UC-NRLF STFlOJfOMY OB SgRVATI ON S~Zmk "'"" ' K^ / BY ELIZA A. BOWEN AMERICAN BOOK COMPANY NEW YORK CINCINNATI > CHICAGO THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA GIFT OF Mrs . Henry J . Miller ASTRONOMY BY OBSERVATION. SPECTRA OF VARIOUS SOURCES OF LIGHT . Lid. Li 8 Cs$ CsQ RhBRbtL Bid BaB IhY PhlPJiS Tfie Surfs edge. 3. Sodium. 4. Patas-sium . 5. Lithium. & Caesium. 2 RaJbiduim. 8. Thatiium. \O.Strontiu,m.llSarmm.\Z. Indium. "SsPhosphorvu. 14 Hydrogen. NEW YORK, D. APPLETONaC9 ASTRONOMY BY OB S ERVATION AN ELEMENTARY TEXT-BOOK FOR HIGH-SCHOOLS AND ACADEMIES BY ELIZA A. BOWEN NEW YORK : CINCINNATI : CHICAGO AMERICAN BOOK COMPANY TO MRS. ZOE DANA UNDERHILL, WHOSE ENLIGHTENED APPRECIATION OF OBSERVATION-STUDY STIRRED ME UP TO WRITE THIS BOOK, AND WHOSE STEADY FRIENDLY ENCOURAGEMENT SUSTAINED ME THROUGH SEVERAL YEARS OF EXPERIMENTAL WORK. COPYRIGHT, 1886, 1890, BY D. APPLETON AND COMPANY. printed bs> S. Hppkton cX Company orfe, -U. B. H. EDUC.- PSYCH t LIBRARY ! Cancer === Libra Y3 Capricornus Taurus ft Leo Til Scorpio Xf Aquarius n Gemini HE Virgo $ Sagittarius X Pisces CHAPTER V. THE MOON AND HER MOTIONS, AND HOW TO OBSERVE THEM. 91. No other heavenly body offers so convenient an opportunity for full observation as the moon. She comes often, and goes through the round of her motions in a reasonable time. It is not only exceedingly interesting ASTRONOMY BY OBSERVATION. work to watch her through a full revolution, it is ex- cellent preparatory training for the observation study of the planets. The positions of the moon in relation to the sun and earth are similar to certain important positions of the planets in regard to sun and earth. In order to observe the moon well, it is necessary to know at least those constellations of the zodiac which are in the evening sky. They can be learned in a few even- ings by drawing them a good deal. (See Introduction.) It is best to begin observation as soon after the date given in the almanac for new moon as the observer can get a sight of her. It is very difficult to see the new moon before she is two days old. There are three lines of observation which must here be treated separately, in order to avoid the confusion which results from describing several things at once. But the student must carry out all three together when he can study nature for himself. If he has time to study the moon through two revolutions, the observa- tion-study of II, p. 33, may be postponed until the last. /. The Moon's Motions. 92. The Diurnal Revolution. At two or three days from the time of new moon, the moon will appear as a slender crescent, seen soon after sunset above the western horizon, and not far from the sunset-point. If, after finding her, the student will return in ten minutes to look again, he will see that she is moving toward the western hori- zon, and will disappear behind it soon after sunset. As all the heavenly bodies which the student has observed exhibit similar motions, he will recognize hers as appar- ent, and due to the earth's rotation on her axis. If so, the moon must cross the sky below the horizon during the twelve hours after her disappearance, pass above the eastern horizon a little after sunrise (invisible because of the sun's blinding light), and, crossing the heavens above the horizon during the day, reappear in the west when the sun's light is withdrawn. This is-the moon's apparent diurnal revolution, and it is the cause of her rising in the east and setting in the west. As it is desir- able to avoid any confusion of this motion with others, it will be best to make observations at the same hour every evening. 93. The Moon's Real Motion. On the second night of observation, the student would find the moon again in the west, but she would be a little higher above the horizon, that is, a little farther east, than before. It is clear that the moon, or the horizon and earth, must have moved. If this motion is apparent, it must be caused by the motion of the observer and earth ; and the earth must move west to cause it. But we know that the varth moves east. For this reason we conclude that this is a real, or, as astronomers call it, a " proper " motion ol the moon. It causes the moon to change place among the stars every evening, while the diurnal revolution makes the moon move with the stars. It should be part of the student's observation-work to note the moon's place among the stars every evening, and thus detect her real motion. 94. The Moon's Real Revolution. Opposition and Conjunction. Since the moon was found moving east, the student will, upon reflection, conclude that she must have come into the evening sky from below the western horizon, and, in doing so, must have passed the sun. She must have crossed a meridian circle running north and south through the sun's position. When she does this, she is said to be in conjunction with the sun.* She does not always cross this line exactly at sunset, but she moves too slowly to get very far east from the sun at the very next sunset after it. So, after conjunction, we always see her near the western horizon at sunset. The moon becomes new after conjunction. If the student continues to watch the moon's real motion at dark, he will find her every evening farther east among the stars ; and in fifteen days she will have crossed the evening sky, and will be found at sunset on or near the eastern horizon. She will have made a half- revolution round the earth. After this she will con- tinue to move east, and, of course, she will at dark be on the heavens below the eastern horizon. 95. It will be evident that she must have passed the point of the ecliptic 180 from the sun, since that point is always on the eastern horizon at sunset. When she crosses a celestial meridian running nearly north and south through this point, she is said to be in opposition with the sun. She may cross this line at any time of the day or night, but she does not move fast enough to get very far from it by the next sunset ; and so, when she is at opposition, she will appear to rise in the east about the time the sun sets. After opposition the moon can be seen only by sit- ting up until she rises, but her rising will come later and later, and finally it will be necessary to get out of bed before light in the morning in order to see her. After her opposition, she approaches the sun on his western side, and she can finally be found in the morning near him. Twenty-nine and a half days after conjunction she would pass the sun, and after that she would again be seen in the west as new moon. Thus, continued ob- * The student can readily understand the position of these bodies in nature, and therefore a diagram is quite unnecessary. It is usually positively pernicious, since students think of the diagram rather than the positions in nature. They may have their heads tilled with diagrams corresponding to nothing whatever in nature thai they know. THE MOON AND HER MOTIONS, AND HOW TO OBSERVE THEM. 33 servation shows us plainly that the moon's proper motion is a revolution round the earth as a center. 96. The Moon's Orbit. During this revolution the moon would not have changed apparent size, so far as the observer could tell without measurement. There- fore it would be evident that the figure of her motion is nearly a circle with the earth as a center.* Meas- urement, however, shows a change ; and for this and other reasons it is believed that the figure of her orbit is an ellipse. The drawings (Fig. 21) show the variation in the moon's apparent size. FIG. 21. When the moon is near the horizon, she looks larger than when she is seen overhead ; but this is an illusion, as is shown by measurement. We estimate her distance from us by so many intervening objects, that she seems to us farther off, and therefore larger. 97. Sidereal and Synodical Revolutions. At con- junction, or new moon; at the beginning of these obser- vations, the moon would be very near the sun ; and, as the student knows, a constellation of the zodiac would be behind, or west of both. Thus the earth, moon, sun, and a star-group would be nearly in line. After the moon had completed its revolution of twenty-nine and a half days, and was again in conjunction with the sun, and on the western horizon at sunset, the same star- group would not be there, but another, since the stars would have been moving west all the time. The star- group which was on the horizon at the first conjunction would have gone below the horizon, and the earth and the moon would have been in line with the first sar- group before the moon came in line with the earth and sun. (See Note on Art. 97, p. 89.) The interval of time after which the moon comes back to the line between the earth and the first star-group is called a Sidereal Revolution of the moon. The in- terval after which the moon comes back to the same position with regard to the earth and sun is called a Synodical Revolution of the moon. Thus the period between two successive conjunctions or two successive * That the moon's orbit is nearly a circle, follows from this, and the fact that her path on the heavens is a circle. (See Art. 43.) oppositions is a Synodical revolution. A sidereal revo- lution is completed in twenty-seven and one-third days, and a Synodical revolution in twenty-nine and a half days. 98. If the student were asked why the sidereal revo- lution is shorter than the synodical revolution, he would perhaps say, " Because the stars move west." But the motion of the stars is apparent, and due to the earth's annual eastern motion round the sun. Therefore astron- omers say that, when the moon has come round to the stars from which she started, she has to go a little farther to catch up with the earth and revolve round the sun. //. The Moons Path and the Ecliptic. 99. The second line of observation is to watch the moon's path among the stars, and learn how it is situated in regard to the ecliptic. The moon is nearer to us than any heavenly body, while the fixed stars are at an im- measurable distance, but she seems to move among them, because they are on the background of the sphere against which we see her. Sometimes the moon's light obscures the stars near her, so that it is somewhat difficult to trace her path with any certainty. But even in this case the student must learn all he can from observation, and must not stop because he can not see everything. 100. The points to be noted in watching the moon and the ecliptic are as follows : The student must note that the moon is always in a zodiacal constellation, and always very near the ecliptic. She sometimes seems to move toward it, sometimes from it. She crosses it. It is somewhat difficult to ascertain the exact point of crossing, but the almanac gives help. The symbols "( in a " signify " The moon to day crosses the ecliptic going north." The symbols " ( in y " signify " The moon to day crosses the ecliptic going south." 101. The paths of the sun and moon intersect at an angle of 5. Therefore she is always very near the circle of the ecliptic, and when at conjunction she passes the sun, she can never be far north or south of the line pass- ing through sun and earth ; and, if she is crossing the ecliptic at the time, she will be on it. Also, at opposi- tion, she can not be very far north or south of the line joining earth and sun, since that line passes through the point of the ecliptic 180 from the sun. She will be on it if she is on the ecliptic. 102. The points where the path of the moon crosses the plane of the ecliptic are called Nodes. ///. The Moon's Phases. 103. This is the third subject to be studied by the observation of nature, beginning at new moon. Fig. 34 ASTRONOMY BY OBSERVATION. FIG. 22. 22 represents the phases of the moon from new to full ; and, reversing the order, from full to new again. First Quarter. If we begin to observe the new moon as soon as she is seen in the west, she will have the ap- pearance of i in Fig. 22, viz., a full circle covered with a faint illumination, but having around the margin, turned toward the sun below the horizon, a slender bright crescent. If we suppose that the moon shines by the light of the sun which she reflects to us, and that the sun is a great deal farther from us than the moon, it will account for the appearances. The sun must en- lighten half the moon, and half the moon must be turned to us ; but these must be nearly opposite halves, since the moon is nearly between the sun and earth. But since the moon is not on a straight line between the earth and sun, but a little above the line, it is clear the halves turned to us and to the sun can not be exactly opposite halves, but must coincide a little. We see the bright crescent because we see a small part of the half that is enlightened by the sun. But the hemisphere of the moon turned toward the earth just at dark is turned toward the portion of the earth just below the western horizon, which is bathed in sunshine. As moonlight causes a faint illumination on the earth, it is clear that sunlight reflected from the earth to the moon could cause the faint light seen over the moon's whole surface. Second Quarter. If we continue to observe the moon, we shall see that, as she moves east, she rises higher, and the crescent increases ; and at seven days after new moon she is overhead at sunset and is a half moon. She has then passed her first quarter, as it is called, and begins the second. In this case, her western side is still turned toward the sun ; but, since she is above our heads at sunset, only half the western hemi- sphere coincides with the hemisphere which we see above us. For this reason we find only a half-hemi- sphere enlightened or visible, and the convex side of that is turned toward the west. Third Quarter. After this, as the moon continues to move east, she increases in size, and about fifteen days after new moon she begins her third quarter. She is then full, or shows a full enlightened circle; and she is rising in the east while the sun is setting in the west. The same side is now turned toward us and toward the sun, for she is now nearly in line with sun and earth, the earth being in the middle. If she were quite in line, it is evident that the earth would cut off the sun's light from her, and we should have an eclipse of the moon. If at full moon she is in that part of her path on the heavens which crosses the ecliptic, she is in a direct line with earth and sun. At full moon she is 180 from the sun on the celestial sphere ; and, as she still moves east, she must approach the sun on the other side, and so she begins to decrease on her western side. After the beginning of her third quarter, she can generally be seen in the daytime, be- cause she is so far from the sun on the sphere that he does not entirely obscure her' light. When, from ap- proaching the sun, she becomes invisible in the daytime, she can best be observed by rising just before light in the morning. We know that, at that hour, the sun is just below the eastern horizon. Fourth Quarter. About twenty-one days after new moon, she begins her fourth quarter. She is then a half- moon again, and when seen while it is still dark in the morning, she is nearly overhead, with her convex side turned toward the sun. It is evident that he illuminates her eastern hemisphere, and that we see only half of it. The half-hemisphere which we do see has its convex- side turned east. After this, we should find the moon drawing nearer the sun at sunrise, and about the twenty-seventh clay after new moon we should again see the faintly illumined full circle with the bright, slender crescent on the side next the sun. Sun and earth would again face opposite sides of the moon, nearly, but not exactly. The moon being a little above the line between earth and sun, the hemispheres opposite each would to a very small extent coincide ; and this is the reason why we should see the crescent. The faint illumination would be turned to- ward the sunny side of the earth below the eastern hori- THE MOON AND HER MOTIONS, AND HOW TO OBSERVE THEM. 35 zon. Finally, the moon would pass nearly between earth and sun, and then \ve should have new moon again. If the moon passed directly between earth and sun, it is clear she would intercept the sun's light from us, and we should have an eclipse of the sun. There is no eclipse unless moon and sun are together at the points where the moon's path on the heavens crosses the ecliptic. The moon's phases are sometimes illustrated by a diagram which is given below (Fig. 23), mainly because some teachers will like to have it. The knowledge which The I' liases of the Moon. such a diagram seems to give is very delusive. The student will be very unwise not to watch the moon herself. From observing the moon it is very clear that she shines by light reflected from the sun, for we do not see any illumination except on the parts which are turned toward the sun. 104. Nothing can give a student of astronomy a com- plete idea of the causes and conditions of eclipses but observation of the relative positions of sun, moon, and earth through a full synodical revolution. He will see that sun, moon, and earth never are nearly in line except at new and full moons ; that at new moon only, they are nearly in line with the moon in the middle ; and there- fore an eclipse of the sun can take place at new moon alone. Also, at full moon only, they are nearly in line with the earth in the middle, and therefore an eclipse of the moon can take place at full moon alone. He will see that they never can be in line unless the moon at full or new is on the ecliptic ; that is, unless the earth and moon are on the line in which the planes of their orbits intersect. The subject of eclipses will be more fully treated further on. 105. Positions of the Moon's Crescent. From new to full, the moon increases gradually from a crescent to a full circle, and back again from full to new ; but, owing to the fact that the moon is sometimes north of the ecliptic, sometimes south of it that is, sometimes north of the sun's path, and sometimes south of it the posi- tions of the crescent vary. The variation is most noticed at new moon, when the sun is known to be just below the horizon. Sometimes a line joining the horns is nearly vertical, as ; sometimes it is nearly horizontal, as @. Superstitious people call the first " wet moon," and the last " dry moon," and suppose they foretell the weather. The variation really depends on the positions of the moon and sun in regard to the horizon. The moon's crescent may have any position inter- mediate between these. It is most nearly horizontal at the new moon near March 2oth,and most nearly perpen- dicular at the new moon near September 2ist. At sun- set on March 2oth the ecliptic has nearly the aspect of Map I. Fig. 24 shows the western hemisphere of the heavens at this time, and it shows that the ecliptic is FIG. 24. HEMISPHERE nearly perpendicular to the horizon. In this case, the sun, which is always on the ecliptic, is nearly below the intersection of ecliptic and horizon. If the moon is at the same time north of the ecliptic, she is directly over the sun, and as the convex side of the crescent is always turned toward the sun, it is nearly horizontal. On September 2ist, at sunset, the ecliptic has the aspect of that circle on Map II. Fig. 25 shows the west- ern hemisphere of the heavens at sunset on September 2 ist, and it is seen that the ecliptic is much inclined to ASTRONOMY BY OBSERVATION. the horizon. The sun, after setting, is far north of the intersection of ecliptic and horizon. If the moon at this S. POtE- HEMISPHERE "* time is south of the ecliptic, she is nearly south of the sun, and therefore the line joining the horns of the cres- cent is nearly vertical. The facts which explain these positions of the cres- cent are: i. The position of the moon in relation to the ecliptic. 2. The two positions of the ecliptic in relation to the horizon. These facts will have little reality to the student unless he sees them in nature. But in order to give them reality, it is not necessary for him to wait and see them in March or September. 106. The Moon's Axial Rotation. The most careless person usually remembers, without special observation, that the moon always presents the same shadings of sur- face, in which many persons have traced a resemblance to a man's face. Thus it is evident that we must see nearly the same hemisphere of the moon all the time. If the student will walk around a chair with his face turned to it all the time, he will imitate the motion. In doing this he turns his face once to every point of the compass. This is precisely what he does when he stands on one spot and turns round or revolves axially. There- fore the moon's motion is usually described by saying that she revolves once on her axis while performing her revolution in her orbit. 107. The Moon's Librations. At different times we really see a little more than one half the moon's surface. The motions to which this is due are called the moon's Librations. In consequence of the elliptical form of the moon's orbit, her motion, like that of the earth, is un- equal. But her motion on her axis is equal, and there- fore her orbital motion sometimes gets ahead of her axial motion, or falls behind it, and thus we see a little farther around her east or west. This is called her Libration in Longitude. The moon's axis, like that of the earth, is inclined to the plane of her orbit, and, like the earth's axis, always moves parallel to itself. We know this, because, when we see the moon through a telescope, she shows a change in the part round the poles. This is called the moon's Libration in Latitude. Owing to the fact that we are about four thousand miles from the center of the earth, which is the center of the moon's motion, we see her from slightly different positions when on our eastern and western horizons ; and there is thus a slight variation in the part of the surface seen. This is called her Parallactic Libration. Parallax is the displacement of an object caused by the observer's change of position. We see in all about .58 of the moon's surface. 108. Times of the Moon's Risings. What we call the moon's rising is due to the earth's rotation on her axis, carrying the horizon east to meet her. But during one rotation of the earth the moon moves 13 east in her orbit, and therefore the horizon must move farther to overtake her. As the earth and horizon revolve through i in four minutes, it takes fifty minutes, on an average, for the horizon to catch up with the moon. Therefore she rises about fifty minutes later every day. But the times of the moon's rising vary a good deal. 109. Harvest Moon. When the constellation Pisces is on the eastern horizon at the time the moon rises, the retardation in her rising on successive nights is much lessened. In latitude 40 the delay may be only about twenty-five minutes. The difference is the more noticed when the moon is full, both because she is then more conspicuous, and because she rises at a more convenient hour for observation. The full moon rises in Pisces within a fortnight of the autumnal equinox in Septem- ber. It is widely noticed by farmers, to whom its early risings are of use in gathering the crops. They call it Harvest Moon. When Virgo is on the eastern horizon at full moon, which happens within a fortnight of the vernal equinox in March, the delay in her rising is increased. In lati- tude 40 the variation in her time of rising on successive nights may equal an hour and a quarter. An explanation (and diagrams) of these phenomena are of little use except to aid the student in observing the facts in nature which cause them. A knowledge of mere diagrams is illusory. In order to have real knowl- edge, however, it is not necessary to see all the facts together, and therefore not necessary to wait until har- vest moon. The facts are as follows: i. The moon is THE MOON AND HER MOTIONS, AND HOW TO OBSERVE THEM. 37 always very near the ecliptic, and we can therefore use the ecliptic to illustrate the angle which her path among the stars makes with the horizon. 2. The moon moves east among the stars ; this causes the delay in her ris- FIG. 26. (t.PT- ing. 3. In March the ecliptic (and the moon's path) seem to curve very little toward the south, and she there- fore moves on a path nearly perpendicular to the hori- zon. Map I shows this, and so does also Fig. 26, exhibit- ing the eastern hemisphere of the heavens and the aspect of the ecliptic in March. Also, in September, the ecliptic is much inclined to the horizon. This is shown in Map II and in Fig. 27, exhibiting the eastern hemisphere of the heavens for September, with the aspect of the ecliptic. It is clear that it would take longer to overtake the moon FIG. 27. "^'SPHERE moving away from the horizon on a path like the ecliptic of Fig. 26, than when she moves on a path like the ecliptic of Fig. 27, even if she moved 13 daily on both. And as her rising is due to the horizon moving down to over- take her, she is less delayed in rising on successive nights in September ; and in March the delay is greater. In December and June, the path of the ecliptic is nearly parallel with the equinoctial, and the angle of the equi- noctial and the horizon does not vary at all, as can be seen from Figs. 26 and 27. Since the ecliptic is not a visible line in nature, it can not be traced so definitely as on the diagram ; but a careful observer, tracing it by the stars, and noting the direction of the zodiacal constellations, will have no difficulty in seeing the difference in the angle, as ex- hibited in March and September. Harvest moon has these peculiarities only in high latitudes, because the great variation in the angles is due to the elevation of the pole above the horizon. The elevation of the pole makes all circles running nearly east and west appear to curve southward at and near the point where they cross the meridian above us. A consideration of Figs. 26 and 27 will show that the difference in the angles with the horizon is due to the elevation of the pole above the horizon. no. The Moon's Motion North and South. Since the moon's path on the heavens is so near the ecliptic, she must vary in position north and south. Also, since she goes through 180 in about fifteen days, she must move from north to south much faster than the sun. This can be seen plainly by watching her risings and settings on successive evenings. Since full moons are always 180 from the sun, they are far north in winter, and longer above the horizon than in summer, when they are far south. Thus they illumine the long winter nights when they are most needed. in. Revolution of the Nodes. Let us suppose that the student, watching the moon, sees her approaching the ecliptic, and, aided by the entries of the almanac, "( in &," and "d> in y," identifies with some degree of ac- curacy a point where she crosses the ecliptic. If, a year or eighteen months afterward, he observes the moon at the same crossing, he will find that she crosses earlier, or farther west. Of course, the intersections, moving on a circle, will finally come round to the point from which they started. Now the moon seems to cross the ecliptic on the heavens, only because she is then crossing the plane of the ecliptic. Thus this revolution shows that the moon's nodes revolve on her orbit. It is completed in about nineteen years. Eclipses. 