary of t-esults. Observations of Governor J. P. Hennessy and Capt. Reed, R.N. Eclipse of August 7, 1869. Observations in America by Prof. Morton and others. Summary of results. Eclipse of December 22, 1870. English expedition in H.M.S. Urgent to Spain. Observations in Spain and Sicily. Summary of results. Rifts seen in the Corona. Sir J. Herschel's inter- pretation of the observations. Characteristics of the Corona. Eclipse of De- cember u, 1871. Observed in India. De La Rue's review of the progress of knowledge respecting Eclipse phenomena. Eclipse of April 16, 1874. Observa- tions by Stone and others in South Africa. Contraction of the Corona in the direction of the Sun's axis. Concluding summary as to the Physical Constitution of the Sun. Enumeration of its several envelopes. .. .. .. 206-18 CHAPTER VII. HISTORICAL NOTICES. Eclipses recorded in Ancient History. Eclipse of 584 B.C. Eclipse of 556 B.C. Eclipse of 479 B.C. Eclipse of 430 B.C. Eclipse of 309 B.C. Allusions in old English Chronicles to Eclipses of the Sun. . . .. . . .. 219-22 CHAPTER Vni. ECLIPSES OF THE MOON. Lunar Eclipses of less interest than Solar ones. Summary of facts connected with them. Peculiar circumstances noticed during the Eclipse of March 19, 1848. Observations of Forster. Wargentin's remarks on the Eclipse of May 1 8, 1761. Kepler's explanation of these peculiarities being due to Meteorological causes. Chaldsean observations of Eclipses. Other ancient Eclipses. Anecdote of Columbus. .. .. .. .. .. .. .. .. .. 223-7 xiv Contents. CHAPTER IX. A CATALOGUE OF ECLIPSES 228-33 CHAPTER X. TRANSITS OF THE INFERIOR PLANETS. Cause of the phenomena. Long intervals between each recurrence. Useful for the determination of the Sun's parallax. List of transits of Mercury. Of Venus. Transit of Mercury of Nov. 7, 1631. Predicted by Kepler. Observed by Gassendi. His remarks. Transit of Nov. 3, 1651. Observed by Shakerley. Transit of May 3, 1661. Transit of Nov. 7, 1677. Others observed since that date. Transit of Nov. 9, 1848. Observations of Dawes. Of Forster. Transit of Nov. u, 1861. Observations of Baxendell. Transit of Nov. 5, 1868. Transit of Venus of Nov. 24, 1639. Observed by Horrox and Crabtree. Transit of June 5, 1761. Transit of June 3, 1769. Where observed. Singular phenomenon seen on both occasions. Explanatory hypothesis. Other phenomena. Transit of Dec. 8, 1874 234-42 CHAPTER XI. OCCULTATIONS. How caused. Table annually given in the " Nautical Almanac." Occupation by a young Moon. Effect of the Horizontal Parallax. Projection of Stars on the Moon's disc. Occultation of Saturn, May 8, 1859. Occupation of Jupiter, January 2, 1857. Historical notices. .. .. .. .. .. 243-7 BOOK III. PHYSICAL AND MISCELLANEOUS ASTBONOMICAL PHENOMENA. CHAPTER I. THE TIDES. Introduction. Physical cause of the Tides. Attractive force exercised by the Moon. By the Sun. Spring Tides. Neap Tides. Summary of the principal facts. Priming and Lagging. . . . . . . . . . . . . . . 248-5 2 CHAPTER II. Local disturbing influences. Table of Tidal ranges. Influence of the Wind. Experiment of Smeaton. Tidal phenomena in the Pacific Ocean. Remarks by Beechey. Velocity of the great Terrestrial Tidal wave. Its course round the Earth, sketched by Johnston. Effects of Tides at Bristol. Instinct of animals. Tides extinguished in rivers. Historical notices. .. .. .. 253-8 Contents. xv CHAPTER III. PHYSICAL PHENOMENA. Secular Variation in the Obliquity of the Ecliptic. Precession. Its value. Its physical cause. Correction for Precession. History of its discovery. Nu- tation. Herschel's definition of it. Connexion between Precession and Nutation .... 259-64 CHAPTER IV. OPTICAL-ILLUSION PHENOMENA. Aberration. The constant of Aberration. Familiar illustration. History of the circumstances which led to its discovery by Bradley. Parallax. Explanation of its nature. Parallax of the heavenly bodies. Parallax of the Moon. Import- ance of a correct determination of the Parallax of an object. Leonard Digges on the distance of the Planets from the Earth 265-71 CHAPTER V. Refraction. Its nature. Importance of a correct knowledge of its amount. Table of the correction for refraction. Effect of refraction on the position of objects in the horizon. History of its discovery. Twilight. How caused. Its duration 272-7 BOOK IV. COMETS. CHAPTER I. GENERAL REMARKS. Comets always objects of popular interest and alarm. Usual phenomena attending the developement of a Comet. Telescopic Comets. Comets diminish in bril- liancy at each return. Period of Revolution. Density. Mass. Lexell's Comet. General influence of Planets on Comets. Comets move in I of 3 kinds of orbits. Elements of a Comet's orbit. For a parabolic orbit, 5 in number. Direction of motion. Eccentricity of an elliptic orbit. The various possible sections of a cone. Early speculations as to the paths in which Comets move. Comets visible in the daytime. Breaking up of a Comet into parts. Instance of Biela's Comet. Liais's observations of Comet iii. 1860. Comets probably self-luminous. Existence of phases doubtful. Comets with planetary discs. Phenomena connected with the tails of Comets. Usually in the direction of the radius vector. Vibration sometimes noticed in tails. Olbers's hypothesis. Transits of Comets across the Sun's disc. Variation in the appearance of Comets exemplified in the case of that of 1 769. Transits of Comets across the Sun 278-88 xvi Contents. CHAPTER II. PERIODIC COMETS. Periodic Comets conveniently divided into three classes. Comets in Class I. Encke's Comet. The resisting medium. Table of periods of revolution. Di Vico's Comet. Pons's Comet of 1819. Brorsen's Comet. Biela's Comet. D' Arrest's Comet. Faye's Comet. Mechain's Comet of 1 790. List of Comets presumed to be of short periods but only once observed. Comets in Class II. Westphal's Comet. Pons's Comet of 1812. Di Vico's Comet of 1846. Olbers's Comet of 1815. Brorsen's Comet of 1847. Halley's Comet. Of special interest. Resume of the early history of Halley's labours. Its return in 1759. Its return in 1835. Its history prior to 1531 traced by Hind. Comets in Class III. not requiring de- tailed notice. 289-307 CHAPTER III. REMARKABLE COMETS. The Great Comet of 1811. The Great Comet of 1843. The Great Comet of 1858. The Comet of 1860 (iii). The Great Comet of 1861. The Comet of 1862 (iii). The Comet of 1864 (ii). The Comet of 1874 (iii) 308-26 CHAPTER IV. COMETARY STATISTICS. Dimensions of the Nuclei of Comets. Of the Comae. Comets contract and expand on approaching to, and receding from, the Sun. Exemplified by Encke's in 1838. Lengths of the Tails of Comets. Dimensions of Cometary orbits. Periods of Comets. Number of Comets recorded. Duration of visibility of Comets. 327-30 CHAPTER V. HISTORICAL NOTICES. Opinions of the Ancients on the nature of Comets. Superstitious notions associated with them. Extracts from ancient Chronicles. Pope Calixtus III. and the Comet of 1456. Extracts from the writings of English authors of the i6th and 1 7th centuries, Napoleon and the Comet of 1 769. S.upposed allusions in the Bible to Comets. Conclusion. .. .. .. .. .. .. 33 J -4 CHAPTER VI. A CATALOGUE OF ALL THE COMETS WHOSE ORBITS HAVE HITHERTO BEEN COMPUTED. .. .... 335-71 Contents. xvii CHAPTER VII. A CATALOGUE OF COMETS RECORDED, BUT NOT WITH SUFFI- CIENT PRECISION TO ENABLE THEIR ORBITS TO BE CAL- CULATED. .. .. .. .. .. 372-430 BOOK V. CHRONOLOGICAL ASTRONOMY. CHAPTER I. What Time is. The Sidereal Day. Its length. Difference between the Sidereal Day and the Mean Solar Day. The Equation of Time. The anomalistic Year. Use of the Gnomon. Length of the Solar Year according to different ob- servers. The Julian Calendar. The Gregorian Calendar. Old Style versus New Style. Romish miracles. Table of differences of the Styles 43 J -43 CHAPTER II. Hours. Commencement of the days. Usage of different nations. Days. Weeks. Origin of the English names for the days of the week. The Egyptian 7-day period. The Roman week. Months. Memoranda on the months. Years. The Egyptian year. The Jewish year. The Greek year. The Roman year. The Roman Calendar and the reforms it underwent. The French revolutionary Ca- lendar. The year. Its subdivisions into quarters. Quarter-days. .. 444-56 CHAPTER HI. Means of measuring Time. The Almanac. Epitome of its contents. Times of Sun- rise and Sunset. Positions of the Sun, Moon, and Planets. The Phases of the Moon. The Ecclesiastical Calendar. The Festival of Easter. Method of cal- culating it. .. .. .. .. .. .. .. .. .. 457-65 CHAPTER IV. The Dominical or Sunday Letter. Method of finding it. Its use. The Lunar or Metonic Cycle. The Golden Number. The Epact. The Solar Cycle. The In- diction. The Dionysian period. The Julian period 466-71 b xviii Contents. BOOK VI. THE STAKKY HEAVENS. CHAPTER I. INTRODUCTION. The Pole-Star. Not always the same. Curious circumstance connected with the Pyramids of Egypt. Stars classified into different magnitudes. Antiquity of the custom of forming them into groups. Anomalies of the present system. Stellar photometry. Distances of the Stars. How distinguished. Antiquity of the custom of naming Stars. Invention of the Zodiac. Letters introduced by Bayer. Effects of the increased care bestowed on observations of the Stars. Ideas of the Ancients respecting the Stars. Remarks by Sir J. Herschel. 472-85 CHAPTER II DOUBLE STARS, ETC. But few known until Sir W. Herschel commenced his search for them. Labours of Sir J. Herschel and F. G. W. Struve. Examples. Optical Double Stars. Binary Stars. Discovered by Sir W. Herschel. Examples. List, of Optical Doubles. Coloured Stars. Examples. Generalisations from Struve's Cata- logue. Stars changing colour. Triple Stars. Quadruple Stars. Multiple Stars... .. .. .... 487-96 CHAPTER III. Variable Stars. o Ceti. Algol. 5 Cephei. /3 Lyrse. R. Coronse Borealis. rj Argus. Miscellaneous remarks. Temporary Stars. Notices of Stars which have disappeared. Proper motion. Motion of the System through space. Summary by W. Struve. Proper motion first suspected by Halley. Wright's hypothesis of a Central Sun. Revived by Madler. Stars which are probably Centres of Systems. .. .. ...... 497-508 CHAPTER IV. CLUSTERS AND NEBULAE. Arranged in three classes. Five kinds of Nebulae. The Pleiades. The Hyades. Mentioned by Homer. Praesepe. Opinion of Aratus and Theophrastus. Coma Berenices. List of Clusters. Annular Nebulae. Elliptic Nebulas. Spiral Nebulae. Planetary Nebulae. Nebulous stars. List of irregular Clusters. Notes to the objects in the list. The Nubeculse major and minor. List of Nebulae in Sir J. Herschel's Catalogue of 1864. Historical statement relating to the observation of Nebulae and Clusters. 