112. Umbra and Penumbra. When a luminous body is larger than a point, the shadows made by intercepting ASTRONOMY BY OBSERVATION. FIG. 28. its light consist of two portions. There is a dark central part which receives no direct rays from the luminary. This is called the Umbra. Surrounding the umbra, there is a fringe of less dense shadow, which receives direct rays from some portions of the luminous body, but not from all portions. This is called the Penumbra. 113. Shadows of Earth and Moon. The shadows formed by intercepting the sun's light consist of an umbra and a penumbra. The umbra belonging to the shadow of the earth or moon is cone-shaped, with the apex of the cone turned away from the sun. The pe- numbra increases in width with the distance from the sun (see Fig. 28). 114. Eclipses of the Moon. An eclipse of the moon takes place when- ever the moon passes through the umbra of the earth's shadow. So small an obscu- ration results from the moon's passage through the penum- bra, that it is not called an eclipse. In a total eclipse of the moon her whole sphere is still faintly visible, and is of a coppery hue. This is due to rays of light refracted by the earth's atmosphere, so as to fall on the moon. 115. Eclipses of the Sun. A total so- lar eclipse takes place in any part of the earth which is in the umbra of the moon's shadow. The umbra of the moon's shad- ow is so small that it does not often cause darkness over a part of the earth's surface more than a hundred miles in diameter, but by the moon's rapid movement this shad- Explanation of Solar and Lunar Eclipses. ow is carried forward over a long zone or band of territo- ry about a hundred miles in width. A total solar eclipse is a very striking event. The stars come out and the animals go to rest. A total eclipse of the sun is an inter- esting event to astronomers, because it gives them an opportunity of studying the sun's corona, of which an account will be given in the chapter on " The Sun." A partial solar eclipse exists in those parts of the earth which are in the penumbra of the moon's shadow. The moon's distance from the earth varies, on ac- count of the elliptical form of her orbit. If an eclipse of the sun occurs when the moon is at her greatest distance from the earth, her apparent diameter is diminished, and she can not entirely hide the sun from us. We have then what is called an Annular Eclipse of the Sun. The moon conceals the central part of the sun's surface, but around her dark body is seen a ring of brilliant light. A total eclipse of the sun lasts but a few minutes, and often only a few seconds. A partial eclipse lasts several hours. A lunar eclipse may be total for two hours, dur- ing which time the moon is crossing the umbra of the earth's shadow. 116. Frequency of Eclipses. Solar eclipses are more frequent than lunar eclipses. The reason can be seen by examining Fig. 28. In a lunar eclipse the moon crosses the cone of the earth's umbra. In a solar eclipse she crosses a prolongation of the same cone toward the sun, which is equal to a broader part of the cone. But in any one place there are more lunar than solar eclipses, because a lunar eclipse is seen wherever the moon is visible, while a solar eclipse is visible over a small ter- ritory. A total or an annular solar eclipse is, in any one place, an event of very rare occurrence. There has been no total solar eclipse in London since 1715, and there was none for more than five hundred years before that date. 117. Causes of Eclipses. Whenever the sun or moon is eclipsed, some parts of the sun, moon, and earth must be in line. It is evident they can not be in line, with the moon in the middle, except at conjunction, and therefore a solar eclipse must take place at the passage of the moon from old to new. The sun, moon, and earth can not be in line, with the earth in the middle, except at opposition, and therefore a lunar eclipse must take place at full moon. But opposition and conjunction do not always bring eclipses. The reason is evident : the moon is not always at the intersection of her orbit with the ecliptic when she is at conjunction or opposition. The moon passes the nodes, or, in other words, crosses the ecliptic, during every revolution round the earth ; and yet there is not always an eclipse. But the reason of this is also plain : the sun and earth arc not always in line with a node when she passes it. THE PLANETS AND THEIR MOTIONS, AND HOW TO OBSERVE THE At. 39 The moon's path on the heavens crosses the ecliptic at an angle of 5. In other words, the plane of her orbit and the plane of the earth's orbit intersect at an angle of 5. It is evident, from Fig. 29, that some parts of the FIG. 29. earth and moon may be in line, though not their centers, both just before and just after she passes a node. A line joining the points where the moon's path on the heavens crosses the ecliptic, passes through the nodes of the moon's orbit. When the earth and sun cross that line, twice a year, they " pass the nodes." Students generally understand the circumstances of eclipses better by studying a few records of eclipses from almanacs of successive years : 1882. May i /th, solar eclipse; November nth, solar eclipse. 1 883. April 22d, lunar eclipse ; May 6th, solar eclipse ; October I5th-i6th, lunar eclipse; October 3Oth, solar eclipse. 1884. March 2/th, solar eclipse; April loth, lunar eclipse; April 25th, solar eclipse; October 4th, lunar eclipse; October 1 8th, solar eclipse. 1885. March 1 6th, solar eclipse; March 3oth, lunar eclipse ; September 8th, solar eclipse ; September 23d, lunar eclipse. From these records the student sees there are two eclipse periods in every year evidently at the passage of the nodes by the earth and sun. These periods come earlier every year, a result plainly due to the backward movement of the nodes on the ecliptic. The record of 1883 can be explained by remembering that the earth and sun move very slowly from the node, while the moon crosses the heavens in a little less than fifteen days. In 1883 the earth remained at the nodes long enough for the moon to be eclipsed and then cross the heavens to eclipse the sun fourteen or fifteen days after- ward. The record of 1884 shows that the moon may partially eclipse the sun on one side, cross the heavens to be eclipsed herself fourteen days afterward, and then cross back to eclipse the sun twenty-nine days after- ward. The student will not be surprised to learn that the slow motion of the sun and earth, together with the rapid motion of the moon, renders it impossible for the two former bodies to pass the moon's nodes without an eclipse. From the record of 1882 it is clear that there is a period of less than six months between two passages of the nodes by the sun and earth. Therefore, if this event takes place just at the beginning of the year, they will pass again in less than six months, and will thus reach the beginning of the third eclipse period within the year. When there are seven eclipses in a year, four must be eclipses of the sun. If there are but two, both are eclipses of the sun. There can not be more than seven or fewer than two. A further account of the moon will be given in the General Account of the Solar System, Part II. CHAPTER VI. THE PLANETS AND THEIR MOTIONS, AND HOW TO OBSERVE THEM. 118. The planets are stars which, like the earth, re- volve round the sun as a center. Five of these bodies, Mercury, Venus, Mars, Jupiter, Saturn, can be seen without a telescope ; and the last four are so often in the sky above us just after dark, that there is no diffi- culty in knowing them by sight, and in seeing and un- derstanding their motions. (See Art. 187.) It is the object of this chapter to give such an intel- ligible account of these movements, in the order in which an observer will see them, that any sensible student may use it to gain what will be a life-long pleasure and ad- vantage, viz., the power to look at the changes in the heavens with intelligent recognition. 119. The Superior Planets. For reasons which will be made clear a little further on, the planets are divided into two classes, called Superior and Inferior Planets. Jupiter, Saturn, and Mars are the superior planets which can be seen with the unaided eye. 120. How to find Jupiter, Mars, or Saturn. Except when they are very near the sun, Jupiter, Mars, and Sa- turn can be seen at some hour of every clear night. The student's knowledge of them will probably be chiefly gained by watching them in the evening before bed- time, and it is well to form a habit of observing them at dark. These planets come into the evening sky from the east. It will give reality to the study of them if the stu- dent who begins this chapter will get a good almanac, and find whether they are, or when they will become, evening stars. Three steps are to be taken: I. To find out when they are evening stars. 2. The student must ASTRONOMY BY OBSERVATION. know where in the heavens to look for them. 3. He must understand how they can certainly be known when seen. A few words will be said on each subject. 121. I. How to tell when a Superior Planet is in the Evening Sky. They come in at opposition, they go out at conjunction. The dates of the oppositions of Jupiter, Mars, and Saturn are given in any good astronomical almanac under the following symbols ; 8 means opposition ; O, the sun; 8 H O, opposition of Jupiter with the sun ; 8 T <> O, opposition of Sat- urn with the sun ; 8 $ O, opposition of Mars with the sun. The publishers of this book will send out with every copy sold a printed slip containing any oppositions coming within three years from the current year. The oppositions of Saturn come once in about a year; those of Jupiter, once in thirteen months ; those of Mars, once in twenty-six months. About two months before opposition, a superior planet can be seen in the evening as early as ten o'clock. For this reason it is best to begin observation about two months before opposition. The planets are not called " evening stars " until they rise at sunset ; and they are called " morning stars " from the time that they rise with the sun. The best almanacs give the times of risings of the planets. 122. II. In what Part of the Sky must these Planets be looked for ? They are always found in the zodiacal constellations, and very near the ecliptic. They are never more than 2^/2 from the ecliptic. ^ j The student who knows the constellations of the zodiac can tell almost at a glance whether Saturn, Jupiter, or Mars is visible. An unknown star of the first magnitude seen very near the ecliptic by a person familiar with that circle is sure to be a planet. Intelligent acquaintance with the heavens is not possible without the knowledge of the ecliptic. In the introduction to this book, " Directions how to learn and teach the Constellations," will be found careful directions for learning the ecliptic. All the zodiacal constellations visible at one time can be learned in a few evenings. It is perhaps well to add a few words about each superior planet, and to say that they can be most easily identified at and near opposition, both because they are then brightest, and be- cause we know we must look for them at dark not far from the eastern horizon. Jupiter can be recognized at any time of night when he is known to be above the horizon, because he will be the brightest star visible unless Venus is present ; and he can easily be distinguished from her by the test of planetary motion, which will presently be explained under III. Near opposition, Mars will be as bright as any star in the east except Jupiter. It is easy to know whether Jupiter is visible, and then Mars can be distinguished by his red color. Mars varies in luster more than any of the three, and could hardly be identified by an inexperienced observer when he is more than eight months from opposition. Saturn varies less than any of the three. An observer must find him chiefly by knowing the zodiacal stars near the ecliptic, and thus recognizing a stranger among them. He is about as bright as the brighter first-magnitude stars. 123. III. How the Planets can certainly be known when seen. When the observer thinks he has found a planet, or wishes to distinguish between two or three stars, one of which he supposes to be some planet, he can make perfectly sure by the test of planetary motion now to be described. The stars are divided into planets and fixed stars. The fixed stars can never be seen to approach' or recede from one another except by trained observers using instruments of the most delicate and extreme accuracy.* For this reason the constellations keep the same figures, coming and going as if painted on a rolling pano- rama. But the planets recede from one star and approach another. Our study of them consists chiefly in watching this motion. It can not be done without care and patience, but, when the student gets fairly at it, it becomes very interesting work. When the student first sees a planet, or a star supposed to be one, he notes very carefully its position in regard to fixed stars near it. Very often it will be found in line with two other stars, as seen in Fig. 30. In. this case any movement will at once be indicated by the planet being out of the line (breaking line), unless the line is nearly parallel with the ecliptic. Sometimes it forms regular figures, as triangles, right or isosceles, and then movement shows FIG. 30. itself very soon by the alteration of the figure. The motion of Mars can usually be detected in forty-eight hours ; that of Jupiter in a week or less. Jupiter and Venus can be readily distinguished from one another by the rapidity of the move- ment of the latter, which becomes evident after an interval of twenty-four hours. Saturn moves more slowly than any of the planets seen with the unaided eye ; but the movement can al- ways be detected when his position in relation to other stars is well observed. All this observation requires perseverance, but the student should be encouraged by knowing that it affords valuable train- ing for a faculty much neglected in our schools, viz., the power of intelligent observation. As fast as a planet moves out of one figure that he has discovered, he seeks to find another. He notes the direction of the motion, and whether the planet is north or south of the ecliptic. A star which does not move among the stars can not be a planet. 124. Motions and Appearances of Superior Planets. There are two important motions which are the key to our knowledge of a superior planet. They are called the Synodical Revolution and the Sidereal Revolution. 125. The Synodical Revolution. Let us suppose the observer begins at the planet's opposition with the sun. It will be found at dark just above the eastern horizon ; and, as the sun is known to be just below the western * The trained observers have detected the motion of only a few. THE PLANETS AND THEIR MOTIONS, AND HOW TO OBSERVE THEM. horizon, the planet is seen to be opposite the sun. If it is on the ecliptic, it is in a straight line with earth and sun; but since it is always on or very near the ecliptic, it is always very nearly in line at opposition. Since the planets, like all other heavenly bodies, have an apparent diurnal revolution owing to the earth's rota- tion upon her axis, we must, to avoid confusion, watch the synodical revolution at the same hour of the even- ing. Some time about dark is most convenient. The observer, beginning at dark, finds the planet then above the eastern horizon. If, after an interval of three or four weeks, he observes the planet at the same hour, he will find it, like the constellations of fixed stars, situated farther from the eastern horizon and nearer the western. If, at intervals of two or three weeks during several months, he takes a look at dark, he will again and again find the planet, like the fixed stars, farther from the eastern horizon and nearer the western ; and finally it would last be seen at dark just above the west- ern horizon. The observer would have reason to think that after its disappearance it was on the western horizon with the sun at sunset ; and, as it was always seen very near the ecliptic, it would be evident that it must at the same time be very nearly in line with earth and sun. This is the planet's conjunction with the sun. If, after this, the student made observation just before day in the morning, the planet would be found just above the eastern horizon. If, during some months, it was oc- casionally observed at that hour, it would gradually be found farther and farther west, until it would finally be seen for the last time just above the western horizon. Then it would again be opposite the sun, or " in opposi- tion" with him. It could again be seen at dark in the evening just above the eastern horizon. Between the two oppositions it would appear to have made a com- plete revolution westward around the earth. This revo- lution would have been so like the annual motion of the fixed stars, that the observer would EAST I suspect that it was also appa- rent, and due to the earth's annual motion around the sun. (See " Talks with Observers," p. 82.) But the fixed stars revolve round the earth in a year, crossing the evening or morning sky in six months, while Saturn would take a few days more than twelve months to complete his revolution ; Jupiter would take thirteen months and Mars twenty-six. 126. Real or Proper Motion. The difference in the times of apparent revolutions of the planets and the fixed stars would be explained by the planet's motion among the stars, which the observer would have seen while watching the synodical revolution. It has already been noticed in 123. The planets move eastward among the stars. It will perhaps sound a little absurd to speak of a planet as having two motions, one of which is east- ward and the other westward in direction. If the western motion were not apparent, it would be absurd. The student can form an idea how the two motions ap- pear to go on together, by imagining the celestial sphere revolving and carrying the planet west, while the planet at the same time moves east on the sphere. Thus, a globe might revolve in one direction, and carry an ant with it, while the ant walked slowly round the globe in the contrary direction. We 'have reason to think the eastern motion of a planet among the stars a real motion, for the earth herself moves east, and so could not make the planet appear to move east. Now the planet would, at opposition, be at a point among the stars, and when this point had in six months revolved to the western horizon, the planet would be at some distance east of it, and would thus be kept longer above the western horizon. Since Mars moves east among the stars faster than Jupiter, and Jupiter than Saturn, Mars would be detained longest in the evening or morning sky, and Jupiter would be delayed longer than Saturn. The apparent western revolution of the superior planets round the earth is called a Synodical Revolu- tion. This is usually defined as a period at the begin- ning and end of which a planet occupies the same position in regard to the earth and sun. 127. Variation in Apparent Size, and Definition of a Superior Planet. If the planet supposed to be watched were Mars, our observer would see another fact very plainly. Mars evidently diminishes in apparent size when crossing the evening sky, and increases while cross, ing the morning sky. Jupiter and Saturn also vary through the same period, but the change is not so marked. FIG. 31. -WEST Let us suppose that the line above (see Fig. 31) repre- sents that drawn from east to west through earth and sun at sunset. S is the sun's place ; E, the earth's place ; and c, the point of the earth's orbit lying west of the sun. At op- position the planet was east of the observer at sunset. As he was evidently farther from the sun then than the earth was, we mark his position at P. For the same reason we put the mark / for his position in the west at conjunction, beyond the sun and also beyond the point of the earth's orbit. Now, if the planet was at the point marked />, it explains why he decreased in apparent size. ASTRONOMY BY OBSERVATION. While P and / are at the same distance from the sun, they are at very unequal distances from the earth at E. When the planet is at P, the observer at E is on the side of the earth's orbit nearest him ; but when he is at /, the observer at E is on the side of the earth's orbit most distant from him. From E to e is a diameter of the earth's orbit, a line more than 185,500,000 miles long. Therefore, Mars appears larger when seen at P than at /, because he is more than 185,500,000 miles nearer. Thus this increase of the superior planets in apparent size confirms the belief that they are at all times farther from the sun than the earth is. This is what we mean by a superior planet. It is always at a greater distance from the sun than the earth is. At opposition the planet and earth are in line with each other and the sun, on the same side of the sun ; at conjunction, on opposite sides. The student must use this line to get the positions in nature, as he can see them. He should make sure of this by pointing to them. The diagram is useless except for this purpose. The fact that Jupiter varies less than Mars in ap- parent size, and Saturn less than Jupiter, can be accounted for by supposing that the two latter are so very far from earth and sun that an approach of 185,500,000 miles nearer is comparatively small. 128. The Sidereal Revolution. A planet always lies in a straight line between the sun and some star-group, since stars are all round the sun ; but at the opposition of a planet we can know the star-group, for we are our- selves in line between it and the sun. At dark a line from the sun, which is just below the western horizon, through the earth, reaches an ecliptic point just above the eastern horizon, and at opposition the planet is seen at dark very near it. We can see that he is in the same direction from us that we are from the sun at sunset. If we sit up till midnight we see an ecliptic point on the meridian, and the planet. at opposition is very near it. The four or five stars nearest the planet and this point are a group. As we are between this ecliptic point and the sun, we are between the sun and this group. So is the planet, though he seems to be among the group, but we know this is the effect of projection. So we can be quite sure the planet is in line between the sun and this group. If we are not quite in line with the sun and planet, we are between the sun and the same star-group. After a year's time the same star-group would be above the eastern horizon at dark, and we should again be between the sun and this group, but the planet would not be there. He would have moved east and would be below the horizon. But after a while he would again be above the eastern horizon at dark, and, of course, op- posite the sun. He would be between the sun and a second star-group. We should know it, for we also should be between the sun and the same group. We should see the first and second group both lying on the evening sky, the ecliptic running through them, and the second group farthest east. Then, after a long interval, the planet would again be in opposition. We should see him between the sun and a third star-group. The three groups would lie on the evening sky along the ecliptic, the last one being farthest east. The two in- tervals between the three would be about equal. After a number of oppositions, there would be a chain of these groups, at equal intervals, extending entirely across the evening sky and along the ecliptic ; and we should know that our planet had been between the sun and every one, for we should have been there at the same time. After a still longer time, the star-groups would make a ring round the whole heavens, both above and below the horizon. We should know that the planet had been between the sun and every one. As they would evi- dently lie in a ring all round the sun, it would be evi- dent that the planet had made a complete revolution round the sun (see Fig. 32). Now, unless the annual motion of the stars is real Fie. 32. G- OPPOSITIONS of unless the earth is at rest, and the sun and stars arc in motion it is clear our supposed planet revolves around the sun. This revolution of a planet from one star-group THE PLANETS AND THEIR MOTIONS, AND HO IV TO OBSERVE THEM. 43 back to the same is called the planet's Sidereal Revolu- tion. 129. It would take Jupiter nearly twelve years to make a sidereal revolution, and it would take Saturn nearly thirty. But the case is quite different with Mars. His motion among the stars is very rapid, and between two successive oppositions he would be seen moving rapidly east through many zodiacal constellations. But when the second came, he would be only 48 in ad- vance of his first position. It is a reasonable conclu- sion that Mars makes a full sidereal revolution, and goes 48 beyond, while he makes one synodical revolu- tion. (See " Talks with Observers," p. 82.) 130. Times of Revolutions. The student of geometry knows that we can not estimate circular or angular mo- tion except from the center. But we are not at the center of the motions of the planets. Fig. 33, in which E represents the earth's position, and /, /', p", p"', posi- tions of a superior planet, shows that we could not see FIG. 33. the planet where it would be seen by an observer at the center, except when we are in line with it and the center. This is the position of sun, earth, and planet at conjunc- tion and opposition. But we can not see the planet at all at conjunction. It is this that makes opposition so important a period in the study of a planet. Now, when a sidereal revolution begins at an opposi- tion, it does not end at one. We know the planet has completed its revolution because we see it at an oppo- sition, in advance of the point from which it started. Therefore, we can not estimate the length of a sidereal revolution by counting days. But we can count the days of a synodical revolution, and measure the angular distance traveled. Since the synodical revolution de- pends on the motion of the earth and planet both, and neither moves through exactly equal spaces in equal times, both the days and distance vary a very little. But we can take the average, or mean, of many obser- vations. Then the length of a sidereal revolution can be found by a question like the following : If the mean motion of Mars is 360-)- 48, or 408, in 779 days, how long will it take Mars to travel 360? The answer, 687 days, is the period of the sidereal revolution of Mars. 131. Discussion of Appearances. The apparent western revolution of the planets is much more con- spicuous than their real eastern motion, because in the first case they move from east to west of ourselves. But the whole matter will be made clearer by consider- ing the appearances produced by motion on earth. If we walk past an object at rest, it appears to move in a direction contrary to our own, and at last passes out of sight. But let us suppose ourselves walking on a straight road, and seeing in advance of us a person walking more slowly in the same direction. He seems to fall back, and we catch up and pass him, but he remains in sight longer than the object at rest. If he quickens his pace, still, however, walking more slowly than we do, he still appears to fall back, but more slowly, and he keeps longer in sight of us. 132. The fixed stars are objects at rest, and Mars, Saturn, and Jupiter are like persons walking in the same direction with ourselves, but at different degrees of speed, round a circle. When a planet and the earth come in line at opposition, it is because the earth, moving east faster, catches up with the planet and passes him. When they come in line at conjunction, it is because the earth has traveled 180 ahead of the planet, and is coming round the circle behind him, to catch up and pass him again. But the earth's motion is without jolt, jar, noise, or exertion on our part, and we do not feel that we are moving, so we take the appearance of the planet falling back to be the real motion. 133. Apparent Retrograde Motion. This is a very important apparent movement; but the description of it has been postponed in order to avoid confusion. Thus far all the facts and appearances can be explained almost equally well on the supposition that the earth is at rest, while sun, planets, and stars move round it with vary- ing degrees of speed, as on the supposition that the sun is the center of motion around which earth and planets revolve. But it is otherwise with the interesting move- ment about to be described. It can be better explained on the theory of the earth's motion. This movement had great influence in making astronomers believe th? 44 ASTRONOMY BY OBSERVATION. Copernican theory, by which the sun is supposed to be the center, and the planets to revolve round him. 134. For a few weeks before and after the opposition of a superior planet, it seems to move west among the stars instead of east. If we supposed that this motion was real, not apparent, we should see, by the advance east among the stars at opposition, that the planets must travel longer and farther east than west. But the move- ment is apparent, not real. 135. The student will best understand the cause of this apparent western or retrograde motion by an ex- periment. On ground, level and open, for fifty feet ra- dius, he must draw two concentric circles with radii of about twelve and eighteen feet. He is to have an as- sistant who walks slowly around the outer circle, while he himself walks around the interior circle, a little fast- er, but in the same direction. At intervals he comes up in line with his assistant and the center, on the same side of the center (or on the same radius), and passes his assistant. While walking on all parts of the circle, he notes the passage of his assistant's head over the back- ground. The apparent and real directions of the walk- er's motion on the outer circle are the same, until just be- fore the two come in line (on the same radius), and then the observer sees his assistant's head retrograde over the background, or move in a direction contrary to that in which the observer knows he is really moving. The position of the three, when in line, is that of S, E, and /, in Fig. 33. This is just the position of the earth and a superior planet at Fia 34 ' opposition. They are in line on the same side of the sun. There is noth- ing mysterious in this. When two walkers on straight paths walk with different degrees of speed in the same direction, the one in advance walking more slow- ly, and the one in the rear catching up and passing the other, the fast walker will see the body of the slow walker move over a distant enough background in a direction contrary to that in which he really walks. In the circles of Fig. 34, the arrows indi- cate the movement of revolution, and it can be seen that a mover at and near a is not moving in the same abso- lute direction with a mover on the outer circle, except when he is at or near b. The reason why the motion of the planet retrogrades near opposition only, is, the earth and planet are not moving in the same direction, except at and near that time. (See Note on Art. 135, p. 83.) 136. There is another point to be noticed. Jupiter retrogrades through a smaller angular distance than Mars, and Saturn than Jupiter. If the experiment with the circles is tried as before, except that the outer cir- cle has a radius of twenty-five or thirty feet, it will be found that the walker on the outer circle retrograde? through a smaller angular distance than before. 137. To explain this retrograde motion, we must make one of two suppositions: i. The earth moves round the sun in a figure nearly a circle, and, catching up in line with the superior planets, makes them seem to move backward, like the walker in the experiment with the circles. 2. The earth is really at rest, and the plan- ets are in motion, but the planets, for no reason at all that we can conceive, imitate the movements they would ap- pear to have if the earth moved. It is clear that the first is the more reasonable suppo- sition. 138. Several conclusions follow: i. This motion of the planets toward the west is apparent. 2. Since the earth's motion gives the planets an apparent motion among the fixed stars, while the fixed stars themselves are too far away to change place among each other in consequence of the earth's motion, the planets must be a great deal nearer to us than the fixed stars. 3. Mars is nearer to us than Jupiter, and Jupiter than Saturn. 139. Summary of what the Student should observe. -The student will probably think that, if he must watch Jupiter twelve years and Saturn thirty, to become satis- fied that they revolve round the sun, he will have to be contented with a blind acceptance of the statements of school-books. In astronomy, as on other subjects, we accept the evidence of witnesses to fact. But on this, as on all sci- entific subjects, there is a great difference between a blind and an intelligent acceptance of testimony. We must know something of the nature of the facts, or we can not tell whether they are rightly interpreted. The motions here described consist largely of repetitions. But we can understand them perfectly, long before we go through with all the repetitions. But the student who learns to look at the heavens with intelligence, will see a great deal more of the repe- titions than he intends at first. The heavens are unrolled before us year by year, without any exertion on our part, and just at the time when we are most at leisure. THE PLANETS AND THEIR MOTIONS, AND HOW TO OBSERVE THEM. 45 We must shut our eyes, or look down, to keep from see- ing them. Thus the person who learns to understand the changes in the heavens is apt to become an observer for life. The following phenomena should be observed: i. The positions of one or two of the planets in regard to earth and sun should be noted near conjunction and at opposition, and thoroughly understood. 2. The de- crease in brilliancy from opposition to conjunction should be noted. This is best seen in the case of Mars. 3. The general eastward motion of all these three planets should be noticed ; also, the western or retrograde mo- tion among the stars near opposition should be noted. 4. The positions among the stars for two successive oppositions should be noted, in the case of Jupiter and Saturn especially. This shows the advance eastward. 5. The apparent motion toward the western horizon should be noted, and especially the period during which each planet is in the evening sky. 6. Besides this, the positions of the planets in regard to the ecliptic must be noted. But the student must clearly understand how much, how little, this proves. It shows with entire truth that the orbits of these planets are in planes very near the ecliptic. But it does not shoiv the exact planes in 'which they move. As we are sometimes a little to one side of the planes in which they move, sometimes a little to the other side, we see them a very little dis- placed. But it is perfectly correct to say that they are always so near the ecliptic because they move in planes differing so little from that in which the earth moves. In no way can the student understand this so well as by watching their position in regard to the ecliptic. (See Note on Art. 139, p. 91.) Tlic Inferior Planets. 140. Definition of an Inferior Planet. Venus and Mercury are never seen near the point of the heavens opposite the sun. If they were not nearer the sun than the earth is, the earth would come nearly between them and the sun at some point of her revolution round that center, for they are always found very near the ecliptic. But we always see them not far from where we know the sun is situated. For this reason, and for others which will appear in studying them, they are supposed to move round the sun in orbits interior to the earth's orbit, and astronomers call them the " Inferior Planets." The student's knowledge of their motions must be chiefly gained from the study of Venus. Venus is very often situated conveniently for observation. 141. When and how to find Venus. Venus is alter- nately evening and morning star for periods of 292 days each. She becomes evening star at what is called her superior* conjunction with the sun, and she becomes morning star at her inferior * conjunction with the sun. The symbol of Venus is ? ; that of conjunction, p ; that of the sun, Q. The entries in the almanac are " P 9 O superior " and " P ? O inferior." They are found at the proper dates. Besides these announcements, most almanacs have, in the beginning, another, giving the evening and morning stars for the year. When Venus is known to be above the horizon in the evening, it is not possible to fail in identifying her, for she is found at dark in the west, and is far the brightest star visible. But she is too near the sun to be visible for some time after conjunction. The length of the delay depends on the angle made by the path of Venus with the horizon, and on clear weather. She should be looked for three weeks after conjunction, and after that once a week, at least, until she is found. If the student prosecutes diligent search, she will be seen at first so early after sunset that no other stars will then be visible. The observer should note and remem- ber the point of the horizon above which he first sees her. 142. Diurnal Revolution. On the evening when Venus is first seen she will set in the west just as the new moon does. As all the other heavenly bodies do this, it will be plain that the motion is apparent and due to the earth's axial rotation, On the next evening she will again be seen in the west, and it will be evident that she must have risen on the same morning a little later than the sun, and during the day revolved (invisible) across the sky, keeping near to the sun on his eastern side, and becoming visible when he has set. 143. Real Motion. After a few weeks the observer would see that, at the same hour of the evening at which Venus was first observed, she was higher above the horizon and farther east. It would be clear that this was due to a motion of the planet, real or apparent. As the earth herself moves, not west, but east, she could not make the planet appear to move east. Thus it would be plain that the motion of Venus from the horizon was her real or proper motion. From this it would also fol- low that she must have come into the evening sky by rising above the western horizon, and must have caught up with and passed the sun. It would be plain that her real motion was faster than the sun's in the zodiac,f and that she must therefore move faster than the earth. When she thus passes the sun, she is at conjunction. After a long time, Venus would be far enough east at sunset to be visible when the stars were seen, and she would be found moving east among the stars. Just as soon as he could, the student should fix her place accu- * This use of the words superior and inferior will be understood in study- ing the planets' motions. f See Art. 47. 4 6 ASTRONOMY BY OBSERVATION. rately in relation to the fixed stars, so that he could de- tect her motion among them. She moves so rapidly that her changed position would be evident the very next evening. This rapid motion makes it very interest- ing work to trace the path of Venus among the stars. 144. Venus and the Ecliptic. One point which should be noticed in watching her is the position of her path in relation to the ecliptic. She keeps very near it, and the student should note whether she is north or south of it. 145. Elongations, etc. The observer, watching Venus, would see her continue to get higher above the horizon, and farther east among the stars. The earth's motion in her orbit would cause many constellations in which Venus had been seen to go out of sight behind the west- ern horizon. Finally, 219 days after superior conjunc- tion, the almanac would contain the notice " ? gr. El. E.," or the greatest Eastern Elongation of Venus. That is, Venus would have reached her greatest height above the western horizon, and would afterward begin to ap- proach it. Venus would be nearly half-way between the zenith and the horizon at the earliest hour at which she could be seen on the evening of her greatest elonga- tion. Lines drawn from the observer's eye to ..Venus and the sun would make an angle of about 47. The angle varies a very little at different elongations, but the almanacs usually give the exact number of degrees. It would be a good many days before the ordinary ob- server without instruments would perceive that Venus was getting nearer the horizon. After a time it would be very plain. But Venus would continue to move east among the stars until about 270 days after superior con- junction, when she would begin to move west among the stars, and would continue to do so while she remained in the evening sky. Finally, 292 days after Venus be- came evening star, the almanac would announce, " f ? O inferior," and Venus would pass the sun again as she went down. But the observer would know the exact time by the almanac only, for he would see the last of Venus some days earlier. The observer should note the point of the horizon above which Venus was last seen. It would perhaps be a little north or south of the point above which she was first seen. Thinking of her motion in relation to the horizon only, and forgetting the movement among the stars, it would seern to the observer that the figure of her motion was a long half oval with its base resting on the horizon. The path of Venus up and down is oblique to the horizon, since it lies very near the ecliptic ; and therefore the half-oval figure which she seems to describe slants northward or southward. 146. Increased Brilliancy. It would be very evident to a person who watched Venus closely, that she in- creased in brilliancy from the time she became evening star. She would be a splendid object when she left the evening sky. These facts would indicate that she was getting nearer to the earth all the time that she was evening star. 147. Explanation, or Theory. If the student will cut a circle of about six inches in diameter from rather stiff paper, he will see that he can hold it west of him in such a position that it will look like a long oval, or even like a line. When held in this position, a body supposed to move around its circumference would be much nearer to the observer when on one side of it than when on the other. If the body moved in the direction in which the earth revolves round the sun, it would be at the greatest distance from the observer when it was going up, and nearest him when moving down. Now the earth's orbit, which is always in the direction of the sun, is always west of us at sunset, and only half of it is above the horizon. If Venus revolves around the sun in an orbit interior to the earth's orbit, it might be situated in such a plane as to look like a long oval, of which half was above the horizon or even like a line. If Venus moves around it in the same direction in which the earth re- volves round the sun, she would seem to move in such a long semi-oval, first up, then down ; and when she passed the horizon moving up, she would be much far- ther from the earth and observer than when coming down. Thus this theory would account for her rising 47 and then coming down. And, as she would be get- ting nearer to the earth from the time that she rose above the horizon, her increased brilliancy would be accounted for. The fact that Venus is never seen oppo- site the sun would also be explained. It is because we never come between her and the sun. 148. Superior and Inferior Conjunction. If Venus moves in an orbit interior to the earth's orbit, and in the same direction in which the earth moves round the sun, she must be farther west than the sun is when she passes him coming up into the evening sky. And, also, when she passes him moving down out of the evening sky, she must be nearer to the earth than the sun is. As Venus is always very near the ecliptic, she must be very nearly in line with sun and earth in both cases. At su- perior conjunction, an inferior planet, the sun and earth are nearly in line, with the sun in the middle. At infe- rior conjunction, they are nearly in line, with the planet in the middle. 149. Phases of Venus. A telescope reveals some facts which strongly confirm this explanation of the mo- tions of Venus. If, at superior conjunction, the earth and Venus are on opposite sides of the sun, she has the same face turned to the earth and sun. This is exactly THE PLANETS AND THEIR MOTIONS, AND HOW TO OBSERVE THEM. 47 the position of the moon when she is full. Also, just after inferior conjunction, if she is then nearer to us than the sun is, she is in exactly the position of the new moon. Also, at her elongation, we see half of the side turned to the sun, just as we see the moon at her quadrature. Now, when Venus is observed through the telescope, she exhibits phases just as the moon does, except that she enters the evening sky a full circle and leaves it a crescent. But these changes proceed concurrently with the increase in apparent size. The full circle is much smaller than that of which the crescent forms part. Fig. FIG. 35. 35 shows the relative proportions of Venus at her supe- rior and inferior conjunctions. About a month before the inferior conjunction, there is an entry in the almanac, " $ at greatest brill- iancy." The brilliancy of Venus depends on two things, her apparent size and the amount of illuminated surface. At the date indicated, the combined effect of the two is greatest. The phases of Venus show that, like the moon, she shines by reflecting the light of the sun.* 150. Transit of Venus. The argument to show that Venus moves round the sun in an orbit interior to the earth's orbit is very strong without the aid of the facts learned from the telescope. It is irresistible with them ; but it is still further strengthened. If Venus is crossing the ecliptic at her inferior conjunction, she is seen to pass across the face of the sun like a small black ball. This is called a Transit of Venus, and it never takes place at superior conjunction. Two transits only oc- cur in a century, with an interval of eight years be- tween them. The last two transits occurred in 1874 and 1882. 151. Venus as Morning Star. After her inferior con- junction, we must look for Venus before sunrise in the morning sky. Her motions are the same as when she * Mars shows a little indication of phases. We do not, in certain posi- tions, see him a perfect circle. 7 is evening star, except that they occur in inverted order. The almanac records them in order. 152. Western or Retrograde Motion. The retro- grade motion of Venus among the stars has not the importance of the retrograde motion of the superior planets. It is not an apparent, or parallactic motion caused by our motion. If the student will take some small object, and revolve it in a circle between himself and the wall, he will see that in the parts of the circle nearest him and farthest from him, it moves in opposite directions against the background of the wall. Thus, Venus really moves up and down. We see both motions against the same background. One is in the order of the signs Aries, Taurus, Gemini, etc. and we are ac- customed to call this eastward and direct, so we call the other retrograde. 153. Synodical Period. Venus does not, like the su- perior planets, make an apparent revolution round the earth. But the period at the beginning and end of which she occupies the same position in regard to the sun and earth is called her synodical period. She does this at successive superior or inferior conjunctions, but she is too near the sun for us to see her. At her elongations, lines drawn between the sun, the earth, and Venus form a right triangle. The reader will see that this must be so, by examining Fig. 36. When the planet in the figure is seen at its greatest alti- tude, the line to the earth is a tangent to its orbit, and makes a right angle with the line from the planet to the sun. But Venus is at her great- est height above the ho- rizon at her elongations. So we can count her synodic period in days, by the time from her eastern or western elongation back to the same. The average time is 584 days. This is also the number of days be- tween her superior conjunctions. She is evening star and morning star 292 days each (on an average). (See Note, p. 51.) 154. Sidereal Revolution. We learn the period of the synodical revolution, as given above. Circular motion is measured from the center; but we are not only not at the center of the orbit of Venus, we are wholly out of it. But at her conjunctions, we can tell where she would be seen by an observer at the sun. At superior conjunction, when she is on the western side of the sun at sunset, an observer at the sun would see her where we do, on the western horizon. But at inferior con- junction, when she is between us and the sun at sunset, CARTH 4 8 ASTRONOMY BY OBSERVATION. an observer at the sun would see Venus just 180 from where we do, viz., on the eastern horizon. Therefore, but for the motion of the constellations, we would say Venus had traveled 180 while evening star. But it is evi- dent that Venus passes through every constellation that passes the sun while she is evening star. That is, Venus, while evening star, travels as far as the sun and 180 360 besides. The sun's mean daily journey is z , or .986, 3 6 5/4 and his mean journey in 292 days is 288. Therefore the mean distance traveled by Venus in 292 days is 180 + 288, or 468. Her mean distance in one day is 1.6, and she would travel 360 in 225 days. This is the period of the sidereal revolution of Venus. (See Note, p. 89.) 