509-42 Contents. xix CHAPTEK V. VARIABLE NEBULAE. Variable Nebula in Taurus. Observations by Hind. Variable Nebula in Scorpio. Observations by Pogson and others. Notes of observations on the other Nebulae suspected to be variable 543~7 CHAPTER VI. THE MILKY WAY. Its course amongst the stars described by Sir J. Herschel. The "Coal Sack" in the Southern Hemisphere. Remarks by Sir W. Herschel as to the prodigious num- ber of stars in the Milky Way. Computation by Sir J. Herschel of the total number of stars visible in an 1 8-inch reflector. Terms applied to the Milky Way by the Greeks. By the Romans. By our ancestors. . . . . . . 648-53 CHAPTER VII. THE CONSTELLATIONS. List of those formed by Ptolemy. Subsequent Additions. Remarks by Herschel, &c. Catalogue of the Constellations, with the position of, and Stars in, each. 554-62 CHAPTER VIII. A CATALOGUE OF CELESTIAL OBJECTS 563-77 CHAPTEK IX. A CATALOGUE OF VARIABLE STARS 578-86 CHAPTER X. A CATALOGUE OF RED STARS 587-94 CHAPTER XI. A CATALOGUE OF KNOWN AND SUSPECTED BINARY STARS. 595-601^ CHAPTER XII. A CATALOGUE OF NEW STARS 602-5 xx Contents. BOOK VII. PBACTICAL ASTKONOMY. CHAPTER I. THE TELESCOPE AND ITS ACCESSORIES. Two kinds of telescopes. Reflecting telescopes. The Gregorian reflector. The Cassegrainian reflector. The Newtonian reflector. The Herschelian reflector. Nasmyth's reflector. Browning's mountings for reflectors. Adjustment of re- flectors. Refracting telescopes. Spherical aberration. Chromatic aberration. Tests for both. Theory of Achromatic combinations. Tests of a good object- glass. The Galilean refractor. Eye-pieces. The positive eye-piece. The nega- tive eye-piece. Formulae for calculating the focal lengths of equivalent lenses. Kellner's eye-piece. Berthon's Dynamometer. Dawes's rotating eye-piece. The diagonal eye-piece. Dawes's solar eye-piece. Airy's eye-piece for atmo- spheric dispersion. Micrometers. The reticulated micrometer. The parallel- wire micrometer. The position-micrometer. Measurement of angles of position. Bidder's micrometer. Slipping-piece. Telescope tubes. . . . . 606-33 CHAPTER II. TELESCOPE STANDS. Importance of having a good Stand. " Pillar-and-Claw " Stand. The "Finder." Vertical and Horizontal Rack Motions. Steadying Rods. Cooke's Mounting. Varley's Stand. Proctor's Stand. Altazimuth stands for reflectors. Brett's Altazimuth mounting for reflectors. . . . . . . . . . . 634-646 CHAPTER III. THE EQUATORIAL. Brief epitome of the facts connected with the apparent rotation of the Celestial Sphere. Principle of the Equatorial Instrument. Two forms in general use. Description of Sisson's form, and of the different accessories to the instrument generally. Description of Fraunhofer's form of Equatorial. In what its supe- riority consists. The adjustments of the Equatorial six in number. Method of performing them. Method of observing with the instrument, reading the Circles, &c. Examples. The Star-Finder. Equatorial mountings for Reflectors. Uni- versal Equatorial. Berthon's Equatorial. .. .. .. .. 647-67 CHAPTER IV. THE TRANSIT INSTRUMENT. Its importance. Description of the Portable Transit. Adjustments of the Transit. Four in number. Method of performing them. Example of the manner of re- cording Transit observations of Stars. Of the Sun. Remarks on observations of the Moon. Of the larger Planets. Mode of completing imperfect sets of transit observations. The uses to which the Transit Instrument is applied. .. 668-81 Contents. xxi CHAPTER V. THE SEXTANT. Description of the instrument. The optical principle on which it depends. Its ad- justments. Corrections to be applied to observations made with it. Method of finding the Sun's zenith distance. The artificial horizon. To find the latitude. To determine the time 682-93 CHAPTER VI. MISCELLANEOUS ASTRONOMICAL INSTRUMENTS. The Altazimuth. Everest's Theodolite. The Mural Circle. The Repeating Circle. Troughton's Reflecting Circle. The Dip-Sector. The Zenith-Sector. The American Zenith-Sector. The Reflex Zenith -Tube. The Horizontal Floating Collimator. The Vertical Floating Collimator. The Heliqmeter. Airy's Orbit- Sweeper. The Comet Seeker. The Astronomical Spectroscope. .. 694-706 CHAPTER VII. CELESTIAL PHOTOGRAPHY. Summary of facts connected with the application of Photography to Astronomical purposes. Description of the apparatus used by Brothers of Manchester. His method of procedure. .. .. 707-22 CHAPTER VIII. PRACTICAL HINTS ON THE CONDUCT OF ASTRONOMICAL OBSER- VATIONS 723-55 CHAPTER IX. HISTORY OF THE TELESCOPE. Early history lost in obscurity. Vitello. Roger Bacon. Dr. Dee. Digges. Borelli's endeavour to find out who was the inventor. His verdict in favour of Jansen and Lippersheim of Middleburg. Statements by Boreel. Galileo's in- vention. Scheiner's use of two double-convex Lenses. Lenses of long focus used towards the close of the i yth century. Invention of Reflectors. Labours of Newton. Of Halley. Of Bradley and Molyneux. Of Mudge. Of Sir W. Herschel. Of the Earl of Rosse. Of Mr. Lassell. Improvements in Refracting Telescopes. Labours of Hall. Of Euler. Of the Dollonds. The largest Re- fractors yet made. .. .. .. ' .. .. .. .. 756-61 BOOK VIII. A SKETCH OF THE HISTOBY OF ASTKONOMY. .. 762-79 xxii Contents. BOOK IX. METEORIC ASTRONOMY. CHAPTER I. Classification of the subject. Aerolites. Summary of the researches of Berzelius, Rammelsberg, and others. Celebrated Aerolites. Summary of facts. Cata- logue of Meteoric Stones. Ara go's Table of Apparitions. The Aerolite of 1492. Of 1627. Of 1795. The Meteoric Shower of 1803 780-87 CHAPTER II. FIREBALLS. General description of them. Fireball seen in 1861. Arago's Table of Apparitions. Results of calculations concerning particular fireballs. .. .. 788-91 CHAPTER III. SHOOTING STARS. Shooting Stars. Have only recently attracted attention. To be seen in greater or less numbers almost every night. Tabular summary of the results of the obser- vations of Coulvier-Gravier and Saigey, and Schmidt. Early notices of Meteoric Showers. Shower of 1799. Showers of 1831, 2, and 3. The Meteors of 1833 divided into 3 groups. The Shower of 1866. Table of apparitions. Singular result. Olmsted's theory. Herschel's theory. Radiant points. . . 792-802 CHAPTER IV. THE THEORY, ETC. OF METEORS. Meteors probably planetary bodies. Their periodicity. Their orbits. The great November showers of Shooting Stars probably caused by a mass of Meteors which revolve round the Sun in about 33 years. The investigations of H. A. Newton. Of Adams. Supposed connection between Meteors and Comets. Recent progress of Meteoric Astronomy. .. .. .. .. 803-16 BOOK X. SPECTROSCOPIC ASTRONOMY. CHAPTER I. INTRODUCTORY. A Valuable Auxiliary to Astronomy. Explanation of a Spectrum. Historical ac- count of the investigations respecting the Solar Spectrum. Dark lines. Kirchhoffs discoveries. Varieties of Spectra. Variations in the width of lines. .. .... .. .. .. .. .. .. 817-23 Contents. xxiii CHAPTER II. SPECTROSCOPY AS APPLIED TO THE HEAVENLY BODIES. The Solar Spectrum described. Atmospheric lines. Janssen's experiments. Spec- trum observations of Sun's Spots. Of Faculse. Of Prominences. Labours of Lockyer and Janssen. Researches of Respighi and Secchi on Solar activity. The Solar Corona. The Zodiacal Light. Spectra of Planets. Of Comets. Of Stars. Secchi's Classification of the Spectra of Stars. Huggins's investiga- tion on the movements of Stars in the line of Sight. Spectra of Nebulae. Of Meteors. .. .. .. .. .. .. .. .. .. 824-46 CHAPTER III. MISCELLANEOUS. Determination of Wave-lengths. Angstrom's investigations. The visible limits of the Solar Spectrum, not its old limits. Researches of Waterhouse and Abney. 847-9 BOOK XL ASTKONOMICAL BIBLIOGBAPHY. CHAPTER I. LIST OF PUBLISHED STAR CATALOGUES AND CELESTIAL CHARTS .. 850-64 CHAPTER II. LIST OF BOOKS RELATING TO, OR BEARING ON, ASTRO- NOMY 865-73 BOOK XII. ASTKONOMICAL TABLES. .. 875-909 VOCABULAKY OF DEFINITIONS .. .. ..910 INDEX 923 LIST OF ILLUSTEATIONS. Fig- Page 1. Coggia's Comet, 1874: on July 13. (Brodu.) . Plate I. Frontispiece. 2. General Telescopic appearance of the Sun .... 7 3. Spot on the Sun, July 2, 1826. (Capocci.) . Plate II. 9 4. Spot on the Sun, September 29, 1826. (Capocci.) ,. 9 5. Spot on the Sun, May 23, 1 86 1. (Birt.) . . 9 6. Spot on the Sun, May 27, 1861. (Anon.) . . 9 7. Paths of Sun Spots at different times of the year . . .13 8. The Great Sun Spot of October 1 865, Oct. ii, n A.M. (Brodie.) Plate III. 16 9. The Great Sun Spot of October 1865, Oct. n, 1 2 -30 P.M. (Brodie.) 16 10. The Great Sun Spot of October 1865, Oct. 12, 9-30 A.M. (Brodie.) 16 n. The Great Sun Spot of October 1865, Oct. 12, 10-30 A.M. (Brodie.) 16 12. The Great Sun Spot of October 1865, Oct. 12, 12-30 P.M. (Brodie.) 16 13. The Great Sun Spot of October 1865, Oct. 12, 2-30 P.M. (Brodie.) 16 14. The Great Sun Spot of October 1865. Pectinated edge on Oct. 12. (Brodie.} . . . . . . . 17 15. Diagram illustrating the connection between Aurorse, Ter- restrial Magnetism, and Spots on the Sun . . Plate IV. 1 1 1 6. Change of form in Spots owing to the Sun's rotation . . .29 17. Ideal View of the Facular Structure of the Sun . . . .32 18. Spot on the Sun, July 29, 1860, shewing the " Willow- leaf" Structure. (Nasmyth.) ...... 33 19. Spot on the Sun, January 20, 1865. (Secchi.) . . . -35 20. Ideal View of the "Granular" Structure of the Sun. (Huggins.) . . . . . . .36 21. Phases of an Inferior Planet ...... 39 22. Diagram illustrating Kepler's Second Law . . . .41 23. The Ellipse ........ 44 24. Belative apparent size of the Sun, as viewed from the different Planets ....... 44 25. Comparative Sizes of the Planets ...... 45 26. unction of Venus and Jupiter, July 21, 1859 . . .48 27. Conjunction of Venus and Saturn, December 19, 1845 . . .49 28. The Ptolemaic System .... .50 29. The Egyptian System .... .51 30. The Copernican System .... .51 31. The Tychonic System . . 52 32. Venus near its Greatest Elongation. (Schroter.) . . . .64 List of Illustrations. xxv Fig. r^e 33. Venus near its Inferior Conjunction. (Schroter.) . . .65 34. View of a portion of the Moon's Surface. (Nasmyth.) . .82 35. The Lunar Mountain Archimedes . . . Plate V. 84 36. The Lunar Mountain Pico .... 84 37. The Lunar Mountain Copernicus. (Nasmyth.) ,, 84 38. Mars, 1858. (Secchi.) . . . Plate VI. faces 96 39. Mars, 1858. (Secchi.) . . 9 6 40. Mars, 1856. (Brodie.) . 99 41. Jupiter, 1857. (Dawes.) . . Plate VII. in 42. Jupiter, 1858. (LasselL) :'.' ., ' . . ,. m 43. Jupiter, 1860. (Jacob.) .... m 44. Jupiter, 1860. (Baxendell.) . . . . m 45. Jupiter, 1856. (DeLaRue.) . . . . . . 