155. Mercury. The synodical period of Mercury occupies 1 16 days, though his sidereal revolution takes only about 88 days ; therefore, he is evening star every alternate 58 days. This event is recorded in the alma- nac by the symbols " 6 5 O sup." But though he comes so often, there are a good many difficulties in getting a sight of him. He is never more than 28 from the sun at his greatest elongation. His orbit differs from a cir- cle more than that of any other planet ;* and, when he is at perihelion and elongation at the same time, he is much nearer the sun. Besides, he gets farther from the ecliptic than any other planet.* The most favorable times for seeing him are when he is evening star in the spring, and morning star in the fall. He is very bright when he is in the field of vision, and can not be mistaken. He, is of course, to be looked for near the horizon and ecliptic. By watching the almanac in spring and fall, for his two conjunctions, the industrious student will be sure to see Mercury in the end. The motions of Mercury are in all respects similar to those of Venus. He moves more rapidly. He does not vary in apparent diameter nearly so much as Venus, be- FIG. 37. Phases of Mercury, and its Comparative Size as seen at Different Times. cause the diameter of his orbit, which measures the vari- ation of his distance from us, is much less than that of Venus. (See Fig. 37.) * Except the asteroids, to be hereafter mentioned. CHAPTER VII. THE ATTRACTION OF GRAVITATION. 156. After learning something of the motions of the planets, it is natural that we should desire to know the force which causes them to move in their orbits. The mere fact that the planets move, does not need to be accounted for. If we roll a ball on a slightly rough surface, it continues to move after the hand is removed, but finally stops. In proportion as we lessen friction and the resistance of the air, its motion lasts longer. Therefore, it is believed that a body moving where there is no friction and no resistance of the air would keep on moving forever. The planets are placed where there is no friction and no resistance from the air, for they carry their atmospheres with them. The continued motion of the planets is regarded as a final proof of the law of iner- tia, viz., a body at rest will continue at rest until some force sets it in motion, and a body in motion will con- tinue to move until some force stops it. But on earth bodies in motion always move in straight lines unless some force deflects their motion. If they move in a curve, they change direction continually ; and therefore the force must be constant, not instantaneous, in operation. It has always been known that some force draws bodies toward the center of the earth, and that they move in straight lines unless some other force acts on a falling body, when it moves in a curve. Thus we may throw a stone horizontally, but, since the attraction of the earth affects it, it falls in a curve. If we revolve a key fastened to the end of a string, we project it in a straight line, as is shown as soon as we let go the string. The force of cohesion in the string, holding it to the center, makes it move in a curve. We have proof that the earth's attraction may be modified, for, when we revolve a bucket of water rapidly, the water does not fall out. So we may surmise that the moon is kept from falling to the earth by her rapid motion, and that the earth's attraction makes the motion a curve. 157. Now the great mathematician, Sir Isaac Newton, proved that the attraction of matter is the force which makes the heavenly bodies move in curves ; but his ar- guments were nothing like these, though he took for granted the known laws of motion in his reasoning. 158. Before the time of Newton, astronomers ob- served the heavenly bodies as we have described in the previous chapters, except that they had instruments for measuring angles, and they subjected everything to exact measurement. They measured the apparent diameters of sun and moon, the angles which inferior planets make THE ATTRACTION OF GRAVITATION. 49 with the sun at their elongations, they measured the angles through which the superior planets retrograde, and a great many other things. -Since Jupiter and Sat- urn revolve very slowly, and since it takes much repe- tition of observation to insure accuracy of result, the reader can see how much slow, patient, unobtrusive work somebody did before anything definite could be known. An astronomer called Tycho Brahe collected a vast mass of accurate information. He was fortunate enough to have some property, and to find a munificent King of Denmark, who built an observatory called Uranienburg, on an island for him, and gave him a com- fortable salary, so that he could watch and measure the heavens in peace and quiet. Me kept at it for more than twenty years. 159. Then an astronomer called John Kepler took Tycho's measurements, and, after a great deal of hard trying and thinking, came to certain very definite results about the times, distances, and orbits of the planets. They are called Kepler's Three Laws, and they are given below : 1. All the planets move from west to east in ellipses which have the sun for a common focus. 2. The radius vector of a planet describes equal areas in equal times. (The radius vector is the moving line from the planet to the sun.) This is illustrated in FIG. 38. Illustration of Kepler's Second Law. Fig. 38. In order that this may be true, the planets must move fastest when near the sun. 3. The squares D of the periodic c times (sidereal rev- olutions) of the planets are to each other as the cubes of their mean dis- tances from the sun. Now, philosophers did not consider it proved that the attraction of matter causes the planets to move in curved lines until the result of the motion had been accurately measured, and the force estimated in numbers, and shown to be mathematically equal to producing the result. To reason otherwise would be as absurd as to pronounce a piece of carpeting exactly sufficient to cover a floor, without measuring the floor and the carpeting. When, by the help of Tycho Brahe and Kepler, the times, the distances from the sun, the orbits, were accurately measured, then, and not before, it was time for some- body equal to the task to try to make a mathematical estimate of the force. 160. Sir Isaac Newton then took up the problem, and by a long chain of difficult calculation belonging to the higher mathematics showed that all the facts would be accounted for by the following law : " Every particle of matter in the universe attracts every other particle with a force proportioned directly to the mass (or quantity of matter) and inversely to the square of the distance between them." When we consider that, besides the sun, there are eight large planets with their moons, of various masses, and at various distances from the sun, we see what a complicated piece of calculation it is to show that this law accounts for the deflection of their motions from straight lines. Sir Isaac Newton did not finish this prob- lem in its details, but he proved enough to make mathe- maticians quite confident that his solution was correct. There were certain irregularities in the motions which were not fully accounted for. These are called pertur- bations. Succeeding astronomers have largely supplied the details notably two Frenchmen, Laplace and La- grange and fresh facts, learned by observation, have aided in this work ; but there is still something to be done. 161. In 1846 there was a curious example of the way in which mathematicians have learned to reason. Some irregularities were detected in the motion of the planet Uranus. There was a very strong suspicion that some unknown planet caused these perturbations of Uranus, and two mathematicians, Mr. Adams, an Englishman, and M. Leverrier, a Frenchman, set themselves, unknown to each other, to calculation in order to find out the di- rection from which the disturbing influence must come. It was a remarkable evidence of their skill that both gentlemen directed attention to the part of the heavens in which the planet was found, and put astronomers to examining it with telescopes. The result was a double discovery, in which Leverrier had a little the advantage of time, and he is therefore called the discoverer of the planet, which was named Neptune. 162. Let us go over the steps of this statement. The merit of Copernicus was that he observed nature, just as is recommended in this book, and reasoned about these observations of himself and others. The merit of Tycho Brahe was that he went systematically to the work of taking mathematical measurements. This was a great step. Then Kepler applied these measurements to mak- ing a mathematical statement of the orbits, the times, the distances, the thing Newton was to account for. Then Newton estimated the force. This is regarded as one of the greatest achievements of the human intellect. 163. The Tides. Twice a day the waters of the ocean move a short distance over the boundaries separating ASTRONOMY BY OBSERVATION. sea from land, and .twice a day they move back. This motion is called the tides. When the water is rising, it is called " flood-tide," and when it is falling it is called " ebb-tide." The highest point reached is called " high water " ; the lowest, " low water." High or low water occurs about fifty-two minutes later every day. It has long been known that the tides depend on the moon's motions ; that they come later every day because the moon rises later. There is a vast swell, or tide-wave, on the half of the earth turned toward the moon. That we should account for this by the attraction of the moon is natural. But the waters not only rise on the part of the earth under the moon : they are at the same time high on the part of the earth turned from the moon, while they are low only between these opposite parts of the earth. We thus account for the high tide on the side of the earth turned away from the moon : The moon attracts most strongly the ocean nearest her, so she draws it up to a tide ; but she attracts the solid earth more strongly than the ocean most distant from her, so she causes a tide in that ocean by drawing the earth away from it. But since the moon attracts the solid earth equally from any side of it on which she is situated, the position of the solid earth does not oscillate as the moon moves, but is permanently affected by her. The attraction of the sun also affects the tides, but, since he is much farther from us than the moon, the effect produced is much smaller notwithstanding his greater mass. But sometimes the attractions of the sun and moon act in the same direction, sometimes in differ- ent directions. Twice a month the sun and moon are on the same or opposite sides of the earth, and then the tides are much higher. These are called spring-tides. Twice a month, when the moon is in quadrature, the attractions of sun and moon act at right angles to each other, and then the tides are lower than usual. These are called the neap-tides. The highest tides occur when sun and moon, or both, are most nearly vertical, and when they are nearest the earth. Thus they vary with every position of the two bodies. The tide does not accompany the moon and sun, but follows a little after them. This is the result of the in- ertia of matter, which can not at once be set in motion. The cause is the same that makes it more difficult for horses to start a wagon than to pull it after it is once set in motion. The tides vary in different places from terrestrial causes. These belong to physical geography rather than astronomy. 164. Refraction. Refraction is the bending of a ray of light in passing obliquely from one medium to another of different density. The subject belongs to physics, but it is necessary to mention the effect the refraction of the atmosphere has on the apparent positions of the heavenly bodies. The effect is much greater nearer the horizon, since it depends on the obliquity of the rays of light, which decreases as we approach the zenith. Its effect is to make bodies on the horizon appear higher than they are. In consequence of refraction we see the sun and stars after they are below the horizon. In the latitude of Nashville, Tenn., observers are indebted to refraction for a good view of the fine first-magnitude star Canopus, which is at that place very near the bound- ary of the circle of Perpetual Disparition. Astronomers, when making accurate observations, have great trouble with refraction. One trouble is that the effect varies with the density of the atmosphere, and thus it is sometimes difficult to allow for it. 165. Celestial Measurements. Lines joining the sun and earth with each other and with a planet or the moon must form one line or a triangle. If we can get the number of degrees contained in two angles of such a triangle, we can draw a tri- angle similar to it, as any student of elementary geometry knows. Also, the sides of the triangle on paper will have precisely the proportions of the triangle in the heavens. One of these sides represents the distance between the earth and sun ; another is the distance between the planet and sun. Thus, by measuring the lines on paper, and getting the ratio, we compare the dis- tances of the earth and the planets from the sun. In this way the proportions of the solar system were long ago learned.. The simplest example of such a triangle is given by FIG. 39. Venus and Mercury at their greatest elongations. The tri- angle, as seen in Fig. 39, is a right triangle. One of the other angles is very easily found by measurement. If lines be supposed drawn from the observer's eye to Venus and to the sun, they form an angle of the triangle repre- sented in Fig. 39. At the time of her greatest elongation, Venus can be seen at sunset with a good telescope, and thus this angle can be measured without difficulty. Nothing will give so much reality in the student's mind to such triangles as to note the positions carefully in nature at the elongation of Venus, and, getting the angle of elongation from a good almanac (which al- ways gives it), to draw a similar triangle on paper. If correct- ly drawn, the lines representing the distances of Venus and the earth from the sun have the proportion of two to three nearly. If we can get one of the sides of such a triangle in miles, it is evident we can get the others, since we can get the propor- tions existing between the lines. This is done by making the EARTH MAP III. For Study of the Stars from October aad to January 2oth. NORTH SCALE OF MAGNITUDES THE September aad at midnight, October 23d at ten o'clock, SOUTH HEAVENS AS SEEN November yth at nine o'clock, November 22d at eight o'clock. CAUTION. Bs sure to read the directions for using these maps given in the Introduction page 3 MAP IV. For Study of the Stars from April a6th to July 22d. NORTH SCALE OF MAGNITUDES & * 12345 CAUTION. Be sure to read the d rections for using these maps, g,ven in the Introduction, page 3. THE March 2 1st at midnight, April aoth at ten o'clock, SOUTH HEAVENS AS SEEN May 5th at nine o'clock, May 2 ist at eight o'clock. CELESTIAL MEASUREMENTS. earth's radius the side of a triangle. In order to understand ihis, it is necessary to discuss parallax a little. 166. Parallax. Let us suppose that we note the direction of some body from us by drawing a line to the center of its position from the center of ours. If we move off the line, the body is displaced on the background against which we see it. If we draw another line between the centers of the bases, it forms an angle with the former line. This change in the direc- tion of a body, as seen from two different positions, is called its parallax. The retrograde motion of the planets is a parallactic motion. The angle measures the parallax, or difference in di- rection. From the previous discussion of these apparent motions, the student knows that bodies undergo parallax in proportion to their nearness to us. The moon is the heavenly body nearest us. If two observers, at widely separated points of the earth, observe the moon at the same time, carefully noting her posi- tion on the starry sphere, so as to compare observations, they will find they do not see her against the same point of the sphere. Thus, the moon undergoes parallax, and, by measuring the an- gular distance between the two points of the sphere, we measure the moon's parallax. When the moon is on the horizon, a line from her to the observer is a tangent to the earth. The earth's radius forms a right angle with it. In Fig. 40 the right triangle moc has for FIG. 40. one side the earth's radius, oc, a. line of known length. The observer at o sees the moon in the direction omz. From the center of the earth the moon is in the direction cms. The dif- ference in the direction of these lines is called the moon's hori- zontal parallax. It is the difference made in the direction of a body by seeing it from the surface, instead of the center, of the earth. The angle smz, or its vertical angle cnto, measures it. This angle bears such a relation to the moon's parallax, ascertained, as has just been related, by observation, that when we know the one we can compute the other. Then, in the triangle cmo, we have two angles, and a side, co, in miles. We easily find another side, cm, which is the moon's distance from the earth's center, in miles. 167. The Sun's Distance from the Earth in Miles. In all the triangles formed by the sun, the moon, and a planet, the earth's distance from the sun is one side. If we know this in miles, we can, from the proportional triangles, find all the other distances in miles. Next to the moon, Mars and Venus are the heavenly bodies nearest us. Both undergo parallax when observed at widely separated stations on earth. When they are nearest to us, the parallax is greatest. Venus is nearest at her inferior conjunc- tion, but we do not see her when the stars are visible, so we have ordinarily no fixed object by which to measure the paral- lax. But twice in a century Venus makes a transit over the face of the sun, and astronomers then use the sun's face as a background on which to estimate the parallax of Venus. The points to be observed are, the first and last contact with the sun, and the distance from the center. When this parallax is known, the horizontal parallax of Venus can be estimated, and her distance from the earth learned, as the moon's distance was found. It is impossible to estimate the sun's parallax directly, but since the distance of Venus from the earth, and the sun's dis- tance from the earth, form sides of the same triangle, we can find the sun's distance by knowing that of Venus. The last transits took place in 1874 and 1882. The correct estimate of the sun's distance from the earth is so important that on both occasions the governments and scientific men of the civilized world interested themselves in fitting out expedi- tions to every quarter of the globe to make observations. There were years of preparation, in which new and accurate instruments were made, and even invented, and modes of in- vestigation studied. When Mars is nearest to us, he shows sensible parallax. He is nearest only at opposition. Every fifteen years the in- crease in the luster of Mars at opposition is much greater than usual. Fig. 32, page 42, shows that the oppositions of a superior planet take place at different points of its orbit. This variation in brightness, therefore, leads us to suspect that its orbit is not a perfect circle. The orbit of Mars is very eccentric, and this is the cause of the remarkable variation in brilliancy. At long intervals Mars is at opposition when he is at the point of his orbit nearest the sun. Mars then rivals Jupi- ter in size. This occurred in 1877. As the apparent in- crease in the size of Mars was due to his near approach, it afforded a good opportunity for estimating his parallax, and from it that of the sun. This was done with great care in 1877. NOTE.' In the next and the succeeding chapters much use will be made of information learned by the telescope. In the Appendix B there will be found an account of the telescope, which is taken principally from Lockyer's " Astronomy." NOTE ON ART. 153, PAGE 47. Elliptical Orbits. In Fig. 36, the line joining Venus and the sun is the radius of the planet's orbit. It is easy to see that a variation in the length of that line would make the angle at the earth vary in size. The angle is that given in the almanacs at the greatest elongation of Venus ; and the almanacs of different years show that it varies. This proves that the line varies, and that the orbit of Venus varies a little from a circle. It is an ellipse. ASTRONOMY BY OBSERVATION. PART II. GENERAL ACCOUNT OF THE SOLAR SYSTEM. 168. The Solar System consists of the sun and the heavenly bodies revolving round him as a center. The revolving bodies are the planets, with their moons or satellites, revolving meteors, and comets. The earth, on which we live, is one of the planets, and therefore the Solar System is much more interesting to us than any other part of the heavens. Chapter VIII will treat of the Sun; Chapter IX, of the Planets ; Chapter X, of Meteoroids and Comets. CHAPTER VIII. THE SUN. 169. The sun * subtends an angle of 32', or a little more than half a degree. That is, it would take about three hundred and thirty suns, placed touching each other, to extend across the sky. The sun's diameter is 866,400 miles. This is nearly four times the radius of the moon's orbit, which is 240,000 miles. If, therefore, the center of the sun were placed at the center of the earth, the sun's circumfer- ence would be nearly twice as far from xis as the moon's orbit now is, and the sun's body would fill the whole intervening space (see Fig. 41). The sun's volume is 1,305,000 times as great as the earth's volume. But the sun is only about one fourth as dense as the earth. Fig. 42 shows the size of the sun as compared with the chief planets. The black * In this book the study of the sun has been deferred until after the dis- cussion of the motions of the solar system has been completed, in order to avoid a wide and long digression on the subject of the sun's telescopic ap- pearance and physical condition. To ordinary students the investigation of the motions is the most important part of astronomy, because not only can all persons see these motions in nature, all must see them, and not to under- stand any important part of the order of nature brought before our eyes en- courages habits of stupidity. circle represents the sun's disk. The mass of the sun is about 750 times that of all the planets and moons put together, and nearly 332,000 greater than the earth's mass. FIG. 42. FIG. 43. Relative Size of the Sun and Planet:. 170. Light. The surface of the sun is ninety thousand times as bright as would be a candle-flame of the same size (Prof. Young). It is a hundred and forty times as bright as the calcium or lime light, and com- pares with the volta- ic arc as three and a half to one. On the sun's disk the bright- ness is greater at and near the center than at the circumference. This is owing to the greater thickness of the sun's atmosphere, which a ray from the margin must penetrate in order to reach us. Rays from the center cross by a shorter line, as is shown in Fig. 43. 171. Heat. Only a small portion of the heat radiated by the sun reaches the earth. Prof. Young sa3 y s : "If THE SUN. 53 we could build up a solid column of ice from the earth to the sun two miles and a quarter in diameter, spanning the inconceivable abyss of ninety-three million miles, and if the sun should concentrate his power upon it, it would dissolve and melt, not in an hour, not in a minute, but in a single second." The Spectroscope. 172. In studying the physical constitution of the sun much use is made of an instrument called a spectroscope, and it is therefore necessary that the principles of its construction should be understood by the student. 173- When light from a luminous gas or vapor (which has not been condensed by high pressure) passes through a prism, and is thrown on a screen in a darkened room, it is found to produce a band, or spectrum, con- sisting of a bright-colored line or lines, separated by dark spaces. In Plate I examples of such spectra are given. Spectra like this are called Bright-lined Spectra. The various chemical elements can be brought to the state of vapor and made to produce spectra. Each substance has its own spectrum, different from all the others, and it always presents the same appearance when magnified to the same degree. The numbers of the lines in the spectra of the different elements differ greatly, and are of different colors, and thus each element can be identified by its spectrum. If a compound is used, its spectrum will exhibit the lines of all its elements. 174. When the light from an incandescent solid or liquid passes through a prism, the spectrum consists of bands of color containing no transverse lines. Gases condensed by high pressure give similar spectra. If, however, this light passes through a luminous vapor be- fore entering the prism, it will consist of a colored band with transverse dark lines on it ; and the dark lines will correspond exactly in position with the bright lines which would be contained in the spectrum formed by passing the light of the luminous gas through a prism. The position of the lines is due to the greater or smaller refrangibility of the rays producing them, and thus it is evident that the vapor absorbs rays having the same degree of refrangibility as those which pro- duce bright lines. This is called a Reversed Spectrum. 