112 46. Jupiter, 1863. (Gorton.) .... .113 47. Jupiter, 1871. (Lassell.) . . . . . .114 48. Jupiter and its Satellites . . . . . .118 49. Jupiter and its Satellites, seen with the Naked Eye, 1863. (Mason.) ........ 118 50. Jupiter and its Satellites, seen with a Telescope, 1863. (Mason.) ...... .119 51. The IV th SateUite of Jupiter, 1873. (Roberts.). . . .123 52. The III rd Satellite of Jupiter, 1860. (Dawes.) . . . .123 53. The IV th Satellite of Jupiter, 1849. (Dawes.) . . . .123 54. Plan of the Jovian System . . . . . .125 55. Saturn, 1856. (De La Rue.) ...... 132 56. Saturn, 1853. (Dawes.) Plate VIII. 135 57. Saturn, 1848. (W. C. Bond.) .... 136 58. Saturn, 1856. (Jacob.) ..... 135 59. Phases of Saturn's Rings ....... 140 60. Saturn, 1861. (Anon.) ..... Plate IX. 141 61. Saturn, 1 86 1. (Wray.) ..... 141 62. Saturn, 1862. (Wray.) ..... 141 63. Saturn, 1 86 1. (De La Rue.) .... Plate X. 144 64. Saturn, 1861. (Jacob.) ..... 144 65. Saturn, 1861. (Jacob.) ..... 144 66. Diagram illustrating the phenomenon of Saturn's Ring "Beaded" .146 67. Diagram illustrating the phenomenon of Saturn's Ring "Beaded" ........ 147 68. General View of Saturn and its Satellites . . . .150 69. Plan of the Saturnian System .... Plate XI. 155 70. Plan of the Uranian System ...... 162 71. Diagram illustrating the Perturbation of Uranus by Neptune ........ 168 72. Plan of the Orbit of Neptune's Satellite ..... 169 73. Theory of a Total Eclipse of the Sun . . .172 74. Theory of an Annular Eclipse of the Sun . . . .172 75. Theory of an Eclipse of the Moon . . . . 173 xxviii List of Illustrations. Fig. 172. The Spiral Nebula 51 M Canum Venaticorum. (Smyth.} . . 521 173. The Nebula 65 M Leonis. (Sir J. Herschel.) . . Plate XXIV. 522 174. The Nebula 65 M Leonis. (Earl of Rosse.) . 522 175. The Nebula 4058 H Draconis. (Earl of Rosse.) . 522 176. The Nebula 3165 H Comse Berenices. (Sir J. Herschel) 522 177. The Nebula 3165 H Comse Berenices. (Earl of Rosse.) 522 178. The Spiral Nebula 51 M Canum Venaticorum. (Sir J. Herschel.) . . . . . . . , 523 179. The Spiral Nebula 51 M Canum Venaticorum. (Earl of Rosse.) ........ 524 1 80. The Spiral Nebula 57 # I Leonis. (Sir J. Herschel.) . Plate XXV. 525 181. The Spiral Nebula 57 ^ I Leonis. (Earl of Rosse.) . 525 182. The Spiral Nebula 99 M Virginis. (Earl of Rosse.) . 525 183. The Nebula 55 $ I Pegasi. (Sir J. Herschel) . 525 184. The Nebula 55 ^ I Pegasi. (Earl of Rosse.) . ., 525 185. The Planetary Nebula 97 M Ursse Majoris. (Sir J. Herschel) . . . . . ... . 526 186. The Planetary Nebula 97 M Ursse Majoris. (Earl of Rosse.) . . . . . , . 526 187. The Planetary Nebula 3614 H Virginis. (Sir J. Herschel.) 527 188. The Nebulous Star t Orionis. (Earl of Rosse) . . , 528 189. The Nebulous Star 45 Jg IV Geminorum . . ^ -.528 190. The Crab Nebula in Taurus. (Sir J. Herschel.) . ' . . 530 191. The Crab Nebula in Taurus. (Earl of Rosse) '.'.. . . 530 192. The Great Nebula in Orion. (Tempel.) . . . . 531 193. The Trapezium of Orion. (Huggins.) . . . . 532 194. The Nebula 30 Doradus. (Sir J. Herschel.) . . . -533 195. The Nebula surrounding 77 Argus. (Sir J. Herschel) . . -534 196. The Nebula surrounding K Crucis. (Sir J. Herschel.) . Plate XXVI. faces 535 197. The Nebula 1 7 M Clypei Sobieskii. (Chambers) . . . 536 198. The Dumb-bell Nebula in Vulpecula. (Sir J. Herschel.) . . 536 199. The Dumb-bell Nebula in Vulpecula. (Earl of Rosse) . . 537 200. The Dumb-bell Nebula in Vulpecula. (Earl of Rosse.) . . . 538 201. Diagram illustrating Herschel's Stratum theory of the Milky Way . . , * 552 202. The Gregorian Telescope . . . . . .607 203. The Cassegrainian Telescope . . " '* . . 607 204. The Newtonian Telescope . . . . . .607 205. TheEarlofEosse'83-ft. Eeflector . . Plate XXVII. 608 206. The Herschelian Telescope ...... 609 207. The Earl of Rosse's 6-ft. Eeflector . . Plate XXVIII. 610 208. Bed for Mirrors adopted by Browning . . . . .611 209. Browning's method of mounting the smaller mirror . . .612 210. Browning's method of mounting the smaller mirror . . .612 211. The Galilean Telescope . . . . . . .617 212. Opera-Glass . . . . . . . .617 213. The Positive Eye-piece . . . . . . .618 214. The Negative Eye-piece . . . . . .618 List of Illustrations. xxix Page Berthon's Dynamometer . . . . . .621 The Diagonal Eye-piece . . . . . .622 Airy's Prismatic Eye-piece . . . . . .624 Airy's Prismatic Eye-piece . . . . . .624 The Keticulated Micrometer . . . . .625 The Parallel- Wire Micrometer . . . . . .626 Diagram illustrating the Measurement of Angles of Position ........ 629 Bidder's Micrometer ....... 630 Barlow-Lens Double-Image Micrometer . . . .631 Telescope mounted on a Pillar -an d-Claw Stand . . . 634 Telescope mounted on a Pillar-and-Claw Stand, with Finder and Vertical Rack Motion . . . . -635 Telescope mounted on a Pillar-and-Claw Stand, with Finder, Vertical and Horizontal Rack Motions, and Steadying Rods . . . . . . 637 Telescope on Tripod Stand, with Vertical and Horizontal Rack Motions ....... 638 Telescope Stand with Motion in Altitude and Azimuth, devised by Proctor ....... 639 Altazimuth Mounting for Reflectors, devised by Brett . . . 640 Altazimuth Mounting for a small Reflector . . . .642 Adjustible Altazimuth or Equatorial Mounting for a medium-sized Reflector ...... 643 Altazimuth Mounting for a large Reflector .... 644 Altazimuth Mounting for a large Reflector, by Browning . . 645 The English Equatorial ....... 648 German Equatorial, by Home and Thornthwaite . . .651 The German Equatorial, as modified by Brodie . . . 65 2 Equatorially-mounted Refractor . . . Plate XXIX. 656 The Equatorial of the Uckfield Observatory .... 659 The Star-Finder ........ 660 Equatorially-mounted Reflector, by Browning . . Plate XXX. 662 Universal Equatorial for a Reflector ..... 663 Equatorially-mounted Reflector, by Browning . . Plate XXXI. 664 Berthon's Equatorial for Reflectors . . Plate XXXII. 665 Equatorially-mounted Reflector belonging to Lord Lindsay Plate XXXIII. 666 245. The Portable Transit Instrument ..... 669 246. The Portable Transit Instrument ..... 670 247. Arrangement of Wires in a Transit Instrument . . . 670 248. The Transit Instrument of the Uckfield Observatory . . . 671 249. The Sextant ........ 683 250. The Artificial Horizon . . . . . . . 688 251. The Portable Altazimuth ...... 695 252. Everest's Theodolite ....... 696 253. Zenith Telescope of the U. S. Coast Survey . . . .698 254. Airy's Orbit-Sweeper ....... 701 xxx List of Illustrations. Fig. Page 255. The Astronomical Spectroscope ... . 702 256. Browning's Star Spectroscope ...... 703 257. Browning's Automatic Spectroscope ..... 705 258-60. Apparatus for taking Astronomical Photographs. (3 views.) . ..... 717 261. Dawes's Observing Chair . . . . . '73 262. Observing Chair, devised by Knobel . . . . 731 263-4. Browning's Observing Box . . . . . 732 265. Sidereal Time Indicator ....... 734 266. Plan for curing Triangular Star Discs . . . . .744 267. Knobel's Astrometer ....... 748 268. Meteor of Nov. 12, 1 861. (Webb.) ..... 789 269. The Meteor radiant point in Leo ..... 805 270. Diagram illustrating the Theory that Meteors are small Planets revolving round the Sun . . . . .807 271. Kadiant Point of Geminids . . . . . .812 272. Radiant Point of Orionids . . . .. . . 813 273. The Solar Spectrum . . . . . . .819 274. Sun Spot and part of its Spectrum . . . . .826 275. Contortion of F line on disc of Sun ..... 827 276. Spectrum of the Sun near line C ..... 830 277. Spectrum of the Sun with Slit tangential .... 831 278. Slit placed radially to view a Prominence . . . .831 279- Slit placed tangentially to view a Prominence .... 831 280. Changes in Prominences noticed by Young .... 832 281. Bending of the line H/3 in the Spectrum of a Prominence . . 833 282. Spectrum of Uranus ....... 837 283. Spectrum of Winnecke's Cornet 1868 and Olefiant Gas . . . 838 284. Secchi's Star Types . . . Plate XXXIV. 841 285. Line F in the Spectrum of Sirius ..... 843 286. Spectrum of the Nebula 37 $ IV Draconis .... 845 ADDENDA ET CORRIGENDA. Page 12, line 9, for 24^ i4 m 59 s read 25* 5 h 37 m . 71, line 5 from bottom of text, for "discovered by Sir G. B. Airy in 1846" read "first surmised by Sir G. B. Airy about 1828, and fully expounded in 1831. See Phil Trans., 1828 and 1832." 87. For "Sept. 21 " read "about Sept. 23." 97. Observers interested in Mars should consult a most interesting and valuable memoir entitled Areographie presented to the Academic Koyale de Belgique in June, 1874, by M. F. Terby of Louvain. It reached me too late to be mentioned in the text. 98, line 9 from bottom, for " one conjunction and one opposition" read " two conjunctions or two oppositions." 105, line 4. The minor Planet Hilda is more distant than Freia. no, line 10 from bottom, after "planet" insert "except Saturn." 124, paragraph 4. A letter from Mr. Barneby, received too late to be made use of in preparing p. 124 for press, has cleared up the difficulties noted in this paragraph. In line 5 " third " is a mis- print by the B,. A. Society's printer for " first." 127, line 9 from bottom, for "synodical" read "sidereal." 129, footnote (t),for 185,500 read 186,660. 129, line i of note (t),for "reduction" read "augmentation." 148. By an oversight no mention is made of Prof. J. C. Maxwell's highly important essay on The Stability of the Motion of Saturn s Rings, published at Cambridge in 1859. See Month. Not., vol. xix. p. 297. 169. Prof. Newcomb has informed me by letter that Hind's elements of the satellite of Neptune, " at least 8 and i, are far wrong." 235, line 5,/or 8io5i; 8121^; 8105^; read 8,105^; 8,121*; 8,1054. 297. Some conclusions by Yon Asten respecting Encke's Comet should have been noted. He finds that inexplicable irregularities in the motion of the Comet were manifested in 1875, whilst in 1868 there were no such irregularities traceable. He deems the xxxii Addenda et Corrigenda. Page "Resisting Medium" theory only partially sufficient, and falls back upon an opinion of Bessel's that the remarkable disturb- ances to which this Comet is subject have their origin in the internal constitution of the Comet. (Ast. Nach, vol. Ixxxv. No. 2038. May 25, 1875.) 546, line 20. Wolf considers that the Merope nebula is variable with a short period. As regards the Stars in the Pleiades generally he thinks Merope and Atlas to be decidedly variable, and Maia perhaps so. (Month. Not., vol. xxxvi. p. 196. Feb. 1876.) 736, line 30. In the remarks on Astronomical Periodicals the following sentence was omitted : " The Germans have a somewhat similar periodical called Sirius; Zeitschrift fiir populdre Astronomie, edited by R. Falb and published by C. Scholtze, Leipzig, about yd. monthly, delivered in England." THE GREEK ALPHABET. * # * The small letters of this alphabet are so frequently employed in Astronomy that a tabular view of them, together with their pronuncia- tion, will be useful to many unacquainted with the Greek language. a Alpha. v Nu. ft Beta. Xi. y Gamma. o O-mlcron. 