175. By comparing the positions of the lines in bright and dark lined spectra, and finding the lines which have the same position in both, the vapor causing the dark lines can always be identified. This is shown in Plate I, where the spectrum of light coming from the sun is compared with the spectra of light coming* from various chemical elements in the state of vapor. The knowledge of these facts has given rise to a new method of chemical analysis, of extreme delicacy and great accuracy, which has the further advantage that it can analyze substances by light which is brought from any distance, however great. Thus it can be applied to FIG. 44. A COLLIMATOR Arrangement of Prismatic Spectroscope. the light coming not merely from the sun, but also from the fixed stars. It has given rise to an entirely new branch of astron- omy. Spectrum analysis is thus far the great discovery in the lifetime of the present generation of middle-aged FtG. 45. people. The essential parts of a spectroscope (Figs. 44 and 45) are (i) the prism; (2) the collimator-tube, through which the light under study passes to the prism ; and (3) the telescope. The lens A collects the light, and the telescope is used to magnify the image. Sometimes the light is made to pass through several prisms. 176. Motion detected by the Spectroscope. The sensations of light and sound are both the results of wave- motion. The undulations of the air convey sound ; those 54 ASTRONOMY BY OBSERVATION. of the supposed ether convey light. A sound of high pitch is produced by the shorter waves, and a sound of low pitch by the longer waves. The shorter waves of ether produce the more refrangible colors of light ; the longer waves the less refrangible. Now a prism separates and arranges the colors on a spectrum, from red to vio- let, in the order of their refrangibility. It is a well-known fact that the whistle Oi a moving train of cars has its pitch gradually raised if the train is approaching the hearer, and lowered if the train is re- Changes in the C Line (September 22, 1870.) ceding from him. It is generally true that sounds mov- ing swiftly from us gradually fall in pitch ; those moving toward us rise in pitch. It is found, when we examine the spectrum of a rapidly moving light, that the lines are displaced. If the light is moving from us, the lines are bent toward the end of the spectrum at which the red rays appear ; but, when the light moves toward us, the lines are bent toward the violet rays (the most re- frangible). Thus the spectroscope enables us to tell whether a luminous gas is in motion, and also whether it moves toward us or from us. Fig. 46 illustrates this displacement of lines. 177. Solar Spectrum. The spectrum formed by pass- ing the sun's rays through a prism is a reversed or dark- lined spectrum. It is crossed by a number of dark lines, and we can match in it the bright lines of many elements that we know. Prof. Young has given a table of the elements, some lines of which have been found in the solar spectrum. List of elements certainlv known to be in the sun: hydrogen, titanium, calcium, vanadium, iron, copper, silver, silicon, manganese, magnesium, chromium, platinum, nickel, cobalt, barium, sodium. The existence of oxygen in the sun was at one time regarded as certain, but the results of the latest re- search, while not perfectly sure, are unfavorable to a belief in its presence there. List of elements for whose existence in the sun there is some evidence, but less conclusive : palladium, uranium, carbon, molybdenum, aluminium, lead, zinc, cadmium, Of these elements, all except carbon are metals. We lack proof that some of the best known terrestrial ele- ments exist in the sun, but it is thought that the result may be due, not to their absence, but to the sun's high temperature, since the spectra of luminous bodies are much affected by temperature. Within a few years Mr. Lockyer has been studying the spectra of the same sub- stances under different degrees of heat, in order to ap- ply the facts in explaining the spectra of the heavenly bodies. Tlie Sun's Telescopic Appearance and Physical Constitution. 178. The most competent authorities now believe that the sun is a great sphere of gas, extremely condensed at the center by the weight of the outer parts. The sun's Photosphere, or visible surface, is a stratum or coating of luminous clouds floating in the sun's atmos- phere, and surrounding the gaseous portion at the cen- ter. The Chromosphere surrounds the photosphere, and consists of very red flames extending about six thousand miles in every direction from the photosphere. The Corona is a faint pearly halo resembling the tails of com- ets, and it radiates from the sun to a distance about equal to the sun's radius. 179. The Photosphere. In a telescope of moderate power the photosphere seems to be composed of small incandescent grains separated by a some- what darker medium. They form streaks and groups. With higher power of the telescope these gran- ules seem formed of still smaller grains. They must, at differ- ent times, vary some- what in form, having been compared by different observers to rice-grains and willow-leaves. They are really incan- descent clouds floating in the sun's atmosphere, and com- posed of metallic vapors. Like the clouds on the earth's surface, they are partially condensed. With a low power of the telescope, the surface of the sun covered by the grains looks like curdled milk. The light of the FIG. 47. THE SUN. 55 sun comes chiefly from these granules. They are seen in Figs. 47, 48. FIG. 48. Granules and Pores of the Sun's Surface. (After Huggins.) 180. Besides the granules, there are found on the sun brighter streaks looking a little like foam, and called faculce. They are seen in Fig. 49. The faculse are por- tions of the photosphere elevated above the rest, as is seen when they are on the edge of the sun. They then FIG. 49. Sun-spots and Faculie. (From a J'/iotsgrap/i.) project a little beyond the circumference. They are most conspicuous near the edge of the sun, which, as was said, is darker than the middle. 181. Sun-Spots. In addition to the granules and 8 faculse on the surface of the sun, there are spots which have been the subject of a great deal of study. Three large and several small ones are on the portion of the sun's surface shown in Fig. 49. A sun-spot consists of two parts, the umbra and the penumbra (see Fig. 50). The umbra is in the center and appears dark, but this is merely in contrast to the brightness of the granulated photosphere. It is in reality filled with bright clouds. FIG. 50. r~ _~laf lJMp!WW^N| The Great Sun-spot of 18651. The spot entering the Sun's disk, Oct. jth (foreshortened view). 2. Its appearance, Oct. loth. j. Central view, Oct. I4th, sfiou>ing the formation of a bridge, and the nucleus. 4. Its appear- ance, Oct. itoth. The penumbra consists of gray filaments, or long-drawn- out granules of photospheric matter, arranged so as to radiate from the center (see Figs. 50, 51, 52, 53). An irregular but well-marked outline separates the penum- bra from both umbra and photosphere. The photosphere around the penumbra is often intensely bright. Some penumbral filaments end in granules of vcrv bright matter. These appear to sink and dissolve, while others ASTRONOMY BY OSBERVATION. FIG. 51. take their places. The penumbra seems to be drawing in luminous matter all the time. In a few of the spots, the inner ends of the penumbral filaments curve spirally, and the spots revolve as if affected by 'a cy- clone. Large spots sometimes seem to have two different centers of cyclonic action. But the rev- olution does not last, and in fact such spots are not numerous. Sun-spots are usu- ally circular when ful- ly formed. Their for- mation is gradual; their coming being indicated by faculas spot of July 16, 1866. at the point, and by small black dots which grow larger, and finally a spot is developed which lasts, on an average, two or three FIG. 52. months. Seventeen months have been about the longest duration of any sun-spot known. Sun-spots are some- times of immense size. The largest spot was observed in 1858. It had a diameter of a hundred and forty -three thousand miles. One of thirty or forty thousand miles can easily be seen with no further aid than a bit of smoked glass. They generally come in small groups. They move across the face of the sun which we see, in about twelve or thirteen days, disappearing on one edge ; but when another period of the same length has elapsed, sun-spots which can unquestionably be identified as the same, are found coming back on the opposite edge of the sun. This is evidently the result of the sun's rota- tion on its axis. It is a curious fact that the motion of the sun-spots in different latitudes indicates that the part of the sun near the equator rotates more rapidly than the portion farther removed from that circle. At the equator the rotation seems to be performed in about twenty-five days. The spots are not found equally distributed on all parts of the sun. They occur chiefly in two zones on each side of the equator, as shown in Fig. 54. These zones are between the latitudes 10 and 30. Sun-spots have been seen but once beyond lati- tude 45. We see the spots move in the direction of a line from the east to the west point of our horizon, but it is evident that the side of the sun's face turned toward us corresponds to the part of the earth's surface which is below the horizon of the person observ- ing him. It is evident that the part of the sun's surface which we see, rotates on his axis in the same direction with the part of the earth's surface which is below our horizon. It is clear from this that the sun's axial rotation is in the same direction as the earth's motion. To avoid am- biguity in the use of words, we must call the direction of the sun's rotation eastward. It is evident that the sun-spots have a motion of their own, as well as that caused by the sun's rotation. The curves made by the spots, as seen in Fig. 55, show that the THE SUN. 57 sun's axis is inclined to the plane of the ecliptic. That the sun-spots are depressions in the photosphere is shown by the changes wrought by perspective in one which FIG. S3- FIG. 54. Hun-spots as seen by Prof. Langley. travels across the sun's face. This is seen in Fig. 56. While the spot is on the western side of the sun, nothing is visible but the western side of the penumbra. As it advances east, the umbra begins to be seen ; then, as it passes directly in front of us, we see the whole umbra and the penumbra all round it ; then the western side of the penumbra goes out of sight ; then the umbra, and finally we see only the east- ern side of the pe- numbra. Observation shows that during periods of about eleven or twelve years there is an alternate increase and decrease in the activity which creates sun-spots. In size and number they reach a maximum and minimum. It also shows that the increase and decrease coincide with the increase and decrease of magnetic disturbances on earth, which have a period of eleven years. 182. The Chro- mosphere. - This word signifies color sphere. The chro- mosphere consists of fiery rolling flames of a vivid scarlet color. From the chromosphere, red clouds, called the " solar promi- nences," rise into the region of the corona. Owing to the blinding brill- iancy of the photo- sphere, they can not be seen upon the sun's disk, and were first discovered at total solar eclipses, around the circle of sun and moon. It was supposed im- possible to examine them at any other time than on these rare occasions, on account of the diffused reflected light in the earth's atmosphere. But the spectroscope, which has revealed so many secrets, came to the aid of the astronomers. FIG. 55. Apparent Paths of the Spots across the Sun's Disk, as seen from the Earth at different times of the year. The arrows show the direction in which the Sun rotates. Dispersion is, as the name shows, a separation or spreading out of the rays of light. A spectroscope of high dispersive power* makes the band of the spectrum * The dispersive power of a spectroscope is increased by passing the light through a great number of prisms. ASTRONOMY BY OBSERVATION. Scale, 75,000 miles to the inch. Prominence as it appeared at half-past twelve o'clock, September 7, 1871. As it appeared half an hour later, when the up-rushing hydrogen attained a height of more than 200,000 miles. Spikes. ERUPTIVE PROMINENCES. Flames. Cyclone. Spot near the Sun's limb, with accompany- ing jets of hydrogen, as seen October 5, 1871. As seen at 3.30 P. M. Vertical Filaments. As seen at 245 r. M. Three figures, of the same prominence, seen July 25, 1872. 100,000 miles to the inch. Scale, 75,000 miles to the inch. QUIESCENT PROMINENCES. Stemmed. Horns. Filamentary. Plumes. Cloud THE SUN. 59 FIG. 56. longer, and the spaces between the transverse lines broader. The light between the lines is greatly weak- ened by it, but the brill- iancy of the lines is not at all diminished. The spectrum of the solar prominences is a bright- lined spectrum, and that of the direct and re- flected solar light is a reversed spectrum. Therefore, by using a spectroscope of great dispersive power, as- tronomers weaken the light which obscures the Diagram illustrating t/ie Fact that Sun- , , r u spots are Hollows in t/ie Photosphere. ll g nt ol the promi- nences, so that they are now seen and studied at any time with as much ease as during a solar eclipse. The solar prominences are of two kinds, the Quies- cent and the Eruptive Prominences. 183. The Quiescent Prominences change very grad- ually and look like masses of red clouds, which, when fully seen, are found joined to the chromosphere by slender trunks, as seen in Fig. 57. The spectroscope FIG. 57. shows that they owe their red color to hydrogen. These prominences are seen all round the sun. 184. The Eruptive Prominences are flames which burst forth near sun-spots, and are therefore not found near the poles. They change very rapidly, showing a great variety of transformations. The spectroscope proves that they are largely due to the vapors of sodium, magnesium, barium, iron, and titanium. For this reason they are called metallic prominences (see Figs. 58, 59). The chromosphere always shows the lines of hydrogen, and sometimes the lines of the elements belonging to the metallic prominences. It also contains some lines which have not been identified as belonging to any ele- ment that we know. Astronomers have agreed for the present to call this element helium. The spectrum of the red prominences consists of hydrogen and helium, The student must not suppose that this hydrogen is burning in the sense of chemically combining with any other substance, as, for example, oxygen. Such combi- nations would not occur in so high a temperature. FIG. 58. FIG. 59. 185. The Corona. (See Figs. 60, 61, 62, 63.) The corona can be seen only when there is a total eclipse of the sun, and, as this lasts but for a few minutes, the op- portunities for observing it have been limited. At the time of a total eclipse, the moon looks like a dark sphere in the middle of a halo formed of " radiant filaments, beams and sheets of pearly light. The portion nearest the sun is of dazzling brightness, but still less brilliant than the red prominences which blaze through it like carbuncles. Generally, this inner corona has a pretty uniform width, forming a ring three or four minutes of an arc in width, separated by a somewhat definite outline from the outer corona, which reaches to a much greater distance and is more irregular in form. Usually there are several rifts, as they have been called, like narrow beams of darkness extending from the edge of the sun to the outer night, and much resembling the cloud-shadows which radiate from the sun before a thunder-shower ; but the edges of these rifts are frequently curved, showing them to be something else than real shadows. Sometimes there arc long bright streamers, as long as the rifts, or longer. On the whole, the corona is usually less extensive and less 6o ASTRONOMY BY OBSERVATION. brilliant over the solar poles, and there is a tendency to accumulations above the middle latitudes or spot-zones." FIG. 60. Corona as observed by Liais in Prof. Young says : " The portion of the corona near- est the sun has a greenish pearly tinge, which contrasts FIG. 61. Corona of iSfi. (Cuf/ain 'J iifinan.) THE SUN. 6l finely with the scarlet blaze of the prominences. . . . For the most part the streamers have a length not ex- FIG. 62. Corona of 1860, (Tempel.) ceeding the sun's radius, but some of them go far be- yond this limit. In the clear air of Colorado, during the FIG. 63. Corona of 1871. (J-'rom Photographs of Mr. Davis.) eclipse of 1878, two of them could be traced for five or six degrees, a distance of more than nine million miles from the sun." The spectrum of the corona is remarkable for con- taining one bright line sometimes called " 1474," more generally " the coronal line,", which comes from some element with which we are entirely unacquainted ; and which when in vapor (if it ever is anything else) is far lighter than hydrogen, the lightest substance we know. There is a dark line corresponding to it in a spectrum of light from the face of the sun. For the present, astronomers have agreed to call this ele- ment coronium. The corona also shows faint traces of hydrogen. The corona must be composed chiefly of luminous gas, but it probably contains some minutely divided par- ticles which reflect sunlight. This matter must be of inconceivable tenuity, for comets have passed through the corona without changing their motions. It is probably less dense than the matter in any vacuum we can produce. Prof. Young says, " The corona can not be a true solar atmosphere in any strict sense of that word." NOTE. The Sun's Heat. Of late years there has been much discussion among astronomers and physicists as to the way in which the sun's heat is maintained. As plants and ani- mals have not changed since the Christian era, the sun's heat can not have diminished much. This heat can not be due to combustion, for, as Prof. Young says, the sun would burn out in six thousand years if made of solid coal. Also, if the sun were only a hot body cooling, its heat would have lessened greatly. There have been two theories. That of Mayer supposes that the sun's heat is maintained by meteors falling upon it, the swift motion being converted to heat by sudden resistance. But it is not believed that the number of meteors falling on the sun can be sufficient to keep up its heat in this manner. The theory of Helmholtz is now generally accepted. A short account of it may interest students of this book, but they could not, of course, understand the mathematical reasoning which supports it. Helrnholtz thinks that the sun, losing heat by radia- tion and contracting by its own gravity, is similar to a body fall- ing against resistance. The contraction produces heat, and thus lessens the loss. It was shown by Lane, in 1870, that a pres- ent result of this loss of heat may be increase of temperature in the contracted mass ; that is, that the heat remaining in the smaller and denser mass may maintain a higher temperature than was maintained by a larger quantity of heat when distributed through the greater volume of the more expanded mass. It follows, from the theory of Helmholtz, that the sun must in time lose all its heat. Only an approximate estimate can now be given of the period in which the sun will continue to radiate heat sufficient for the support of life such as we know it. Prof. Newcomb reckons it to be about ten million years. 62 ASTRONOMY BY OBSERVATION. CHAPTER IX. THE PLANETS GENERAL. ACCOUNT. 186. The planets are arranged according to size in two classes, called respectively the Major and the Minor Planets. 187. The Major Planets are Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury, Venus, Mars, Jupiter, and Saturn were all known to the ancients. Uranus was discovered by Sir William Herschel in 1781. It can be seen by the naked eye as a star of the sixth magnitude. It can, however, hardly be identified by any but an experienced observer who knows where to look for it, as its apparent size is so small and it moves so slowly. Neptune can be seen without difficulty with a good opera-glass. 188. The Minor Planets. Besides the planets just named, there are a large number of small planetary bod- ies revolving round the sun between the orbits of Mars and Jupiter. They are called the Minor Planets, or Asteroids. (See " Asteroids," p. 79.) In the following list, the planets are named in the order of their distances from the sun, beginning with the one nearest the center : Mercury, Venus, the Earth, Mars, the Asteroids, Jupiter, Saturn, Uranus, Neptune. The distances from the sun increase with some regular- ity, and it was long noticed that there seemed to be a gap between Mars and Jupiter, so it was thought that some unknown planet might fill it. Early in the present cen- tury four very small planets were discovered, and called Juno, Ceres, Pallas, and Vesta. In 1843 another, called Astraea, became known ; and since then more than three hundred have been found. New ones are often discov- ered. Many of them are very minute bodies. Except Mercury and Venus, all the known planets are superior planets ; that is, they revolve round the sun in orbits exterior to the earth's orbit. Their motions correspond to the motions of the superior planets de- scribed in Chapter VI. The angle through which their movement appears to retrograde, decreases as their dis- tances from the sun increase. In this book, the forms, volumes, densities, etc., of the planets are described together under these respective heads, because the student gains much more definite ideas by comparison. 189. Forms of the Planets. The earth is one of the planets, and it will be best to speak of her figure first. The earth is round, or a sphere. We know this from a variety of facts : i. The shadow of the earth, as seen on the moon when she is eclipsed, is always round. 2. The horizon is always a circle. If we are on a plain, our horizon is limited ; if we ascend to a slight elevation, our view is more extended ; and, if we go up a high mount- ain, we have still a wider horizon ; but through all the changes our horizon is still a circle (see Fig. 64). This makes it quite certain that the earth is a sphere. 3. FIG. 64. Horizons of the Same Place, at Different Heights. When vessels at sea come in view of an observer, we see first the top of the mast, then the upper sails, next the lower sails, and finally the hull. This is represented in Fig. 65, and shows conclusively that the surface of the ocean is spherical. FIG. 65. / 'roof of the Curvature of the Earth's Surface. But the earth is not a perfect sphere. Her figure is flattened like an orange at the poles. Mathematicians describe the earth's figure as an "oblate spheroid." In order to understand some arguments made from these facts, the student is reminded of twirling a key, tied to THE PLANETS GENERAL ACCOUNT. FIG. 66. a string, in a circle round the hand. That the key has a tendency to move off in a straight line is shown at once when we let go the string. It is retained in place by the hand-grasp and the cohesive force of the string. If the string is not very strong, the motion may be rapid enough to break it. Now, there is a great deal of evi- dence to show that the earth was once in a semi-fluid or plastic condition from heat ; that is, the earth was once revolving on its axis with the force of cohesion much weaker than it now is. Every particle moved, as now, in a circle like the key. The particles about the surface of the equator move most rapidly, since they move through larger circles in the same time. This increases their tendency to fly off, and to resist all forces of attrac- tion ; and, as cohesion was weak then, they pulled out the sphere about the equator, and thus the earth became an oblate spheroid. The excess of matter about the earth's equator, and the attraction of the sun and moon for the parts nearest them, are said by astronomers to cause the precession of the equinoxes. The planets in different degrees show this same pe- culiarity of figure ; and, as will be seen, we have evidence that some of them are still plastic from heat. The polar diameter of Jupi- ter is five thou- sand miles shorter than his equatori- al diameter. 190. Volumes of the Planets. The word volume refers to mere size, without re- gard to weight. Jupiter is the lar- gest planet. His diameter is 86,500 miles, and his vol- ume is thirteen hundred times that of the earth. Saturn comes next, with a di- ameter of 75,000 miles, and his vol- ume is seven hun- dred times great- er than that of the earth. The The diameter of Comparative Sizes of the Planets. earth's mean diameter is 7,918 miles. Neptune measures 35,000 miles, that of Uranus 32,000, 9 Venus is a very little smaller than the earth in volume, and her diameter is only about 300 miles less than that of the earth. The diameter of Mars is about 4,200 miles, and his volume is about one seventh that of the earth. Mer- cury has a diameter of about 3,000 miles, and a volume about an eighteenth that of the earth. The diam- eter of the moon is a little more than 2,000 miles, and her volume Earth and Moon. is about one fiftieth that of the earth. These comparative volumes are illustrated in Figs. 66, 67, 68, 69. 191. Densities. The density of a body is its weight in proportion to its size, or in comparison with an equal volume of some other body. Thus a pound of iron is denser than a pound of wood. The earth is five times and two thirds as heavy as would be a globe of water of the same FIG - 68 - size. The density of Mercury is more than twice that of the earth ; that of Venus is a lit- tle less than that of the earth. Mars has only three fourths the earth's density. Jupi- ter is a little heavier, and Saturn a little lighter, than would be a globe of water of the same size. Saturn is the least dense planet. Uranus is a little less dense than Jupiter, and Neptune than Uranus. FIG. 69. 192. Eccentricity of Orbits. The orbits are all el- lipses. We know that the earth's orbit is an ellipse be- 6 4 ASTRONOMY BY OBSERVATION. cause the sun varies in apparent size ; and we attrib- ute this to a variation in his distance. The moon also varies in apparent diameter. So do the superi- or planets vary in diameter at different oppositions. The angle made by Venus and the sun at her great- est elongations varies in consequence of the varying distance of Venus from the sun, and the same is true of Mercury. All these facts show that the orbits are not perfect circles. Accurate measurement is, of course, required to ascertain the figures of the differ- ent ellipses. The eccentricity of an ellipse is the degree in which FIG. 70. it differs from a circle. The orbits of the earth and moon can hardly be distinguished from circles. That of Venus is the least eccentric of all. This is shown by the fact that the angle of her great- est elongation differs very little from 47. The al- manac always re- ports it. But the other inferior plan- et, Mercury, some- times reaches 28 from the sun, some- times only 17. This makes it at times very difficult to see Mercury. Thus the two inferior planets, Venus and Mercury, have the leas\ and the mo^eccentric orbits bf^ any of the major planets. Mars comes next to Mercury. Every fifteen years he is much brighter at opposition than at the in- tervening oppositions. The orbit of Jupiter has about half the eccentricity of that of Mars. The asteroids are remarkable for the very great eccentricity of some of their orbits. The orbits of some of the planets are illustrated in Figs. 70 and 71. The orbit of Polymnia is the most eccentric orbit to be found among the asteroids. 193. Inclination of the Planes of the Orbits to the Ecliptic. The ecliptic is the plane of the earth's orbit, as the student remembers. Also Mercury, Venus, Mars, and Saturn are to be looked for very near the ecliptic. This is because the planes of their orbits dif- fer so little from the ecliptic. This subject becomes interesting as soon as we begin to watch the planets in nature. They are so near the great circle that we wish to know the exact angle made by the planes, since that marks their greatest distance from it. It must be re- membered, however, that from the very fact that we are not in the same plane, we see them a little dis- placed from the positions in which an observer at the sun would see them. Uranus crosses the ecliptic at an angle of a little less than i; Jupiter, a little more than i ; Mars and Neptune, less than 2 ; Venus, a little more than 3. Mercury wanders farther from the eclip- tic than any major planet. He is sometimes 7 from it, and as the zodiac extends only 8, he barely keeps within it. Mercury deals in extremes. He is the densest planet, the nearest to the sun, has the most eccentric orbit, and moves farther from the plane of the ecliptic than any other planet except the asteroids. Many of the minor planets are sometimes more than 10 from the ecliptic ; and one, Pallas, gets as far as 34 from it. Fig. 72 shows the angles made by the various planes The Plane of tlie Ecliptic and the Planetary Orbits. with the plane of the earth's orbit. A number of these differ from the ecliptic so little that they are not sepa- rately represented. THE PLANETS GENERAL ACCOUNT. Distances from the Sun. Greatest distance miles. Least distance miles. Mean distance miles. 43,OOO,OOO I53,OOO,OOO 5O3,OOO,OOO 28,000,000 127,000,000 457,000,000 Mercury, Venus, The Earth Mars, Jupiter, Saturn, Uranus,. Neptune, The moon is 239,000 miles from the earth. Fig- 73> which shows the comparative size of the sun as he would appear to an observer on the different plan- FIG. 73. 36,000,000 67,000,000 92,900,000 141,000,000 483,000,000 886,000,000 1,781,000,000 2,791,000,000 The Relative Size of the Sun, as seen from, the Planets. ets, enables the student to get a realizing idea of the dis- tances from that luminary. 194. Sidereal and Synodical Revolutions. When we are interested in observing the planets, we very soon become familiar with the meaning of a synodical revolu- tion, since we learn from it when to look for any planet in the evening sky. The planets are evening stars through half a synodical revolution; morning stars through the other half. The synodical period of Mer- cury is 1 16 days ; that of Venus, 584 days ; that of Mars, 779 days, or 26 months; that of Jupiter, 13 months. The synodical periods of Saturn, Uranus, and Neptune differ so little from a year that they seem to come and go with the fixed stars. The sidereal periods have an interest of another kind. The sidereal period of a planet is its year, containing an entire revolution of its seasons. These periods increase in length with the distances of the planets from the sun. They are as follows (nearly) : Mercury, 88 of our days. Jupiter, 12 years. Venus, 225 " " Saturn, 29^ " The Earth, 365^ " Uranus, 84 Mars, 687 Neptune, 165 " 195. Rotation of the Planets. The planets all ro- tate on axes from west to east. We know this by the motion of spots or inequalities upon the surface. By the curves in which the spots move we learn the inclina- tion of a planet's axis to the plane of the ecliptic, and from that we find its inclination to the plane of its own orbit. (See Fig. 55, p. 57. Also " Rotations," p. 80.) Account of each Planet. 106. Mercury. We know little of Mercury, owing to his great nearness to the sun. He appears to have no moons. He receives on an average seven times as much light and heat as the earth. But he must, at opposite points of his orbit, vary much in regard to both. The presence of water vapor in his spectrum in- dicates that he has an atmosphere, but it must be less dense than that of Venus, since he does not exhibit the ring described in the next paragraph. 197. Venus. As Venus enters on the sun at her tran- sit, a ring of light round her shows the presence of a re- fracting atmosphere. The vapor of water is found in her atmosphere. She resembles the earth in size and density, but receives twice as much light and heat from the sun. We know little of her. 198. Mars. (Fig. 74.) In a very good telescope Mars appears to have his surface marked by what appear to be islands and continents of a dull-red color, and darker intervening spaces of greenish hue give the impression of water. By means of the spectroscope we find that there is certainly water in the atmosphere of Mars. There appears to be a great deal more land than water on Mars. The oceans are long, with narrow seas like canals 66 ASTRONOMY BY OBSERVATION. FIG. 74. running up into the land, so that water communication must be very general on Mars. At the poles there are white circles (probably ice- caps), which di- minish in size when the sun is alternately turned toward them in the varying sea- sons of Mars. We have evidence that the axis of Mars is inclined to the plane of his orbit about as much as that of the earth is ; and therefore the sea- sons must differ much as ours do, except that they are longer, in con- sequence of the greater length of the year of Mars, which is 687 of our days. The day of Mars is not far from twenty- four hours in length. The red color of Mars has been attributed to the cause which produces our sunset-red, viz., the ab- sorption of certain rays by the atmosphere of Mars. But there are many speculations about it. The general appearance of Mars, the evidence of land and water, his seasons, his days, make him appear to be a globe similar to ours, and not unsuited to the support of life. But we have no evidence at all of its existence. Mercury, and especially Venus, come near us, but they are then so partially illuminated (being crescents) that we have no good opportunity to see their surface. Mars at opposition is near us, and in good position to be seen, since we see his side turned to the sun. He comes into this position only once in twenty-six months, but astron- omers are then very assiduous in observing him. About every fifteen years he is still nearer. Much was learned in 1877, when he was at perihelion and the earth at aphelion, at the time of his opposition. Maps have been made of his surface. (See " Mars," p. 79.) Mars has two small moons, discovered in 1877 by Mars in 1862. Prof. Hall, and called Phobos and Deimos. The diame- ter of the inner moon, Phobos, is probably seven miles. It is not quite six thousand miles from Mars. Our moon is about forty times as far from the earth. Pho- bos occupies nearly eight hours in his sidereal revolu- tion, but, since Mars rotates on his axis more slowly in the same direction, Phobos moves round Mars in nine hours, and must rise in the east and set in the west twice in a Martial day of twenty-four hours. Deimos makes a sidereal revolution in thirty hours. Its diameter is probably five miles. 199. Jupiter. (See Fig. 75.) The axis of Jupiter is nearly perpendicular to the plane of his orbit, and there- fore there can not be much variation of seasons. From the small density of Jupiter, and the constantly varying character of his surface, it is believed that a large part of the planet that we see consists of an atmos- phere filled with great clouds and heavy vapors. The changes are so great and sudden that it is supposed that heat must be the cause of such activity. We have abun- dant evidence that our own globe was once in a plastic condition from heat ; and it is supposed that Jupiter may be in that condition, not having cooled off sufficiently to be covered with a solid crust. Belts formed of rolling clouds seem to stretch across the disk of Jupiter from east to west. At the equator, when not too strongly magnified, they have the appearance of two parallel belts. In a large telescope Jupiter shows many colors brown, red, olive-green, and sometimes purple. Jupiter, like the sun, is darker at the edges than near the center, so that his moons, when first seen passing over him, appear bright by contrast, but, as they ap- proach the center, the contrast diminishes. It has been supposed that Jupiter may have a little light of his own, but this is unproved, and the moons do not reflect any when Jupiter is between them and the sun. Some- times there are spots on Jupiter which pass away very slowly. In 1878 there appeared a great oval red spot not far from the equatorial belts, and it was seen at inter- vals for several years. There are indications that, in the case of Jupiter as in that of the sun, the equatorial parts rotate with a different velocity from that of the parts near the poles. This spot, much faded, was seen in 1889. There is every reason to think that Jupiter is in no condition to maintain life ; but he is of great interest to us, because he seems to be in the condition in which our earth was while undergoing the changes which fitted it to be the abode of men and animals. Jupiter has four large moons. The largest has a di- ameter of about 3,600 miles, being nearly as large as the planet Mars. The smallest is about the size of the earth's moon. The shortest revolution around Jupiter takes a THE PLANETS GENERAL ACCOUNT. 6 7 little less than two days, and the longest nearly seventeen days. When Jupiter passes between his satellites and the sun, they are eclipsed and, of course, darkened. But FIG. 75. Jupiter can pass between us and his satellites without passing between them and the sun. They are then said to be occulted, because they are hidden from us ; but they are not darkened. When a moon passes between Jupiter and the sun it casts a shadow on him. The bright side of the moon is turned toward us, and is seen to pass over the planet's face. This is called a transit of the moon. As we are not often in a straight line with the planet and a moon, we usually see both the bright spot and the shadow. (See Fig. 75.) The axis of Jupiter is very little inclined to the plane of his orbit, and the orbits of his moons are very nearly in the plane of his equator, so that they are all, except one, eclipsed at every revolution. If there were an ob- server on Jupiter, he would see in a year over four thou- sand eclipses of the sun, and about the same number of eclipses of the moons. The moons of Jupiter can be seen through a very small telescope, or through a good opera-glass. 200. Saturn. The axis of Saturn is much inclined to the plane of his orbit. The telescopic appearance of Saturn has long been of great interest on account of his rings. Figs. 76, 77, give pictures of Saturn and his rings. The rings are thin and flat, and lie in the plane of Saturn's equator. They consist of three parts: i. A dusky ring nearest the planet, and therefore in a position where it can not reflect much sunlight upon us. It is therefore difficult to see it. The outline of the planet has been seen through this interior dark ring. 2. Next to this dusky ring there is a bright ring. 3. Outside of this bright ring, there is an- other, not quite so bright, but still brighter than the dusky ring ; and between the two outer rings there is an open space which is black in the picture. The inner edge of the dusky ring is about ten thousand miles from the planet's surface. The rings are in mo- tion round the globe ; and, with the axis, are inclined about 27 to the plane of the planet's orbit. In moving, both axis and rings keep this inclination. The rings of Saturn are now generally believed to consist of a multitude of small satellites so close together that, like a swarm of bees, they seem at a distance to be con- tinuous. In the dusky ring, the particles are supposed to be farthest apart. The interior dark ring is sometimes called " the crape ring." The diameter of the outermost ring is one hundred and sixty-eight thousand miles. From the movements of Saturn and the earth, his FIG. 76. Saturn ami the Earth Comfarativ 68 ASTRONOMY BY OBSERVATION. rings are turned in a great variety of positions as regards the observer. These are called the phases of the rings, and they are FIG. 77. shown in Fig. 78. The rings and their phases may be seen in a tele- scope of small power, but a good view of the three rings can be had only in a telescope of high power. As Sat- urn is 29^ years in making a complete revolution round the sun, he takes the whole period to show all the phases. At two opposite points in Saturn's orbit, the rings are turned edgewise toward us, and they disap- FIG. 78. Saturn with the North Surface of its Rings pre- sented to the Earth. FIG. 79. Different Appearances of Saturn's Kings. pear, except in the most powerful telescopes; and in them look like a thin line of light. The satellites of Saturn are nearly in the plane of the ring system, and when it looks thus they are seen moving on its edge "like golden beads on a silver thread." The disappearance of the system shows its very great thinness. This phase is shown in Fig. 79- Saturn's sphere seems to be very much in the condi- tion of Jupiter. It is covered with clouds in which some faint traces of belts are distinguished, and is probably still hot and the seat of great activity. But Saturn Appearance of Saturn when the Plane of its Kings passes through the Earth. is so far away that it is difficult to know much about him. Saturn has eight moons. The largest of these, Titan, is larger than the planet Mercury, and can be seen in very small telescopes. But they are not all visible ex- cept in telescopes of the largest size. 201. Uranus and Neptune. Nothing is known in re- gard to their physical features. In a large telescope both are said to have a sea-green appearance. Uranus receives ^fj- as much light and heat as the earth ; while Neptune receives only -^ as much. (See " Neptune," p. 80.) Uranus has four moons, whose orbits are inclined nearly 80 to the plane of the planet's orbit. Neptune has one. The satellites of Uranus and Neptune have one remarkable peculiarity : they revolve around their planet from east to west; all other planets and satel- lites revolve from west to east. 202. The Moon. That the moon has no appreciable atmosphere is clearly shown in several ways. At half- moon, the diameter which forms one boundary is the dividing line between day and night on the moon. If there were an atmosphere, there would be some indica- tion of twilight near this line ; but there is none. The shadows of the mountains are pitchy black, showing no trace of the diffused light which would result from an atmosphere. Besides, if the moon had any atmosphere, we should expect, at a solar eclipse, that her edge would be surrounded by a dusky ring or border, such as are seen around Venus and Mercury when making a transit across the sun's face. But there is nothing of the kind. Also, in the moon's course through her orbit, she often passes over or occults a star. If she had any atmosphere, we should proba- bly see these stars FIG. 80. become dimmed just before disap- pearing ; but they seem suddenly blotted out. As- tronomers have made careful ob- servations to see whether there were any traces of refraction. The conclusion is, that the moon has no appreciable atmo- sphere. There never is any appearance of clouds passing over the moon, except clouds near the Moon at the First Quarter. (From Photographs taken by Prof. H. Draper, New York.) THE PLANETS GENERAL ACCOUNT. 69 FIG. 81. Moon Scenery. earth. There is, thus far, an absence of indication of water. There might be ice at a very low temperature. The moon's surface, to an observer with the naked eye, seems spotted with dusky patches, in which imagi- nation sometimes sees a resemblance to a human face popularly called " the man in the moon." Under a telescope of low power, the dusky patches ap- pear smooth, but with higher power, elevations and depressions become visible. The moon's face seems to be thickly pitted with the craters of extinct volcanoes. Many of them have central cones which have every ap- pearance of having arisen from erup- tive action, which would have great power on the moon, since the force of gravity would be less than on earth, owing to the moon's smaller mass. Some of the craters are, however, different from those of any volcanoes that we know, appearing to be mere plains surrounded by irregular circu- lar walls. In the formations on the moon there is a very general tendency to circular shape. . The greater num- ber of the craters are depressed below the surface, but some are hollowed out in elevations. In some places they stand singly on the plain ; in others they are crowded and heaped upon one another ; sometimes there are small craters on a plain which is surrounded by the wall of a large crater. They are of all sizes, from the small one just visible to us, to the great one with a diam- eter of more than a hundred miles. The great crater Ptolemy incloses a space equal to the State of Massa- chusetts. These formations are generally regarded as due to volcanic origin, but it is not believed that there are now any active volcanoes on the moon. The moon has been compared to a burned-out cinder. The inequalities of the moon's surface are best shown at or near her quadrature. The line which separates light from darkness is called the terminator. On this line the sun is all the time rising or setting, and there- fore long shadows are thrown which bring out details and show the heights of mountains. (Something of this irregular appearance of the terminator can be seen at quadrature without a telescope.) Besides this, there are elevated portions which catch the light both before sunrise and after sunset. These effects can be seen very plainly with a telescope of even moderate power. Fig. 80, showing the shadows, and also the light on the FIG. 82. Moon Scenery. ASTRONOMY BY OBSERVATION. elevated points, is a reduced copy of one of Prof. H. Draper's photographs of the moon. There are also chains of mountains on the moon. FIG. 83. Moon Scenery. Their heights have been ascertained by means of their shadows ; and it is found that in proportion to the size of the moon, her mountains are higher than those on the earth. There are chains which have been named re- spectively the Alps, the Apennines, and the Caucasus. Fig. 8 1 shows the region of the lunar Alps, with the great crater Plato. The diameter of Plato's ring is about seventy miles, and the mountains surrounding it are five or six thousand feet high. To the left of Plato there is a remarkable valley, the valley of the Alps. It is as level as if it were a roadway made by engineers; but it is bounded by very tall mountains. It is six miles wide and seventy-five miles long. The sun is on the left of the picture, and the mountains throw long shad- ows on the right. Fig. 82 is a representation of the lunar Apennines and the great crater Archimedes. These mountains rise to nearly eighteen thousand feet. On the plain there are black lines representing the chasms, cracks, or canals, which form a curious feature of the moon's surface. Some of them are a hundred miles long. They are sup- posed to be of volcanic origin, like so many other feat- ures of the moon's surface. Fig. 83 shows an ideal lunar landscape, taken from the work of Nasmyth and Carpenter on the moon. There is another very curious feature of the moon's surface seen when she is full, and when, of course, the perpendicu- lar rays of the sun shine on her. There are seen, radiating from some of the craters, bright streaks, which run over the sur- face of the moon for hundreds of miles, crossing mountains and valleys without seeming to be stopped by any obstacles what- ever. Fig. 84 shows the full moon and these bright streaks. They radiate especially from three great craters, Tycho, Co- pernicus, and Kepler. They do not appear to be elevations or depressions on the moon's sur- face. It has been well said, " They look as if, after the whole surface of the moon had received its final configuration, a vast brush charged with a whitish pigment had been drawn over the globe, leaving its trail upon everything it touched, but ob- scuring nothing." An effort has been made to account for these appearances by sup- posing that the moon, in cooling, suddenly cracked ; that these cracks afterward became filled with melted lava, which, when cool, presented a smooth surface ca- pable of reflecting light. This, of course, is not much more than conjec- ture. It is supposed that the moon rep- resents a body like the earth, in a much more advanced stage of cooling than the planet on which we live. It has, astronomers think, reached the stage when it can no longer support any form of life that we know. Moon. (From Photographs taken by Prof. H. Draper, New York) METEOROIDS AND COMETS. CHAPTER X. METEOROIDS AND COMETS. 203. Shooting-Stars. When we are out of doors after dark on a clear evening, we frequently see what appear to be stars moving swiftly across the sky and vanishing ; sometimes leaving, for a few seconds, a long train of light, and sometimes breaking into pieces without any noise. These bodies are called shooting-stars. 204. Meteors. Occasionally we see larger moving bodies giving a brilliant light, which in some instances is bright enough to illuminate the whole heavens. Some of these explode with a loud noise. These larger shoot- ing-stars are commonly called meteors, though the name applies in strictness to both classes. 205. Aerolites. Besides this, stony or metallic bodies are, at rare intervals, known to fall through the air, penetrating a short distance into the earth, and being hot if found soon after the fall. A very few of these stones are large bodies. One in the cabinet of Yale Col- lege weighs nearly a ton. Sometimes they fall in num- bers. These falling bodies are called aerolites. They are composed of chemical elements well known on earth, but sometimes in combinations not seen here except under circumstances which furnish good evidence that they have fallen from the skies. (See " Meteors," p. 80.) 206. Composition. It is now believed that these bodies can be distinguished by their composition when there is nc other evidence of their fall. They always contain iron in the metallic state, which is very rarely found on earth. They are called meteoric irons and meteoric stones, according as they are composed largely of metallic iron, or as they contain a larger proportion of other elements. They also have in them compounds of iron not known on earth. Meteoric iron has a crys- talline structure, and aerolites in general look as if they had been melted to some distance below their sur- faces. They are covered with a black crust. 207. Origin. It is now thought that a large number of small particles and masses of matter revolve round the sun, and, coming within the sphere of the earth's attraction, they are brought into forcible contact with our atmosphere. The collision produces heat and like- wise light, and in some cases the effect is great enough to produce explosion. The atmosphere is very attenu- ated matter but the velocity is so great that the result is like that of striking flint with far less velocity. To these bodies, called meteoroids, are due shooting- stars, meteors, and aerolites, which thus all belong essen- tially to the same class. Meteoroids are supposed to be exceedingly numerous. It has even been estimated that eight million pass through our atmosphere in twenty-four 10 hours. The visible path of shooting-stars is usually be- tween fifty and seventy-five miles from the earth. 