3 Delta. n Pi. e Epsllon. p Rho. C Zeta. f the perihelion. K OllOW- CHAP. II.] The Planets. 45 ing these steps, it is not a matter of much difficulty to form a general conception of a planetary orbit in space, for though the method is perhaps rather crude, it is so far strictly accurate. The following scheme will assist the reader to obtain a correct notion of the magnitude of the planetary system. Choose a level field or common ; on it place a globe 2 feet in diameter, for the Sun ; Vulcan (?) will then be represented by a small pin's head, at a distance of about 27 feet Fig. 25. from the centre of the ideal Sun; Mercury by a mustard seed, at a distance of 82 feet ; Venus by a pea, at a distance of 142 feet; the Earth also by a pea, at a distance of 215 feet; Mars by a small pepper-corn, at a distance of 327 feet; the minor planets by grains of sand, at distances varying from 500 to 600 feet : if space will permit, we may place a moderate - sized orange nearly \ mile distant from the starting-point to re- present Jupiter ; a small orange f of a mile for Saturn; a full-sized cherry | mile distant for Uranus; and lastly a plum I i miles off for Neptune, the most distant planet yet known. COMPARATIVE SIZES OF THE PLANETS. Extending this scheme, we should find that the aphelion distance of Encke's Comet would be at 880 feet ; the aphelion distance of Donati's Comet of 1858 at 6 miles ; and the nearest fixed star at 7500 miles. According to this scale the daily motion of Vulcan (?) in its orbit would be 4! feet ; of Mercury 3 feet ; of Venus 2 feet ; of the Earth i|- feet; of Mars ij feet; of Jupiter io inches ; of Saturn 46 The Sun and Planets. [BOOK I. 7i inches ; of Uranus 5 inches ; and of Neptune 4 inches. These figures illustrate also the fact that the orbital velocity of a planet decreases as its distance from the Sun increases. Connected with the distances of the planets, Bode of Berlin in 1778 published the following singular " law" of the numerical re- lations existing between them, which, although not discovered by him but by Titius of Wittemberg, usually bears his name. Take the numbers o 3 6 12 24 48 96 192 384; each of which (the second excepted) is double the preceding; adding to each of these numbers 4, we obtain 4 7 10 16 38 52 100 196 388 ; which numbers approximately represent the distances of the planets from the Sun expressed in radii of the Earth's orbit, as exhibited in the following table : Planets. True Distance from Distance by Bode's Law. 3.87 A-OO 7-23 7-oo Earth IO-OO IOOO I *\-23 16-00 27-66 28-00 Jupiter .. C2-O3 K2-OO QC.2Q IOO-OO IQI -8? 196-00 aoo-s? 388-00 Bode having examined these relations, and noticing the void between 16 and 52 (Ceres and the other minor planets not being then known), ventured to predict the discovery of new planets; and it may reasonably be believed that this conjecture guided or induced the investigations of subsequent observers ; though some have disputed this 1 . In the above table the greatest deviation > As far back as 450 B.C. Democritus of Abdera thought it probable that even- tually new planets would, perhaps, be discovered. (Seneca, Quasi. Nat., lib. vii. cap. 3 and 13.) Kepler was of opinion that some planets existed between the orbits of Mars and Jupiter, but too small to be visible to the naked eye. The same philosopher conjectured that there was another planet between Mercury and Venus. CHAP. II.] The Planets. 47 between the assumed and the true distance is in the case of Neptune ; it is possible, however, that when more complete ob- servations of this planet shall have been made, the above difference may be somewhat reduced. "We may sum up Bode's law as fol- lows : That the interval between the orbits of any two planets is about twice as great as the inferior interval, and only half the superior oneK Separating the major planets into two groups, if we take Mer- cury, Venus, the Earth, and Mars as belonging to the interior ; and Jupiter, Saturn, Uranus, and Neptune to the exterior group, we shall find that they differ in the following respects : 1. The interior planets, with the exception of the Earth, are not, as far as we know, attended by any satellites, while the exterior planets all have satellites. "We cannot but consider this as one of the many instances to be met with in the universe of the benefi- cence of the Creator that the satellites of these remote planets are designed to compensate for the small amount of light their pri- maries receive from the sun, owing to their great distance from that luminary. 2. The average density of the first group considerably exceeds that of the second, the approximate ratio being 5 i. 3. The mean duration of the axial rotations, or mean length of the day, of the interior planets, is much longer than that of the exterior ; the average in the former case being 23 h 58, but in the latter only IO 11 I2 m . In the Appendix will be found a full tabular summary of infor- mation concerning the Sun, Moon, and Major Planets, brought up to the latest possible date. The following singular coincidences deserve to be mentioned : 1. Multiply the Earth's diameter (7912 miles) by 108, and we get 854,496= +the Sun's diameter in miles. 2. Multiply the Sun's diameter (852,584 miles) by 108, and we get 92,079,072= + the mean distance of the Earth from the Sun. k Many attempts have been made by the series o, i, 2, 4, 8, 16, 32, and 64: ingenious dabblers in Astronomy to dis- add 4 to each, and the resulting figures cover other arithmetical coincidences represent with some approach to accuracy formed after the spirit of Bode's law. the relative distances from their primary The following is the only one I have met of the satellites of Saturn, with which deserves reproduction. Take 48 The Sun and Planets. [BOOK I. 3. Multiply the Moon's diameter (2160 miles) by 108, and we get 233 ,280=+ the mean distance of the Moon from the Earth. Fig. 26. A phenomenon of considerable interest, es- pecially on account of its rarity, is the con- junction, or proximity, of two or more planets within a limited area of the heavens. A no- ticeable instance is de- picted in fig. 26. It oc- curred on the morning of July 21, 1859, when Venus and Jupiter came very close to each other ; at 3 h 44 m A.M. the dis- VENUS AND JUPITER, July 21, 1859. tance between the two planets was only 13", and they accordingly appeared to the naked eye as one object. On Jan. 29, 1857, Jupiter, the Moon, and Venus were in a straight line with one another, though not within telescopic range. On Dec. 19, 1845, Venus and Saturn appeared in the same field of the telescope. [See fig. 27, next page.] On Oct. 3, 1 80 1, Venus, Jupiter, and the Moon were in close proximity in Leo, and Saturn was not far off. On Dec. 23, 1769, Venus, Jupiter, and Mars were very close to each other. On March 17, 1725, Venus, Jupiter, Mars, and Mercury appeared together in the same field of the telescope. On Nov. IT, 1544, Venus, Jupiter, Mercury, and Saturn were enclosed in a space of 10. On Nov. n, 1524, Venus, Jupiter, Mars, and Saturn were very close to each other, and Mercury was only 1 6 distant. In the years 1507, 1511, 1552, 1564, 1568, 1620, 1624, 1664, 1669, 1709, and 1765, the three most brilliant planets Venus, Mars, and Jupiter were very near each other. CHAP. II.] The Planets. 49 Fig. 27. On Sept. 15, 1186, Mercury, Venus, Mars, Jupiter, and Saturn were in conjunction between the Wheat-ear of Virgo, and Libra. The earliest record we possess of an occurrence of this kind is of Chinese origin. It is stated that a conjunc- tion of Mars, Jupiter, Saturn, and Mercury, in the constellation S/ii, was assumed as an epoch by the Emperor Chuen- hio, and it has been found by MM. Desvig- noles and Kirch that such a conjunction ac- tually did take place on Feb. 28, 2446 B.C., be- tween 10 and 1 8 of Pisces 1 . Another calcu- lator, De Mailla, fixes upon Feb. 9, 2441 B.C., VENUS AND SATURN, Dec. 19, 1845. as the date of the conjunction in question ; and he states that the four planets named above, and the Moon besides, were comprised within an arc of 12, extending from 15 to 27 of Pisces. It deserves mention that both the foregoing dates precede the Noa- chian deluge. It can therefore only be that the planetary con- junction in question was after-ascertained. De Mailla gives the following positions m : R.A. 56 2 18 39 45 16 12 21 47 1 1 Mercury .., ... ... ... 344 Jupiter ... 347 The Moon ... ... ... ... 353 Saturn ... ... ... ... 354 Mars ... ... ... ... 356 A few general remarks on the different theories of the solar system which have at various times been current will appropriately conclude this chapter. 1 Bailly, Axtron. Ancfenne, p. 345. p. 166, and Kirch's in vol. v. p. 193 of Desvignoles's original memoir appeai-s the same series, in Mem. de VAcad. de Berlin, vol. iii. m Hist. Gen. de la Chine, vol. i. p. 155. E 50 The Sun and Planets. [BOOK I. THE PTOLEMAIC SYSTEM. The Ptolemaic system claims the first place in consequence of its wide acceptance and the fame of the astronomer whose name it bears. It would, however, be more correct to say that Ptolemy 2 8. reduced it into shape rather than that he actually origi- nated it. The earth was re- garded as the centre, and around this the Moon ( j) ), Mercury ( $ ), Venus ( ? ), The Sun (0), Mars (<^), Jupiter ( 11 ), and Saturn ( T? ), all called planets, were assumed to revolve in the order in which I have given them. More accurate ideas were, however, current , even before Ptolemy's time, but they found few supporters. Aristarchus of Samos, who lived about 280 B.C., supposed, according to Archimedes and Plutarch, that the Earth revolved round the Sun, for which " heresy " he was accused of impiety. Cleanthus of Assos, who flourished but 20 years later, was, according to Plutarch, the first who sought to explain the great phenomena of the universe by supposing a motion of translation on the part of the Earth around the Sun, together with one of rotation on its own axis. The historian relates that this idea was so novel and so contrary to the received notions that it was proposed to arraign Cleanthus also for impiety. The Egyptian system differed from the Ptolemaic only in regard- ing Mercury and Venus as satellites of the Sun and not primary planets. A long period elapsed before any new theories of importance were started, but in the i6th century of the Christian era Copernicus came forward and propounded his theory, which ulti- mately superseded all others, and is the one now (in substance) adopted. It places the Sun in the centre of the universe as the point around which all the primary planets revolve. It must not be supposed, however, that the renowned Pole attained to our existing amount of knowledge on the subject. Far from it : his CHAP. II.] The Planets. 51 ideas were defective in more than one important particular. In order to account for the apparent irregularities in the motions of the planets, as seen from the Fig 29. Earth, he upheld theories which subsequent advances in the science shewed to be unnecessary and to rest on no substantial basis. Amongst other things he retained the theory of Epicycles. The ancients considered that the planetary motions must be effected uniformly and in circles, because uniform mo- tion appeared the most per- fect kind of motion, and a circle the most perfect and THE EGYPTIAN SYSTEM. most noble kind of curve. There is at any rate a reverential spirit in this idea which, notwithstanding our enlightenment, we need not despise. Copernicus Fig. 30. announced his system in a treatise entitled De Revolu- tionibus Orbium ccelestium, the actual publication of which he did not live to see ; for him this was perhaps fortu- nate rather than otherwise, because the work was con- demned by the Papal " Con- gregation of the Index." Had it been possible for those reverend gentlemen to have got the author within their clutches, it is more than THE COPERNICAN SYSTEM. likely that he would have suffered as well as his book; as did Galileo after him. Tycho Brahe was the last great astronomer who ventured on any original speculations in this field. Influenced either by bond fide scruples resulting from an erroneous interpretation of certain E 2 52 The Sun and Planets. [BOOK I. passages in Holy Scripture, or it may be, simply by a desire to Fig. 31. perpetuate his name, he chose to regard the Earth as immoveable, and occupying the centre of the system : the Moon as revolving immediately round the Earth : and, exterior to the Moon, the Sun doing the same thing the various planets revolving round the latter as solar satellites. Kepler and Newton finally set matters right by perfecting the Copernican system, and so nega- THE TIOHONIO SYSTEM, tivin g a11 the others - CHAP. III.] Vulcan. 53 CHAPTER III. VULCAN. (?) Le Verrier' s investigation of the orbit of Mercury. Narrative of the discovery of Vulcan. Le Verrier's interview with M. Lescarbault. Approximate elements of Vulcan. Concluding note. BEFOTIE entering upon the story of the supposed discovery of a new planet to which this name has been given, a brief prefatory statement seems necessary. M. Le Verrier having conducted an investigation into the theory of the orbit of Mercury, was led to the conclusion that a certain error in the assumed motion of the perihelion could only be ac- counted for by supposing the mass of Venus to be at least T V greater than was commonly imagined, or else that there existed some unknown planet or planets, situated between Mercury and the Sun, capable of producing a disturbing action. Le Verrier offered no opinion on these hypotheses, but contented himself with laying them before the scientific world in the autumn of 1 859 a. On these views being made public, a certain M. Lescarbault, a physician at Orgeres, in the Department of Eure-et- Loire, France, came forward and stated that on March 26 in that year (1859), he had observed the passage of an object across the Sun's disc which he thought might be a new planet, but which he did not like to announce as such until he had obtained a confirmatory observa- tion ; he related in writing the details of his observation, and Le Verrier determined to seek a personal interview with him. a Compt. Rend., vol. xlix. p. 379. 1859. 54 The Sun and Planets. [BOOK I. The following account of the meeting will be read with interest. " On calling at the residence of the modest and unobtrusive medical practitioner, he refused to say who he was, but in the most abrupt manner, and in the most authoritative tone, began, ' It is then you, Sir, who pretend to have observed the intra-Mercurial planet, and who have committed the grave offence of keeping your observation secret for nine months. I warn you that I have come here with the intention of doing justice to your pretensions, and of demonstrating either that you have been dishonest or deceived. Tell me then, unequivocally, what you have seen.' The doctor then explained what he had witnessed, and entered into all the particulars regarding his discovery. On speaking of the rough method adopted to ascertain the period of the first contact, the astronomer inquired what chronometer he had been guided by, and was naturally enough somewhat surprised when the physician pulled out a huge old watch with only minute hands. It had been his faithful companion in his professional journeys, he said ; but that would hardly be considered a satisfactory qualification for performing so delicate an experiment. The consequence was, that Le Verrier, evidently now beginning to conclude that the whole affair was an im- position or a delusion, exclaimed, with some warmth, ' What, with that old watch, shewing only minutes, dare you talk of estimating seconds? My suspicions are already too well founded.' To this Lescarbault replied, that he had a pendulum by which he counted seconds. This was produced, and found to consist of an ivory ball attached to a silken thread, which, being hung on a nail in the wall, is made to oscillate, and is shewn by the watch to beat very nearly seconds. Le Verrier is now puzzled to know how the number of seconds is ascertained, as there is nothing to mark them ; but Lescarbault states that with him there is no difficulty whatever in this, as he is accustomed 'to feel pulses and count their pulsations/ and can with ease carry out the same principle with the pendulum. The telescope is next inspected, and pronounced satisfactory. The astronomer then asks for the original memoran- dum, which, after some searching, is found ' covered with grease and laudanum.' There is a mistake of four minutes on it when compared with the doctor's letter, detecting which, the savant declares that the observation has been falsified. An error in the watch regulated by sidereal time accounts for this. Le Verrier now wishes to know how the doctor managed to regulate his watch by sidereal time, and is shewn the small telescope by which it is accomplished. Other questions are asked, to be satisfactorily answered. The doctor's rough drafts of attempts to ascer- tain the distance of the planet from the Sun ' from the period of four hours which it required to describe an entire diameter' of that luminary are produced, chalked on a board. Lescarbault's method, he beinjf short of paper, was to make his calculations on a plank, and make way for fresh ones by planing them off. Not being a mathematician, it may be remarked he had not succeeded in ascertaining the distance of the planet from the Sun. " The end of it all was, that Le Verrier became perfectly satisfied that an intra- Mercurial planet had been really observed. He congratulated the medical practi- tioner upon his discovery, and left with the intention of making the facts thus obtained the subject of fresh calculations b ." b Epitomised from the North British Cosmos, vol. xvi. pp. 22-8, 1860; see Review, vol. xxxiii. pp. 1-20, August 1860. also Cosmos, same vol., pp. 50-6. A full account will also be found in CHAP. III.] Vulcan. 55 In March or April, 1860, it was anticipated that the planet would again pass across the Sun, which was carefully scrutinised by different observers on several successive days, but no trace of "it was obtained then, and in a certain sense Lescarbault's obser- vation continues unconfirmed. However, this proves nothing, and many are prepared to regard the existence of this planet as a fact, to be fully demonstrated on some future occasion. The following approximate elements were calculated by Le Ver- rier from Lescarbault's rough observations : Longitude of ascending node . . . . . . = 1 2 59' Inclination of orbit .. .. .. .. = 12 lo' Semi-axis major (0 = i) .. .. .. = -i43 Daily heliocentric motion .. .. .. = 18 16' Period = 19* i; 11 Mean distance .. = 13,082,000 miles. Apparent diameter of Q from Vulcan . . = 3 36' Do. do. do. ( = i) = 6-79 Greatest possible elongation . . . . . . =8 The application of Kepler's third law yields, as has already been shewn, a result sufficiently consistent with the results in the cases of the other planets to demand attention ; but, as will now be seen, additional evidence can be adduced as to the reality of the discovery, much as it has been called in question. On March 20, 1 862, Mr. Lummis, of Manchester, was examining the Sun's disc, between the hours of 8 and 9 A.M., when he was struck by the appearance of a spot possessed of a rapid proper motion. He called a friend's attention to it, and both remarked its sharp circular form. Official duties most unfortunately inter- rupted him, after following it for 2O m ; but he has not the slightest doubt about the matter. The apparent diameter was estimated to be about f, and in the 2o m it moved over about 12' of arc. The telescope employed was 2 inches aperture, and was charged with a power of 80. Mr. Lummis communicated with Mr. Hind on the subject of what he had seen ; and the latter, by the aid of the diagram sent, determined that 12' was too great an estimate of the arc traversed by the spot in the time, and that 6' would be a nearer value . Two French calculators deduced elements from Lummis's obser- c Month. Not., vol. xxii. p. 232. April 1862. 56 The Sun and Planets. [BOOK 1. vations : the orbits which they obtained, though necessarily very imperfect, are fairly in accord both with each other, and with Le Verrier's earlier orbit. The first result is adopted from Valz's elements, the second from Radau's. I. II. Longitude of ascending node .. = 2 52' .. Inclination of orbit .. .. .. = 10 21' .. Semi-axis major (0 = i -o) .. .. = 0-132 .. 0-144 Daily heliocentric motion .. .. = 20 32' .. 18 5' Period .. .. .. .. = I7 d I3 h .. 19* 22 h Mean distance in miles .. .. = 12,076,000 .. 