208. Meteoric Showers. There are also periodical showers of shooting-stars. About November i4th there is a noticeable shower every year. The meteors seem to radiate from the constellation Leo (i. e., their paths ex- tended backward would cross in Leo), and therefore they are called the Leonids. The radiation is apparent. The small bodies move in parallel lines, and they seem to diverge from Leo because all parallels seen a long way off appear convergent. The orbit of these meteors is exhibited in Fig. 85. They evidently move in their orbit retrograde, or from east to west. There must be a dis- tribution of meteors along the whole line, since they are seen every year, but every thirty -three years there is an unusual exhibi- tion of them, and therefore it is supposed a great num- ber are collected in one part of their orbit. The earth crosses this orbit every year, but only en- FIG. 85. counters the great body of meteors every thirty-three years, because that is their period of revolution. In 1833 there was a striking display of these meteors in the United States. The sky was covered with lines of light in every direction, and great alarm was ex- cited among ignorant people. The last great exhibition occurred in 1866, but it did not equal that of 1833. It is supposed that the dense mass is of such extent that tin- earth gets into some part of it for three successive years. There was a lesser recurrence of the shower of 1866 in the two following years. The November meteors do not generally explode. They are small bodies. Another annual shower of less brilliancy comes in August, and, as these seem to radiate from the constel- lation Perseus, they are called the Perseids. It is sup- posed that the earth passes through the orbit of the Perseids in August. Since there is no variation in dif- ferent years, it is supposed that the Perseids are pretty regularly distributed along their orbit. The perihelion points of the orbits of both Leonids and Perseids touch the earth's orbit. The orbit of the August meteors reaches far beyond Neptune, and they take one hundred and twenty years to make a revolution. ASTRONOMY BY OBSERVATION. Fir.. 86. Comets. 209. Description. When comets are first seen with the telescope, before they come near enough to be visi- ble to the naked eye, they look (as do the nebulas hereafter to be described among the fixed stars) like small clouds. They can be distinguished from the nebulas only by their motion. There are many com- ets called telescopic comets, because they can not be seen by the naked eye. Their approach to the sun makes them large and conspicuous. In or- der that we may see them, especially with the naked eye, their nearest point to the sun must be not very far from the earth's orbit. 210. Parts of Com- ets. Comets which can be seen without a telescope have, when near the sun, three parts. They have a nu- cleus, which is very like a bright star, and which is sur- rounded by a cloudy, shining envelope called their Coma. They have also a tail, which is a long stream of light ex- tending from the coma (see Fig. 86.) Their approach to the sun develops these parts. As they draw near that luminary, they throw out jets toward the sun which seem to be alternately attracted and repelled by him. The appearances are very like those exhibited by bodies in op- posite states of electricity. It has also been noticed that large comets in the neighborhood of the sun throw off from the jets, toward the sun, a succession of apparently vaporous envelopes. These were observed with great care in the case of Donati's Uonati s Lvtiiel (s/u>ii.'in^ ///< COmet, Which appeared 111 1858. 1 lead and Envelopes). Comet of 1264. FIG. 87. FIG. Fig. 87 shows these envelopes. Donati's comet was one of the most brilliant of modern times (see Fig. 88). It came so near the earth's orbit that, had the earth been on the same side of the sun, it must have passed through the comet's tail. The same enve- lopes were seen in Coggia's comet in 1874. 211. Tails of Comets. As comets draw near the sun, their tails are developed with great rapidity on the side turned away from the sun. Thus, when approaching the sun, the tail follows the nucleus and coma, but in receding from the sun the nucleus and coma fol- low the tail. The tails of com- ets vary in apparent length, a few extending more than half across the heavens above the horizon. Their tenuity is very great, very faint stars being seen through them. The earth is believed to have passed through the tail of a comet in 1861, without the fact being known to its inhabitants. This was a FIG. Donati's Comet (general view). Comet of 1861. very brilliant comet, fan-shaped. It is shown in Fig. 89. The comet of 1744 had five tails (see Fig. 90). 212. Origin of Comets. Comets are supposed to come from the stellar spaces beyond the solar system, and to be drawn into the sphere of the sun's attraction, which, it is believed, changes the direction of their motion, making them travel on curved lines round the sun. Some METEOROIDS ANT) COMETS. 73 move in ellipses and are called periodical comets, since they return ; while others appear to move in curves FIG. 90. Comet of if 44. which lead them around the sun to go again beyond his attraction.* Whether the figure is an ellipse depends on its velocity in proportion to its distance from the sun. Where the ratio of the velocity to the distance is small, the figure is an ellipse, and the degree of eccentricity of the ellipse depends upon the same proportion. But comets are apt to have their speed increased or dimin- ished by the attraction of the planets which they pass, and thus their orbits become changed. The attraction of the greater planets produces decided effect in chan- ging the orbits of the comets to ellipses. In that case the aphelion points of their orbits are found not very far from the orbit of the planet. There are a number of com- ets thus connected with each one of the larger planets.f 213. Return of Comets. The first comet of which the return was successfully foretold is called Halley's comet. He saw it in 1682, and found that its orbit was about the same as that of two comets which had previously ap- peared, one in 1531, the other in 1607. The orbit of the latter had been investigated by Kepler. The comet re- turned in 1759, according to Halley's prediction, only it was one month behind time, but it was shown by an- other astronomer that its delay was occasioned by the attraction of Jupiter and Saturn. It was proved to be identical with a comet seen and recorded in 1066, 1456, and 1531. It was last seen in 1835, when it was very brilliant and excited great interest. In 1456 it caused much alarm, as this was soon after the Turks took Con- stantinople and threatened the rest of Europe. It was this comet that caused the prayer, " From the comet, * These curves are known to the students of conic sections as parabolas and hyperbolas. f See Note, p. 91. the Turk, and the devil, good Lord, deliver us." It was 60 in length, and was thought to resemble a saber. 214. Meteors and Comets. There are a number of facts showing a close connection between comets and meteors. There is a comet which has the same orbit as the November meteors, while another has that of the August meteors. Biela's comet furnishes further evi- dence of this relation. This is a periodic comet, and when it came again in 1845 it was found, after its ap- pearance, to have divided into two parts, of which one disappeared before the other. In 1852 both returned, and were found still farther apart. After their disap- pearance, they were not again seen. But, in 1872, the comet was due, and on November 27th, the time" when the earth's orbit crosses that of the comet, there was a great shower of meteors, which seemed to come from the part of the sky where the comet would have been situated. (See Note, p. 74.) It is thought that the nucleus of a comet may consist of a collection of meteoroids. Through some telescopic comets, stars can be seen, and in this case the meteoroids must be small and far apart. But in large comets they must be very dense, if the nucleus is not solid. The tail and coma are produced by vaporization as they draw near the sun. In 1843 a brilliant comet was visible to the naked eye in sunlight. Its tail stretched 65. It passed very near the sun, being within his exterior atmosphere, and, while thus near, passed half round the sun in two hours, revolving its tail through 180 in that short time. From this it would appear that the matter of which the tail is made is all the time changing, like that of smoke from a chimney or vapor from a kettle. 215. Comets and the Spectroscope. Comets have been examined in the spectroscope ; the brilliant comet of June, 1 88 1, being subjected to thorough study. They show a spectrum in which are indications of hydro- carbon vapors. Their light is probably due partly to reflection of the sun's light, and partly to these va- pors when acted upon by an electric discharge passing through them. 216. Numbers of Comets. There have been about five hundred comets seen by the naked eye since the beginning of the Christian era. Over two hundred tele- scopic comets have been seen since the instrument was invented. Doubtless a much larger number has existed beyond the limits of our observation, which are very narrow. There are accounts of comets in ancient times, but the description is so colored by the alarm they excited that it can not be trusted. There was a great comet visible before the assassination of Julius Caesar, 43 B. c. It was seen for several hours before sunset. It was supposed by Halley to be the same 74 ASTRONOMY BY OBSERVATION. FIG. 91. Cjmet of i Si i. comet which appeared in 1680, when its orbit was in- vestigated by Sir Isaac Newton. A very remarkable comet was seen in 1811, just before the invasion of Russia by Napoleon Bona- parte. It was very brilliant, and was seen for seventeen months. (See Fig. 91.) (See " Comets," p. 80.) 217. The Zodiacal Light. If, on a clear evening in late winter or in spring, we look, just at the close of twilight, at the part of the horizon at which the sun has set, we may see a sort of au- rora of faint pearly light, of a half-oval figure, with its base resting on the hori- zon, and its axis coinciding with the ecliptic. It is visible for nearly 90 from the sun, growing fainter as the distance from him increases. It is represented in the wood - cut, Fig. 92. It is a lens -shaped appendage surrounding the sun and lying nearly in the plane of the ecliptic. It is called the Zodiacal Light. The zodiacal light is also visible in the autumn at the beginning of the morning twilight. It is seen at evening on or near March 2ist, and in the morning on or near September ist, because at those seasons and hours the ecliptic makes its greatest angle with the horizon. At other seasons, when the ecliptic makes a smaller angle, the zodiacal light is so near the horizon that it can not clearly be dis- tinguished, and also it sets before the sunlight has entirely faded. People who live within the trop- ics, where the air is very clear, can sometimes trace it entirely across the sky from east to west. It obscures very small stars within its area on the heavens. Various explanations of the zodiacal light have been suggested. Some observers have supposed that it might be an extension of the sun's coro- na. The most common opinion now attributes it to a collection of very minute meteoroids re- volving about the sun nearly in the plane of the ecliptic and reflecting his light. When the zodia- cal light is visible, it marks the course of the ecliptic very clearly. FIG. 92. NOTE. Biela's comet had a period of 6% years. It would have been due in 1879, but neither comet nor meteors were seen. In November, 1885, it was again due. The comet was not seen, but on the nights of November 25th, 26th, 27th, there was a brilliant meteor-shower radiating from Androm- eda and reaching its maximum on November 27th. It is now regarded as nearly certain that Biela's comet has broken up into a collection of meteo- roids revolving round the sun. This meteor group is called the Andromedes, or Bielids. Over a hundred meteor swarms are known, but the Perseids, Leonids, and Andromedes are the most important. During the shower of November 27, 1885, a piece of meteoric iron fell at Mazapil, in northern Mexico, but in no other shower has any aerolite fallen. Many astronomers regard this as a piece of Biela's comet. THE HEAVENS OUTSIDE OF THE SOLAR SYSTEM. 75 PART I II. THE HEAVENS OUTSIDE OF THE SOLAR SYSTEM. CHAPTER XI. 218. The stars are usually divided into fixed stars and planets. The latter class includes the stars revolv- ing round the sun, both primaries and secondaries, or moons. All other stars were called fixed stars, because they showed no change of place as regards each other, though they all apparently revolve round the earth. 219. Movements of the Fixed Stars. The ancient astronomers at Alexandria ascertained with some accu- racy the relative positions of the chief fixed stars, and, in the early part of the last century, Halley compared the records left by them with the results of observatibn, and he was led to believe that the so-called fixed stars have changed relative place. Owing to their enormous dis- tances from us, we perceive this motion very slowly, and so it was only detected by the combined work of men living many centuries from each other. Other astron- omers began to investigate the subject. Finally, they came to the conclusion that the stars examined seemed all to be moving farther from a point in Hercules, and nearer to a point of the celestial sphere situated exactly opposite Hercules, or 180 from him. This is exactly the appearance which would be produced if our sun were moving toward Hercules, carrying with it all the bodies dependent on it. Thus they were led to think it probable that the sun has a real motion. This motion, called the Secular Motion of the Sun, appears to be very slow, only because it is measured by bodies at such enormous distances from us. (See " Hercules," p. 80.) 220. Secular Motion of Stars. Besides these changes of place, which could be explained by the supposition that we ourselves, with the solar system, are in motion, there were others which could not be thus accounted for, and they led to the belief that the stars have a very slow real motion of their own, which is called the Secular Motion of 'the Stars. The slowness of the motion is ap- parent, resulting from the enormous distance of the stars. The student who has seen a train of cars moving at a very great distance remembers that they seem to creep over the earth. On the other hand, a traveler may move rapidly for a whole day, but, if he measured his motion by a far-distant mountain only, he would not see that he had changed place. After the invention of the spectroscope, it was used in studying this problem. The displacement or bending of lines toward the red or the violet end of the spectrum indicates motion from or toward the observer. The results in regard to the motion of the stars confirm the previous conclusions. 221. Star-Drift. It is found that stars in certain parts of the heavens have motions in a common direction. Mr. Proctor, who has specially studied this subject, pro- poses for this motion the name of Star-Drift. Thus five stars in the Great Dipper have a common motion in the same direction, while the two others move in another direction (see Fig. 93). This must, in the course of ages, FIG. 93. alter the figure of the Great Dipper. The spectroscope confirms these conclusions by showing that these five stars recede from us. 222. Motion of First-Magnitude Stars. It will inter- est students to know that, of the first-magnitude stars, Sirius, Regulus, Betelguese, and Rigel are found by the spectroscope to be receding from us, while Arcturus, Vega, and Pollux approach us. The rates of motion even are computed, but the results are not more than an approximation. The spectroscope, the student must re- member, only tells about approach toward the observer or recession from him, but nothing as to the general di- rection. A little reflection about motions of persons (or bodies) whom he sees on earth will make the student understand that they may move in many different direc- tions, any one of which might make them draw near us or recede from us. Nothing, therefore, can at present be known, from the observed facts, in regard to the figure of the sun's motion, or that of the stars. The epithet fixed stars is still used for distinction. 223. Physical Constitution of the Stars. The spec- troscope shows that they resemble our sun. All show some dark Fraunhofer lines, and thus it is evident that the luminous spheres are enveloped in vapors absorbing the light from some of the rays. The lines show that 7 6 ASTRONOMY BY OBSERVATION. they contain elements known to us, and existing in our sun. The spectra of different stars vary somewhat. They have been arranged in four classes. The stars vary in color, as any observer may see. Thus, Antares and Al- debaran are red ; Vega Lyrae, Altair, and Spica Virginis are pale blue ; Capella and Sirius are white ; Arcturus, Pollux, and Procyon are yellow. The colors correspond generally to differences in the spectra, but these may be partially due to differences in the heat and density. The yellow stars are more like our sun. 224. Distances of the Stars. For a long time it was thought that no fixed star showed any apparent change of place, due to the earth's change of position, in six months, between points 185,000,000 miles apart. But refined and accurate instruments and methods of observa- tion have shown that some of the stars exhibit a very small displacement, not in any instance amounting to i". The star a Centauri has a pretty well-ascertained paral- lax, and is supposed to be the nearest of the fixed stars to us. But the distance must be about twenty billions of miles. These numbers convey little idea to us, and it will perhaps give a more definite notion to the student if he is told that it would take light more than three years to reach us from a Centauri. Other stars are probably nearly all much more distant. (See " Light Year," p. 80.) 225. Volumes of the Stars. None of the fixed stars show any disk, even when examined with the most pow- erful telescopes. This is one way in which observers with the telescope readily distinguish planets from stars. The telescope aids us in observing a star merely by its power of collecting light. It does not magnify the stars, so we can not measure their diameters. Magnitudes. (See p. 80.) 226. Numbers of the Stars. Ob- servers with the naked eye see over six thousand stars. With the great telescopes of modern times they see many millions, but no estimate has been made approaching exactness. 227. Double and Multiple Stars. Some stars which are single to the naked eye are resolved by a telescope into two or three or sometimes more stars. In some cases this is due to the fact that stars not near together are on the same line of vision, and in such cases they are called optically double stars. But in many instances it is found that these stars are connected FIG. 94. Orbit of a double Slur. FIG. 95. by revolving around a common center ; and in that case they are called physically double stars. The motion and even the period of some stars are clearly determined, as in of the Great Bear, which com- pletes a revolution in sixty years. e Lyrae can be resolved into a double star by a good opera-glass (young people with good eyes see it double without a glass), and a powerful telescope shows that each of these stars is double. (See Figs. 94 and 95.) Double and multiple stars are very often of different col- ors, and sometimes of comple- mentary colors. (See " Masses of Binary Stars," p. 80.) 228. Variable Stars. This name is applied to stars which change in brightness. Many of these have periodical varia- tions. The most noted are the stars Algol or ft Persei ; Mira in Cetus, or the Whale ; and Eta, or t), in the ship Argo. The changes of Algol, or /3 Persei, are of such short period that any observer may detect them if he knows FIG. 96. The double-double Star in the Constellation Lyra. i. As seen in an cpera -glass. 2. As seen in a small tele- scope, j. As seen in a telescope of great power. 'J he Pleiades in a large Telescope. THE HEAVENS OUTSIDE OF THE SOLAR SYSTEM. 77 when to look for them. Algol occupies seven hours out of every sixty-nine in making a gradual change in luster or brilliancy. Algol is usually a faint star of the second magnitude, but during twenty minutes at the middle of the seven hours, it becomes a star of the fourth magnitude. The change is very gradual. In order to remark it, it is necessary to compare Algol, before the beginning of the variation, with some other star of the same size. This gives a standard by which to observe the change. The exact period in which the variations of Algol are completed is two days, twenty hours, forty- nine minutes. (See " Algol," p. 80.) Mira in the Whale, or o Ceti, as the star is called by astronomers, is a star whose variations can be seen with the naked eye. It has a period of nearly a year. From a star of the second magnitude it becomes invisible. Its variations are not altogether regular. Eta, in the ship Argo, or the star rj Argus, is an irreg- ular variable, which is never visible in the United States. It was seen by Sir John Herschel brighter than any other star except Sirius, and he says it then began to decrease and passed slowly out of sight.* 229. Temporary Stars. There are instances of stars which have suddenly appeared and shone for a time with great brilliancy, disappearing gradually. In the year 1572 such a star appeared in the constellation Cassiopeia, and was described by the astronomer Tycho Brahe. It outshone Jupiter and Venus, and could be seen at noon. After six months, it disappeared, and has not been heard of since. It underwent several changes of color while visible. In 1604 a new star of the first magnitude appeared in the constellation Ophiuchus. It was visible more than a year, and then disappeared. This is sometimes called Kepler's star, because this as- tronomer observed and recorded its changes and appear- ance. In 1866 a telescopic star in Corona Borealis sud- denly increased to the second magnitude and afterward faded. It was examined with the spectroscope, which gave evidence that hydrogen was the chief agent in the great outburst. The red flames of the sun's chromo- sphere are due to hydrogen, and it is supposed that this star showed phenomena like the red prominences, only on a much larger scale.f 230. Star-Clusters. The Pleiades are a cluster of stars. The ordinary observer sees six stars ; good eyes sometimes detect seven ; a small telescope brings out many more ; and one of the large telescopes shows over four hundred. (See Figs. 96, 97.) The telescope shows fine globular clusters in Hercules, in Aquarius, and in Toucan, in the southern hemisphere. One of the finest * See " Variable Stars," p. 80. f See " Temporary Stars," p. 80. is in Centaurus. In the sword-handle of Perseus, there appears to be a star which is hazy to the naked eye, but a telescope of moderate power shows that it is a very brilliant double cluster. (See " Pleiades," p. 80.) 231. The Galaxy, or Milky- Way. This has been de- scribed in Chapter I. When examined through the tele- scope, it is seen to be composed of very small stars whose combined light creates the milky appearance. It FIG. 97. Star-Clusters. I. In Libra. 2. In Hercules, j. In Capricorniis. 4. In Ser- pent, j. In Aquarius. 6. In Gemini. contains many clusters of stars. The numbers of the fixed stars in general increase in the direction of the Milky- Way and in it. This is more evident when we use a telescope, for the telescopic stars greatly out-num- ber the others. 232. Nebulae. A nebula looks like a patch of cloudy light. There are two classes of cloud-like patches : those whose light is due to a great collection of telescopic stars, and those which no telescope yet used by us has ASTRONOMY BY OBSERVATION. FIG. 98. FIG. 99. Planetary Nebula in Ursa Major. Elliptical Nebula near y Andromeda, FIG. 100. resolved into stars. The latter are nebulas, and of these many have been shown by the spectroscope to owe their light to masses of glowing gas. (See " Nebulae," p. 80.) There is a remarkable nebula in the sword-handle of Orion. It can be seen by the naked eye sur- rounding the middle of the three stars in the handle. It was examined by Professors Secchi and Huggins with the spectroscope, and they found lines leading them to believe it composed of glowing gas. Another remarka- FlG. 101. Ring-Nebula in Lyra. FIG. 102. ble nebula which can be seen with the naked eye is the great nebula of Andromeda. The triangle in Cancer contains a small cloudy patch called Prassepe, and sometimes the Bee-hive Nebula, but it is a star-cluster. Great numbers of nebulae .are seen with the telescope, but they are most numerous at a distance from the Milky - Way. Some, as seen in Fig. 98, are round in Nebulous star, appearance like a planet, and are there- fore called Planetary Nebulge. Other nebulas are elliptical (Fig. 99), while still others are ring- shaped (Fig. 100). Some are oval (Fig. 101). Somecon- Crab Nebula in Taurus. sist of a hazy circle surrounding a star, which is there- fore called a nebulous star (Fig. 102). Besides these, FIG. 104. there are nebulas of many irregular shapes (Fig. 103). Some are spiral, and look as if they might be in rotation around some central , /0 T FIG. I0 5- point. (See Figs. 104 and 105.) Fig. 1 06 is a representa- tion of the nebula of Orion. 233 . The Ma- gellanic Clouds. Travelers in the southern hemi- sphere of the earth long ago brought accounts of two large masses of cloudy light which they saw near the south pole of the heavens. They are Spiral Nebula in Canes I 'enatifi. THE HEAVENS OUTSIDE OF THE SOLAR SYSTEM. 79 Fir;. 106. called the Magellanic Clouds, and they are further dis- tinguished from each other as Nubecula Major and Nubecula Minor. Nubecula Ma- jor is, of course, the larger, and it is so bright that it is not obscured by the full moon, which causes Nu- becula Minor to become invisi- ble. When seen through a pow- erful telescope, they are both re- solved into stars and separate neb- ulae. Of the latter, Herschel counted 278, besides more than fifty outlying nebula;. (See Fig. 107.) Fir,. 