13,174,000 From the heliocentric position of the nodes, it appears that transits can only occur between March 25 and April 10 at the descending, and between September 27 and October 14 at the ascending node. Instances are not wanting of observations of spots of a planetary character passing across the Sun which may turn out to have been transits of Vulcan d . On October 10, 1802, Fritsch saw a round spot pass over the Sun. In 3 m it had moved 2', and after a cloudy interval of 4 n had disappeared. On October 9, 1819, Stark saw a well-defined and truly circular spot, about the size of Mercury, which he could not find again in the evening. On October u, 1847, Schmidt saw a small black point rapidly pass across the Sun. On October 14, 1849, the same observer saw a dark body, about 15" in size, pass very rapidly from East to West before the Sun. " It was neither a bird nor an insect." In the works whence these instances are cited, others are given ; but, though suspiciously suggestive of planets, the dates do not come within the necessary limits for them to have been apparitions of Vulcan, so it is not worth while to transcribe them ; but never- theless they are interesting, and worthy of attention. It is right here to state that M. Liais asserts that being in Brazil he was watching the Sun during the period in which Lescarbault professes to have seen the black spot, and that he is d Month. Not., vol. xx. p. 100. Jan. 1860; also pp. 192-4; Webb, Celest. Objects, p. 40. CHAP. III.] Vulcan. 57 positively certain that nothing of the kind was visible, though the telescope he employed was considerably more powerful than that of the French physician. He adds that parallax will not explain the discrepancy 6 . Though it is the fashion to repudiate the reality of Vulcan's existence, yet it is scarcely prudent to dogmatise on the subject as some have done, considering that an astronomer of Hind's experi- ence leans to the affirmative side. He says : " It is a suspicious circumstance that the elements as regards the place of the node, or point of intersection of the orbit with the ecliptic, and its inclination thereto, as worked out by M. Valz of Marseilles, from the data I deduced from a diagram forwarded to me by Mr. Lummis, are strikingly similar to those founded by M. Le Verrier upon the observations, such as they were, of Dr. Lescarbault. It is true if the place of the node and inclination were precisely as given by this astronomer, the object which was seen upon the Sun's disc on the a6th of March could not have been projected upon it as early as the 2oth of March. But, considering the exceedingly rough nature of the observations upon which he had to rely, perhaps no stress need be placed iipon the circumstance. Now the period of revolution assigned by M. Le Verrier from the observations of 1859 was 19-70 days. Taking this as an approxi- mate value of the true period, I find, if we suppose 57 revolutions to have been performed between the observations of Dr. Lescarbault and Mr. Lummis, there would result a period of 19-81 days. On comparing this value with the previous observa- tions in March and in October, when the same object might have transited the Sun at the opposite node, it is found to lead to October 9, 1819, as one of the dates when the hypothetical planet should have been in conjunction with the Sun. And on this very day Canon Stark has recorded the following notable observation, 'At this time there appeared a black, well-defined nuclear spot, quite circular in form, and as large as Mercury. This spot was no more to be seen at 4-37 P.M., and I found no trace of it later on the 9th, nor on the I2th, when the sun came out again.' The exact time of this observation is not mentioned, but appears likely to have been about noon, one of Stark' s usual hoars for examining the solar disc. Hence I deduce a corrected period of 19-81 2 days." In the communication from which this is taken f Hind throws out suggestions for a scrutiny of the Sun at certain dates. It must be admitted that the scrutiny took place and that no planet was found, and here the matter rests. Notwithstanding, however, the strong negative evidence against the existence of Lescarbault 's planet Vulcan, Le Verrier has quite recently (December 1874) announced that the orbit of Mercury is perturbed to an extent rendering it necessary to augment the movement of the perihelion by 31" in a century. " The con- e Ast. Nach,, vol. liv. No 1281. Nov. i, 1860. f Letter in the Times, Oct. 19, 1872. 58 The Sun and Planets. [Boox I. sequence " (he says) " is very clear. There is, without doubt,, in the neighbourhood of Mercury, and between that planet and the Sun, matter hitherto unknown. Does it consist of one, or several small planets ? or of asteroids, or even of cosmic dust ? Theory cannot decide this point s." s Compt. Rend., vol. Ixxix. p. 1424. 1874. CHAP. IV.] Mercury. 59 CHAPTER IV. MERCURY. Period, &c. Phases* Physical Observations by Schroter and Sir W. Herschel. De- termination of its Mass. When beet seen. Acquaintance of the Ancients with Mercury. Copernicus and Mercury. Tables of Mercury. MERCURY is, of the old planets a , the one nearest to the Sun, round which it revolves in 87 d 23 h I5 m 43*9 I s , at a mean distance of 35,392,000 miles. The eccentricity of the orbit of Mercury amounting" to 0*205, the distance may either extend to 42,665,000 miles, or fall as low as 28,1 19,000 miles. The apparent diameter of Mercury varies between 4'5 // in superior conjunction, and 1 2*9" in inferior conjunction : at its greatest elongation it amounts to about 7". The real diameter is about 3050 miles. The compression, or the difference between the polar and equatorial diameter, has usually been considered to be too small to be measure- able, but Dawes, in 1848, gave it at ^. Mercury exhibits phases resembling those of the Moon. At its greatest elongation (say W.) half its disc is illuminated, but as it approaches superior conjunction the breadth of the illuminated part increases, and its form becomes gibbous ; and ultimately when in superior conjunction circular : at this point the planet is lost in the Sun's rays, and is invisible. On emerging therefrom the gibbous form is still apparent, but the gibbosity is on the a In case it should be thought that undesirable to encumber these pages with these accounts of the planets are too de- too many figures. The statistics alluded ficient in statistical data, it may here be to are, in this edition, placed in the Ap- remarked that they are intended to be pendix, to allow of the incorporation read in connexion with the tabulated of the results of the Transit of Venus statistics in the Appendix of this volume, to the latest attainable degree of com- as it has been thought for several reasons pleteness. 60 The Sun and Planets. [BOOK I. opposite side, and diminishes day by day till the planet arrives at its greatest elongation East, when it again appears like a half-moon. Becoming more and more crescented, it approaches the inferior conjunction ; and having passed this,, the crescent (now on the opposite side) gradually augments until the planet again reaches its greatest Westerly elongation. Owing to its proximity to the Sun, observations on the physical appearance of Mercury are obtained with difficulty, and are there- fore open to much uncertainty. The greatest possible elongation of the planet not exceeding 2745' (and it being in general less), it can never be seen free from strong sunlight b , under which con- ditions it may occasionally be detected with the naked eye during ij h or so after sunset in the spring (E. elongation) and before sun- rise in the autumn (W. elongation), shining with a pale rosy hue. With the aid of a good telescope equatorially mounted, Mercury can frequently be found in the daytime. Mercury does not appear to have received much attention from astronomers of the present day, and the observations of Schroter, at Lilienthal, and of Sir W. Herschel, are the main sources of in- formation. The former observer and his assistant Harding obtained what they believed to be decisive evidence of the existence of high mountains on the planet's surface : one in particular, situated in the southern hemisphere, was supposed to manifest its presence from time to time, in consequence of the southern horn, near inferior conjunction, having a truncated appearance, which it was inferred might be due to a mountain arresting the light of the Sun, and preventing it from reaching as far as the cusp theoretically extended c . The extent of this truncature would serve to determine the height of the mountain occasioning it, which has been set down at 1 0-7 miles, an elevation far exceeding, absolutely, anything we have on the Earth, and in a still more marked degree relatively, when the respective diameters of the two planets are taken into consideration. Schroter, pursuing this inquiry, announced that b When Mercury's elongation is the greatest possible elongation is a W. one greatest possible, the planet's position is which happens at the beginning of April, (in England) South of the Sun, and there- The least (17 50') an elongation (also fore the chances of seeing it are not so W.) which happens at the end of Sep- good as when an elongation coincides tember. with a more Northerly position, albeit the c This has also been seen by Noble elongation is less considerable. The (Ast. Register, vol. ii. p. 106. May 1864). CHAP. IV.] Mercury. 61 the planet rotated on its axis in 24 h 5 m 489. Sir W. Herschel was unable to confirm these results either in whole or even in part. The alleged period of rotation, especially, cannot be relied on. The phases of Mercury are noticeable, as it has sometimes been found that the breadth of the illuminated portion is less than ac- cording to calculation it should be. This does not rest on the testimony of Schroter alone, but is supported by Beer and Madler, from an observation made on September 29, 1833. Mercury is not known to be possessed of an atmosphere ; at least, if one exists, it is too insignificant to be detected. Sir W. Herschel, contradicting Schroter and Harding, pronounced against its existence. [But see Book II, chap, on " Transits," post.] Mercury is, as far as we know, attended by no satellite, and the determination of its mass is a difficult and uncertain problem. However, the small comet of Encke has furnished the means of learning something, and from considerations based on the dis- turbances effected in the motion of this comet by the action of Mercury, it has been calculated by Encke that the mass of the latter is T ^ TTT that of the Sun. Le Verrier gives ^OTRTO cy ; Littrow 2 OTOTO ; and Madler TFT infir . The ancients were not only acquainted with the existence of this planet d , but were able to approximate with considerable accu- racy to its period, and to the nature of its motions in the heavens. " The most ancient observation of this planet that has descended to us is dated in the year of Nabonassar 494, or 60 years after the death of Alexander the Great, on the morning of the I9th of the Egyptian month Tkotk, answering to November 15 in the year 265 before the Christian era. The planet was observed to be distant from the right line, joining the stars called (3 and in Scorpio, one diameter of the Moon ; and from the star ft two diameters towards the North, and following it in Right Ascension. Claudius Ptolemy reports this and many similar observations ex- tending to the year 134 of our era, in his great work known as the Almagest*" We have also observations of the planet Mercury, by the Chinese astronomers, as far back as the year 118 A.D. These ob- servations consist, for the most part, of approximations (appulses) d Pliny, Hist. Nat., lib. ii. cap. 7 ; Cicero, De Natura Deonim, lib. ii. cap. 20. Hind, Sol. Syst., p. 23. 62 The Sun and Planets. [BOOK I. of the planet to stars. M. Le Verrier, the French astronomer, has tested many of these Chinese observations by the best modern tables of the movements of Mercury, and finds, in the greater number of cases, a very satisfactory agreement. Thus, on June 9, 1 1 8, the Chinese observed the planet to be near the cluster of stars usually termed Prsesepe, in the constellation Cancer ; calcu- lation from modern theory shews that on the evening of the day mentioned Mercury was less than i distant from that group of stars. " Although the extreme accuracy of observations at the present day renders it unnecessaiy to use these ancient positions of the planets in the determination of their orbits, they are still useful as a check upon our theory and calculations, and possess, more- over, a very high degree of interest on account of their remote antiquity f ." La Place says : " A long series of observations were doubtless necessary to recognise the identity of the two bodies, which were seen alternately in the morning and evening to recede from and approach the Sun ; but as the one never presented itself until the other had disappeared, it was finally concluded that it was the same planet which oscillated on each side of the Sun." Arago considers that " This remark of La Place's explains why the Greeks gave to this planet the two names of Apollo, the god of the day, and Mercury, the god of the thieves, who profit by the evening to commit their misdeeds." The Greeks gave Mercury the additional appellation of 6 2n'A- /3cor, ' the Sparkling One.' When astrology was in vogue, it was always looked upon as a most malignant planet, and was stigma- tised as a siclus dolosum. From its extreme mobility chemists adopted it as the symbol for quicksilver. It is rather difficult, in a general way, to see Mercury, and Copernicus, who died at the age of 70, complained in his last moments that, much as he had tried, he had never succeeded in detecting it ; a failure due, as Gassendi supposes, to the vapours prevailing near the horizon on the banks of the Vistula where the illustrious philosopher lived. An old English writer, of the name of Goad, in 1686, humorously termed this planet " a squirting f Hind, Sol. Syst., p. 23. CHAP. IV.] Mercury. 63 lacquey of the Sun, who seldom shews his head in these parts, as if he were in debt." In computing the places of Mercury, the Tables of Baron De Lindenau, published in 1813, were long employed, but they are now superseded by the more accurate Tables of Le Verrier . * Annales de I'Obs. de Paris, 1859. The Sun and Planets. [BOOK I. CHAPTER V. VENUS. 9 Period, &c. Phases resemble those of Mercury. Most favourably placed for observa- tions once in eight years. Daylight observations. Its brilliancy. Its Spots and Axial Rotation. Suspected mountains and atmosphere. Its "ashy light." Phase irregularities. Suspected Satellite. Observations on it. The Mass of Venus. Ancient observations. Galileo's anagram. Venus useful for nautical observations. Tables of Venus. NEXT in order of distance from the Sun, after Mercury, is Venus ; which revolves round the Sun in 224 d i6 h 49 8 s , at a mean distance of 66,131,000 miles. The eccentricity of the orbit of Venus amounting" to only 0*006, the extremes of distance are Fig. 32. only 66,585,000 miles and 65,677,000 miles. This ec- centricity is very small. No planet, major or minor, has an eccentricity so small. The apparent diameter of Venus varies between 9* 7" in superior and 66-5" in inferior conjunction. At its greatest elongation its appa- rent diameter is about 25". A numerous series of careful observations enabled Main to VENUS NEAB ITS GBEATEST ELOKGAT.ON. determine that the planet's (Schroter.) diameter (reduced to mean distance) is i7'55", subject to a correction of 0-5" for the effects of irradiation. Stone, from an elaborate discussion of a large series of Greenwich observations, obtained 16-944", with a probable error CHAP. V.] Venus. 65 of + 0*08". Tennant in 1874 (during the Transit) obtained, as the mean of 68 measures, 16-9036" (reduced) with a probable error of O'OOi6" only a . The real diameter corresponding- to this latter evaluation is about 7500 miles, or, roundly, Venus's diameter is almost the same as the Earth's. The compression must be small, but Tennant thinks he found traces thereof. Great difficulty must ever arise in clearly detecting it, because the planet's diameter in superior conjunction is so small. Venus exhibits phases precisely identical with those of Mercury. Though under the most favourable circumstances Venus is never farther removed from the Sun than 47 15', and is therefore always more or less under the influence of twilight, yet it is difficult to scrutinise this planet for a reason additional to that which obtains with Mercury, namely, its own extreme brilliancy. This is such as to render the planet not unfrequently visible in full daylight and capable of casting a sensible shadow at night. This happened in Jan. 1870, and occurs every 8 years, when the planet is at or near its greatest North latitude and about 5 weeks from inferior conjunction. Its apparent dia- meter is then about 40", and the breadth of the illuminated part nearly TO", so that rather less than Fig- 33- VENUS NEAR ITS INFERIOR CONJUNCTION. (Schrdter.) of the entire disc is illuminated, but this fraction transmits more light than do phases of greater extent, because the latter correspond to greater distances from the Earth. A lesser maximum of brilliancy , due to the same circumstances less favourably carried out, occurs on either side of the Sun at intervals of about 29 months. The planet's angular distance from the Sun on these occasions is rather less than 40 (in the superior part of its orbit) ; so its phase therefore corresponds with that of the Moon I i d and 1 7 d old. Observations of Venus in the daytime were first made at a very early period ; the following are the dates of a few instances : 398, Month. Not., vol. xxxv. p. 347. May 1875. 66 The Sun and Planets. [BOOK 1. 984, 1008, 1014, 1077, 1280, 1363, 1715, 1750. " Bouvard has related to me," says Arago, " that General Buonaparte, upon repair- ing to the Luxembourg, when the Directory was about to give him a fete, was very much surprised at seeing the multitude which was collected in the Rue de Tournon pay more attention to the region of the heavens situate above the palace than to his person or to the brilliant staff which accompanied him. He inquired the cause, and learned that these curious persons were observing with astonishment, although it was noon, a star, which they supposed to be that of the conqueror of Italy ; an allusion to which the illustrious general did not seem indifferent when he him- self with his piercing eyes remarked the radiant body. The star in question was no other than Venus V The dazzling brilliancy of this planet is such that the daytime is to be preferred for observing it, but under the best of circumstances it is far too tremulous for physical observations to be conveni- ently made. J. D. Cassini attacked it in 1667, and some ill-defined dusky spots seen on various occasions during April, May, and June, enabled him to assign 23** i6 m for its axial rotation. Bianchini, in 1736 and 1727, favoured by an Italian sky, observed spots with greater facility : thence he inferred a rotation performed in 24 days 8 hours. Sir William Herschel, desirous of arriving at some more decisive conclusion, devoted much care to the inquiry; but he was unable to assign a precise period beyond generally believing that Bianchini's statement was largely in excess of the true amount. Schroter, by close attention to certain spots, deduced a period of 23 h 2i m 7'98 S , which Di Vico and his colleagues at Rome, in 1840-2, only slightly modified to 23** 2i m 23*93 9 . We may thus assume that the period of the axial rotation of this planet is known to within a very small fraction of the whole amount. Sir W. Herschel's opinion of the spots which he saw was that they were in an atmosphere, and did not belong to the solid body an opinion supported by no analogy, and now, with reason, be- lieved to be groundless, for Di Vico found the spots just as deline- ated by Bianchini. The Roman observers, 6 in number, displayed great diligence in the matter, and Bianchini's drawings were, b Pop. A st. t vol. i. p. 701, English experimentally found to be 10 times as edition. bright as the brightest part of the full c Denison states that Venus has been moon. (Ast., p. 149, 3rd ed.) CHAP. V.] Venus. 67 with one exception, confirmed. Of the 6 observers, the most suc- cessful were those who had most difficulty in catching very minute companions to large stars, the reason of which Webb points out to be obvious enough. A very sensitive eye, which would detect the spots readily, would be easily overpowered by the light of a bril- liant star, so as to miss a very minute one in its neighbourhood. Mountains probably exist on Venus, though the testimony on which the statement must rest is not quite so conclusive as could be desired. In August 1700 La Hire, observing the planet in the daytime near its inferior conjunction, perceived in the lower region of the crescent inequalities which could only be produced by moun- tains higher than those in the Moon. To the same effect Derham, writing in 1715. Schroter asserted d the existence of several high mountains, in which he was confirmed by Beer and Madler, but his details as to precise elevation measured by toises must be ac- cepted with great reserve, amongst other reasons because it is doubtful whether his micrometers were of sufficient delicacy. Sir W. Herschel disbelieved him on some points, and attacked him in the Philosophical Transactions for 1793: his reply was published in the volume for the year but one after f ; it was calm and digni- fied, and vindicated the mountains if not the measurements. Di Vico, at Rome, in April and May 1841, appears to have noticed a surface-configuration akin to that of the Moon ; and Lassell, when at Malta in January 1862, observed the same sort of thing. Browning, on March 14, 1868, saw mottlings on the surface of Venus which reminded him of the look of the Moon as seen in a small telescope through a mist. A bluntness of the southern horn, referred to by Schroter, was also seen by the Roman astronomers, and often by Breen subsequently with the Northumberland tele- scope at Cambridge. That Venus has an atmosphere is generally admitted ; that it is of considerable density is likewise an opinion apparently well founded. During the transits of 1761, 1769, and 1874, the planet was observed by several persons to be surrounded by a faint ring of light, such as an atmosphere would account for. Schroter, too, discovered what appeared to him to be a faint crepuscular light d Phil. Trans., vol. Ixxxii. p. 337. 1793. ^792. f Phil. Trans. f vol. Ixxxv. p. 117. e Phil. Trans., vol. Ixxxiii. p. 202. 1795- F 2, 68 The Sun and Planets. [BOOK I. extending beyond the cusps of the planet into the dark hemisphere. From micrometrical measures of the space over which this light was diffused he considered the horizontal refraction at the surface of the planet to amount to 30' 34", or much the same as that of the Earth's atmosphere. Sir W. Herschel confirmed the discovery as a wholes, and more recently Madler was able to do the same with the mere modification of making the amount somewhat greater, or equal to 43*7'. With this the 1874 Transit results fairly agree. The existence of snow at the poles of Venus has been suspected by Webb and Phillips, but the idea awaits confirmation, though there is no prima facie reason why it should not be well founded ; indeed rather the reverse. A phenomenon analogous to the tumiere cendree, or { ashy light,' of the Moon is well attested in observations of Venus when near inferior conjunction. Many observers have noticed the entire contour of the planet to be of a dull grey hue beyond the Sun- illumined crescent. Webb uses the expression the phosphorescence of the dark side: I cannot but regard this as an objectionable phrase, for phosphorescence notably conveys the idea that some inherent light is spoken of, whereas there can be little doubt that reflection is in some way or other the cause of what is seen in the case of Venus, though it may be difficult at present to specify the precise nature of it h. Derham noticed this appearance, and refers to it in his book ; and Schroter, Sir W. Herschel, Di Vico, and Guthrie * are amongst those that have seen it. The chief objection however to the term phosphorescence lies in the fact that Green, Winnecke, Noble, and several others have repeatedly seen the un- illuminated limb of Venus distinctly darker than the back-ground on which it was projected. The peculiarity about Mercury's phases already pointed out (the measured breadth being different from the calculated) obtains also with Venus. At the greatest elongations, the line terminating the illumination ought to be straight, as with a half-moon, but several observers have found a discrepancy of between 3 d and 8 d between the first, or last, appearance of the dichotomisation (according as one or other elongation was referred to). Thus, at the Westerly s Phil. Trans., vol. Ixxxiii. p. 214. 1793- Borealis seems a rather poor hypothesis. h The supposition of the existence of > Month. Not, vol. xir. p. 169. March some such phenomenon as our Aurora 1854. CHAP. V.] Venus. 69 elongation of August 1793, Schroter found the terminator slightly concave, and it did not become straight till 8 d after the epoch of greatest elongation. Previous to the present century testimony was not wanting that Venus had a satellite, but nothing has been ascertained about it in recent times, and Webb, with great propriety, calls the matter " an astronomical enigma." On Jan. 35, 1672, J. D. Cassini saw, be- tween 6 h 52 m and 7 h 2 m A.M., a small star resembling a crescent, like Venus, distant from the Southern horn on the Western side by a space equal to the diameter of Venus. On Aug. 28, 1686, at 4 h i % 5 m AM., the same experienced observer saw a crescent-shaped light East of the planet at a distance of f ths of its diameter. Day- light rendered it invisible after J an hour. On Oct. 23, 1740 (o.s.), Short, the celebrated optician, with 2 telescopes and 4 different powers, saw a small star perfectly defined but less luminous than the planet, from which it was distant 10' 2". On 4 different occa- sions between May 3 and n, 1761, Montaigne, at Limoges, saw what he believed to be a satellite of Venus. It presented the same phase as the planet, but it was not so bright. Its position varied, but its diameter appeared equal to |- th that of the planet. The following extract is from the Dictlonnaire de Physique^ a French work published in 1789. "The year 1761 will be celebrated in astronomy in consequence of the discovery that was made on May 3 of a satellite circulating round Venus. We owe it to M. Mon- taigne, member of the Society of Limoges, who observed the satel- lite again on the 4* and 7 th of the same month. M. Baudouin read before the Academy of Sciences of Paris a very interesting memoir, in which he gave a determination of the revolution and distance of the said satellite. From the calculations of this .expert astronomer we learn that the new star has a diameter about i th that of Venus, that it is distant from Venus almost as far as the Moon is from the Earth, that its period is 9 d 7 h , and that its ascending node is in the 22 nd degree of Virgo." Wonderfully circumstantial! In March 1764 several European observers, at places widely apart, saw a supposed satellite. Eodkier, at Copen- hagen, on March 3 and 4, saw it : Horrebow, with some friends, also at Copenhagen, saw it on the io th and II th of the same month, and they stated that they took various precautions to make sure there was no optical illusion. Montbaron, at Auxerre, 70 The Sun and Planets. [BOOK I. on March 15, 28., and 29, saw the satellite in sensibly different positions J. This is the plaintiff's case, if I may be pardoned for using such an expression : on the other side it can only be said that no trace of a satellite has ever been seen by any subsequent observer with larger telescopes. And with the care bestowed on Venus by Sir W. Herschel and Schroter during so many years, it is difficult to understand that, if it existed, they should not have seen it at some time or other. Lambert combined all the observations in a very tolerable orbit k , but, as Hind points out \ notwithstanding its agreement with the observations, there is one fatal objection to it if it were correct, the mass of Venus would be 10 times greater than what other methods shew it to be, namely TmrVrr that f tne Sun. Encke gives OTTWs-j Littrow T^STY* Madler T^TTID and L e Verrier TT^TTO"' There are several methods of ascertaining this quantity, the most obvious of which is based on the disturbing influence exerted by Venus on the Earth's annual motion. Venus has ever been regarded as an interesting and popular planet, and it is somewhat remarkable that it is the only one whose praises are sung by the great Greek bard, who thus apostrophises it: " "EffTTfpos, bs Ka\\iaros Iv ovpava> tararcu affrrjp." This refers to it as the Evening Star, but elsewhere in the Iliad 11 we meet with it in its other function of the 'Etoo-^o'pos, to which the Latin Lucifer corresponds. Some have thought, and perhaps not without reason, that it is the object referred to in IsaiaJi xiv. 12. The earliest recorded observations of Venus date from 685 B.C. (Ast)., and appear on an earthenware tablet now in the British Museum . " Claudius Ptolemy has preserved for us in his Almagest many observations of Venus by himself and other astronomers before him, at Alexandria in Egypt. The most ancient of these observa- tions is dated in the 476 th year of Nabonassar's era and 13 th of i Scheuten says he saw a satellite ac- m Homer, Iliad, lib. xxii. v. 318. company Venus across the Sun during n Lib. xxiii. v. 226. Pythagoras (or, the transit of 1761. See Ast. Jahrbuch, according to others, Parmenides) deter- 1778. mined the identity of the two "stars." k Bode's Jahrbuch, 1777. Month. Not., vol. xx. p. 319. June 1 Sol Syst., p. 27. 1860. CHAP. V.] Venus. 71 the reign of Ptolemy Philadelphia, on the night of the 17 th of the Egyptian month Messori, when Timocharis saw the planet eclipse a star at the extremity of the wing of Virgo. This date answers to 271 B.C., Oct. 12 A.M. P " As this was not a telescopic observa- tion, it and all others recorded before telescopes came into use are open to this uncertainty, that the two objects may merely have been in juxta-position so as to have appeared as one without actual super-position taking place. The recorded occultation of Mercury by Venus on May 17, 1737, was no doubt an occultation in the strict sense of the word. The interesting discovery of the phases of Venus is due to Galileo q , who announced the fact to his friend Kepler in the fol- lowing logograph, or anagram r : "Haec immatui-a, a me, jam frustra, leguntur. oy." " These things not ripe [for disclosure] are read, as yet in vain, by me." Or, as another interpretation has it " These things not ripe ; at present [read] in vain [by others] are read by me." The " me " in the former case being the ordinary reader ; in the latter, Galileo. This, when transposed, becomes "Cynthise figuras aemulatur Mater Amorum." "The Mother of Love [Venus] imitates the phases of Cynthia [the Moon]." To the mariner, owing to its rapid motion, Venus is a useful auxiliary for taking lunar distances when continuous bad weather may have prevented observations of the Sun. In computing the places of Venus the tables of Baron De Lin- denau, published in 1810, were long in use, but they have now been superseded by those of Le Verrier, for amongst other causes of error there existed a long inequality (discovered by Sir G. B. Airy in 1846) affecting the heliocentric places of the Earth and the planet to a very sensible amount. This inequality goes through all its changes in about 239?, and when at a maximum displaces Venus by 3" and the Earth by 2", as viewed from the Sun. P Hind, Sol. Syst., p. 32. more distinctly, they would be found to i It was one of the objections urged do so. Prof. De Morgan believes the to Copernicus against his theory of the anecdote to be apocryphal. (Month. Not., solar system that if it were true then the vol. vii. p. 290. June 1847.) But <