107. Gnat Nebula of Orion. Part of the Nubecula Major. 234. The Nebular Theory. A great many astrono- mers believe that suns and planets are formed from neb- ulae. The nebulous matter is supposed to be very hot and rotating rapidly. It gradually condenses and flat- tens into a rotating disk. This throws off rings which revolve and finally condense into planets, the central and large mass constituting a sun. The facts which they adduce in support of this theory are : the evidences of the earth's former fused condition ; the flattening at the poles seen both in earth and planets ; the fact that planets and moons (with two exceptions) all revolve from west to east, or in the same direction with each other ; the rings of Saturn ; the partially fused and vaporous condition of the larger planets, which would cool more slowly ; and the spiral form common among the nebulae. The opponents of this theory consider the fact that the moons of Uranus and Neptune revolve from east to west a serious objection to it, and also the fact that one of the moons of Mars revolves around Mars in a shorter time than Mars revolves on his axis. 235. Southern Circumpolar Constellations. To peo- ple in the United States these are in the Circle of Per- petual Disparition, so that the student can not see them in nature, but he should know something of them. An account of them is placed here, because they may be overlooked in the Description of Constellations in the Appendix, which is intended for reference only : The chief groups are: Ara, the Altar; Crux, the Cross ; Dorado, the Sword-Fish ; Hydrus, the Water- Snake ; Pavo, the Peacock ; the Southern Triangle, and Toucanus, the Toucan. Three viz., Phoenix, Grus, the Crane, and Centaurus, the Centaur can be partially seen in the Southern States, but the two first-magnitude stars in Centaurus are not seen in the United States at all. The Southern Cross contains one first, three second- magnitude stars. It is the glory of the Southern skies. There are six first-magnitude stars around the southern pole. One of these, Canopus, can be seen in Tennessee. It presents a fine appearance in Georgia, and in the clear skies of Florida it is an object of much interest to visit- ors acquainted with astronomy. It is south of Sirius. NOTE. (See " Photography in Astronomy," p. 80.) NOTES TO CHAPTERS IX, X, XI. Mars. In 1877, when Mars was in opposition at his perihelion, an Italian astronomer, Schiaparelli, saw with a telescope a curious network of straight dark lines on the planet, connecting what we regard as seas and oceans. They are called " canals," but the student must not fall into the mistake of suppos- ing that they can be artificial works. At subsequent oppositions, Schiapa- relli's report received some partial confirmation from other observers. Later, Schiaparelli reported that at certain seasons of Mars the lines were doubled, or replaced by parallel lines ; also that a supposed continent, called Libya, seemed to become inundated at intervals. This causes great interest in the prospect of seeing Mars in 1892, when opposition again comes at perihelion. II The Asteroids. Tn 1889, two hundred and eighty-nine asteroids, or planetoids, were known. These bodies cause some irregularity in the motion of Mars, enabling mathematicians to calculate their united mass, which is about one fourth the earth's mass. As the united mass of those we know is so small, there must be many more to discover. The brightest of the known asteroids, Vesta, which is just visible to the unaided eye near opposition, may be from two hundred to four hundred miles in diameter. The undiscovered ones must be very minute, and there may be thousands of them. There arc two theories of their origin. One is based on the now prevalent idea that all the planets were once rings of meteorites resemUing Saturn's ring. The, 8o ASTRONOMY BY OBSERVATION. attraction of so great a mass as Jupiter so near these rings would affect them, and this, it is believed, caused these rings to break up into many small bodies instead of collecting into a large planet. Other astronomers think it more likely that the asteroids were once a large planet which was broken up by a series of explosions. Neptune. Prof. Young says that sunlight at Neptune is equal to the light of a large electric arc lamp seen at a distance of a few feet. He says it would equal the light of six hundred and eighty-seven full moons, and would be abundantly sufficient for visual purposes. Meteors. Twenty-four of the known chemical elements are found in meteorites. In the United States several hundred fragments have fallen at once in the following places : in 1807, at Newton, Connecticut ; in 1860, at Concord, Ohio ; in 1875, at Amana, Iowa ; and in 1879, m Emmett County, Iowa. Comets. Prof. Young says that the total number of comets must be enormous. In 1881 two were seen by the naked eye near together. Fourteen comets have been seen at their return ; the orbits of fifty indicate a return ; twenty-six of these have periods of less than one hundred years ; the period of Encke's comet, the shortest known, is three years and a half. There are several groups of comets of which the members belonging to each have nearly the same orbits, and they come after one another at intervals, as if they might once have been united. We know nothing of the masses of comets, except that they must be very small in comparison with the earth's mass. In volume they are the largest bodies known, therefore their average density must be very small ; and, since part is probably solid, the density of their tails must be inconceivably small. The tails of some large comets have extended a hun- dred million miles. The tails are usually curved, because the particles re- tain their motion after they are thrown off. Hercules. The point in Hercules to which our sun and his attendant planets are moving is called the "Apex of the Sun's Way." " Light Year." This name is sometimes given to the distance light travels in a year. It is a measure of length used to give an intelligible idea of the stellar distances. Light travels 186,000 miles in a second, and the light year is about 63,000 times the distance of the earth from the sun. Magnitudes. This word is used by astronomers to describe the com- parative brightness of the stars. Average stars in each of the six original magnitudes differ from those of the nearest inferior magnitude by about two and a half of their light. But, since those in each class vary, fractional mag- nitudes are much used now. At Oxford (England) and Cambridge (Massa- chusetts) the comparative light of the stars is systematically studied by means of a photometer (light-measurer). Masses of the Binary Stars. The physically double stars, which are by far the most numerous class of the two, are called Binary Stars. When we know the parallax of a binary star, we can estimate its distance from us and the diameter of its orbit. By observation, we find the period of its revolution, and then, assuming that gravity is the cause of its revolution, we can, by calculation, find its mass in comparison with the mass of our sun. Sirius is known to be a binary ; and in this way it is estimated, very roughly, that it has about five times the mass of our sun. If we could know the diameter of Sirius, we could estimate its density ; but Sirius is only a point of light in our best telescopes. Variable Stars. A few stars, "perhaps a dozen," have shown a con- tinuous change in brightness since the earliest time of observing them. Beta Libra?, the most northern of the two second-magnitude stars in Libra, was once brighter than Antares in Scorpio. As the fixed stars are probably, like our sun, hot bodies cooling, changes in them are not surprising. The Pleiades. The stars of this cluster have motion in a common di- rection (with the exception of a few stars which seem simply to be in line with the cluster), and they have spectra alike ; therefore it is believed that they form a connected system of stars of some kind. Temporary Stars. In the summer of 1885 a new telescopic star ap- peared in the nebula of Andromeda. It increased rapidly in luster from its appearance in August to September, when it had attained the size of a seven- and-a-half magnitude star. It then decreased, and after six months could not be seen. This phenomenon is supposed to be due to some sudden evolution of glowing gas in a star previously too small to be seen with the most power- ful telescopes. In 1872 a telescopic star in Cygnus in like manner suddenly blazed up to the second magnitude, and then decreased to its former appear- ance in a month. There are on record eleven such cases of temporary stars, one having con- before the Christian era. The last three, in 1866, 1872, and 1885, were examined with the spectroscope. Photography in Astronomy. More than thirty years ago, good pic- tures of the moon's surface were made by photography. Since the use of the very sensitive plates, which have made instantaneous photographs pos- sible, the applications of photography to astronomy have greatly increased. If the eye looks long at an object, the continued impression on the retina does not make the object more distinct, but the long exposure of the sensi- tive plates accumulates the effect on them ; and, when the heavens are photo- graphed, stars are made visible which can not be seen with the same power of the telescope used to enlarge the photographic picture. Thus, such pictures can make stars visible that we can not see with our present telescopes. Some remarkable charts of parts of the heavens were made by the brothers Henry, at Paris, and by others ; and in 1887 a congress of astronomers was held at Paris which determined to attempt a photographic chart of the whole heav- ens, which should make stars visible to the fourteenth magnitude. This work is divided among a number of observatories in the northern and south- ern hemispheres, and it is estimated that it will occupy five or six years. A photographic chart is, of coarse, of minute accuracy in regard to dis- tances on it. Photography is also used in the systematic study of stellar spectra ; and, as improvements are constantly made in sensitive plates, it is becoming an ally of remarkable value to astronomers. The great obstacle, at present, is that the plates are not equally sensitive to rays of different colors. Nebulae. We know that the light of the nebulce comes from gas ; but this may possibly contain small, solid particles like meteorites. Rotations Mercury and Venus. Schiaparelli announced in 1889 that, by a study of markings on Mercury, he had found that this planet, like our moon in its revolution round the earth, always turns the same face to the center, and thus its rotation on its axis must occupy the exact time of its revolution in its orbit. Schiaparelli's conclusions are generally regarded as correct, though not yet confirmed by others. Schiaparelli's observations of Venus make it probable that she also makes her rotation and revolution in the same period of time. Algol. Vogel has lately explained Algol's obscuration by evidence that the star is a binary, revolving with an opaque companion round a common center, and coming periodically between us and Algol. To show the sup- posed position of Algol's orbit, let the student take a paper circle which has one diameter drawn on it, and hold it in such a position that he sees only its edge, and also so that the diameter points to his eye. Algol at obscuration would.be at the remote end of the diameter, and the dark star at the near end. Study of the circle so placed will show, that, if Algol revolves on an orbit thus situated edgewise to us, the spectroscope ought, before Algol's obscuration, to show that star moving farther from us, and, after obscuration, coining nearer (see 176). By photographing Algol's spectrum, Vogel gained evidence of such motion. Vogel found data to compute the volume and density of this one fixed star Algol. He gives the diameter of Algol as 1,160,000 miles, that of his dark companion as 840,000 miles, and their united density as one fifth that of our sun. The spectra of some stars repeatedly photographed show a periodical doubling of spectroscopic lines, as if spectra from two sources of light were seen overlapping. It is now believed that each such star is a binary, with both of the pair luminous, and situated at the time of the double, like Algol and its dark companion, at obscuration. The pair are too close for telescopic separation, and are called spectroscopic binaries. The star Mizar, at the bend of the Great Dipper's handle, is a telescopic binary, but one of the pair now proves to be a spectroscopic binary, thus making Mizar a triple star. TALKS WITH OBSERVERS. 8l TALKS WITH OBSERVERS. (For end of Art. 45.) Students are warned not to substitute the mere study of the diagram, Fig. 7, page 17, for that obser- vation of nature which it is intended to aid. On the very next clear evening after studying Chapter III, they should go out just as soon as there are stars enough visible to trace the con- stellations, and should make a study of nature. (i.) They must first trace the half ring of the zodiac. The constellations will probably not be just those whose names rep- resent them in Fig. 7. The ecliptic will, nevertheless, extend through them as it does in the diagram, and can be traced by stars on and near it. The semicircle that it makes will, of course, be less definite than the visible line of the drawing (Fig. 7), but the zodiac will be a definite ring, and the student's imagi- nation can help him to represent the line of the ecliptic pass- ing through it. (2.) The observer must next note the zodiacal constellation lying nearest to the eastern horizon. Astronomers tell him, that the earth's center is passing between the sun's cen- ter and some point of that group. His own observation and thought will give to this statement confirmation. The sun, which he knows is not far below the western horizon, was on it at sunset, and the group in the east, from its position, must have been on the eastern horizon at sunset. The earth's rotation on her axis has made both sun and star-group move. The student will thus see clear confirmation of the statement that the earth is passing between the sun and some point of this star-group. If he wishes to satisfy himself further, he can come out at mid- night, when the sun is on the meridian below the horizon, and find this star-group on the meridian above the horizon. Just at this point the observer may think that the diagram of the book (Fig. 7) incorrectly represents, in one particular, the positions of the bodies at sunset. It will seem to him that the sun, as he saw it at sunset, is on the semicircle, or at one side of the figure, whereas, in the drawing, it is placed at the center. The reason is as follows : On what an ordinary observer would call "the western side of the sun at sunset," there is a zodiacal constellation. Anybody could have seen it for a month past, at dark, passing out of sight in the west. Now this group, and in fact every group of the zodiac, has been seen at dark just above the eastern horizon, where the observer now sees another, whose position was discussed in the foregoing paragraph. When each group was seen in that place, it seemed very far from the sun ; no one ever seemed any nearer to the sun than the others. As our theory supposes that the earth is the mover, but the sun and fixed stars have not moved, the sun is represented in the diagram as being at the center of the semicircle, equally distant from all parts of the zodiac. (3.) Let the student next imagine, in the space above him, a great semicircle, part of the earth's orbit, extending round be- tween ecliptic and sun. The star-groups would be little changed from what they were at sunset ; so let the student imagine them to be as they were, and imagine the sun in the west. One end of the semicircle would be beyond the sun, and the other would come down through the observer's position to the earth's center. Let the observer imagine the earth as situated beyond the sun six months ago, and, during those six months, coming round on her orbit to her present position. The facts on which all these suppositions are founded are, that in the successive six months the earth has been seen be- tween the sun and all of these star-groups, in order, beginning in the west. The student must, it is probable, take a good many of these facts on testimony. If he has been observing the heavens he may have seen two, or perhaps three, of these zodi- acal constellations above the eastern horizon at dark. If not, future observation will enable him to depend less on testimony, when at dark he looks up and thinks of the earth's revolution in her orbit. NOTE. In the foregoing discussion of the earth's annual motion, which is to be used after Art. 46 is studied, the reader has been told that the sun is very far away from all the stars of the zodiac ; and in the diagram, page 17, the sun has been placed at the center of the circle which represents the ecliptic. Nevertheless, in Art. 47, and in a part of Chapter IV, the sun is spoken of as " on the ecliptic," just as he would appear to be if we could see sun and stars at the same time. The reason is as follows : The sun's apparent falling back upon the ecliptic is used in measuring the year of our calendar. Although we do not see this movement, we can perfectly measure it, as is shown in Art. 51. It is true the earth, not the sun, is the real mover ; but we do not see the earth move, or know that she does so, except by changes in the position of other bodies. (For Art. 50.) The Ecliptic and the Zodiac. Unless we neglect or refuse to look at the heavens at dark, the groups of the zodiac will be impressed on our organs of sight. The appearances of the visible part of the zodiac, or, in other words, the aspects of the zodiac, have all, at different seasons, their meanings. The observations required in order to see the facts, and also the explanations, are perfectly simple and easy to a person who knows the zodiacal constellations in nature. The following directions tell what to note. Surely every student will desire to look at the heavens with intelligence. During the evenings of November and December we may see an aspect of the zodiac which the student will identify most readily by knowing that Pisces is on the meridian when we see it. This aspect is also above the horizon at sunset, December 22d. To make the meaning of this aspect plain, the student is reminded that the direction in which the zodiac extends from the sun at sunset must be that in which the earth has been moving in her orbit for the previous six months, since the earth's orbit lies between the ecliptic and the sun. On December 22d the earth has traveled northeast for six months, and we know that the sun has appeared to travel south for six months. These facts show that the earth's motion accounts for the apparent movement of the sun. The observer is to note the following points : (i.) That the zodiac extends northeast from the western horizon when Pisces is on the meridian ; (2.) That this aspect 82 ASTRONOMY BY OBSERVATION. comes at an earlier and earlier hour every evening, Pisces having passed the meridian before dark on December 22d, thus con- firming the statement of astronomers that this is the aspect of the zodiac and ecliptic at sunset, December 22d. During the evenings of May and June we can see Virgo on the meridian, giving us the aspect of the zodiac at sunset, June 22d. It is easy to see that the zodiac extends southeast from the western horizon. During the six months before June 22d the earth travels southeast, and we know that the sun appears to travel north during the same six months. The observer can see, at dark, June 22d, that Virgo has passed the meridian, thus confirming the statement of astronomers that this is the aspect of the zodiac at sunset, June 22d. During the fine evenings of February and March we can see Gemini on the meridian. This gives us the aspect of sunset, March aoth. The important point to note is, that the ecliptic and zodiac are more nearly perpendicular to the horizon than at any other season. This is extremely easy to see, and several interesting phenomena, coming near March zoth, and described in this book, depend on the angle made by the two circles. This aspect of the ecliptic, with Gemini on the meridian, shows us the sun's path on the ecliptic during summer, or from March 2oth to September 2ist. The path curves northward, though the student will not think so, unless he remembers that, on the heavens, curving " northward " means nearer to the celestial north pole, not nearer to the north point of the horizon. The whole visible path lies north of the equinoctial, crossing it in the east and west points of the horizon. When the summer solstice in Gemini is on the meridian, it gives us the altitude of the sun at noon on June 22d. It shows that the sun's rays are not vertical, even on June 22d. During the evenings of August and September we can see Sagittarius on the meridian. This gives us the aspect of the zodiac and ecliptic September 22d, at sunset. The important point to note is the long, slanting lines in which the ecliptic and zodiac meet the horizon. It is easy to see that the ecliptic is greatly inclined to the horizon, and this fact explains several interesting phenomena described in this book. Also, the visible half of the ecliptic shows the path traveled by the sun in winter, or from September 2ist to March 2oth. When the winter sol- stice in Sagittarius is on the meridian, it shows the altitude of the sun at noon, December 22d. (For end of Art. 125.) Opposition and Conjunction. The following exercise will visualize* these important positions in the student's mind : Let the student imagine that it is the day on which he knows from the almanac that a planet is in opposition, and that the hour is sunset. The planet would be on the eastern horizon, opposite the sun. Let the student, to aid the imagination, point first in the direction of the sun at sunset, next in the direction * We have a visualized idea of a thing when we can represent it in our imaginations as it exists in nature. A mere representation by diagram or apparatus" does not make us think of it as we see it in nature. of the planet. Let him then answer the following questions : (i.) Which of the three bodies, sun, earth, and planet, is be- tween the others at opposition ? (2.) Is the planet or the earth more distant from the sun ? There is always a point of the earth's orbit on the side of the sun turned away from us. At sunset it is west of the sun. Let the student, still supposing that it is sunset, extend his fore- finger in the direction of that point of the earth's orbit. Let him again point in the direction of the planet at opposition. (3.) Is the earth at opposition on the side of her orbit nearest to the planet, or on the other side ? The student should now draw a horizontal line on a piece of paper. At the middle of the line let him write S for sun. To a person facing south the earth is at the left of the sun. Let the student then make a dot an inch at least to the left of the S, and mark it E. Let him draw a circle with center S and radius S E. Let him find, from his own answers, the place of the planet on the horizontal line. Let him make a dot there, and mark it P at O. Let him, holding the paper with E to the left of S, mark the top of his paper up, and the bottom down. Conjunction. At the conjunction of a planet, it is situated in the same direction from us that the sun is. Let us suppose that it is sunset, and that the same planet is at conjunction. Point in the direction of the sun and planet. Questions : (i.) If you face south, is the planet on your left or your right? (2.) If the sun has not moved, and the planet's apparent change of place, from opposition to conjunction, is due not to its own motion but to the earth's revolution in her orbit, is there any- thing in all this to change the planet's distance from the sun ? If there is not, then draw a dot for the planet on the horizontal line, at the right of the earth, and just as far from the sun as the planet was at opposition. Mark it P at C. If the drawing is correct, a circle which you must draw now, with radius P S, will pass through both the positions of the planet. (3.) Look at your drawing, and tell whether you think a planet is nearer the earth at opposition or at conjunction. (4.) Suppose you saw the planet Mars at opposition, and some months afterward at con- junction, and you found it very plainly diminished in brightness, do you think your drawing would help you to account for the change? (5.) Look at the drawing. Is the planet at conjunc- tion on the side of its orbit nearest the earth, or on the side farthest from the earth ? (6.) Let us again suppose that it is sunset, and that a planet is in conjunction with the sun. Point in the direction of the sun and planet. If the suppositions made were true, could you see the planet ? Give reason. (7.) Where would the earth's rotation carry the sun by dark ? Where would it carry the planet ? (8.) Would you see a planet at all at the precise time of its conjunction ? (See Art. 125.) (For end of Art. 129.) Superior Planets. A student can, to some extent, study in nature the sidereal revolution, on the very first starry night, provided he has already studied in nature the earth's motion as directed in a previous " Talk with Observers." He should go out as soon as the stars become visible. The earth is always between the sun and a point of TALKS WITH OBSERVERS. the ecliptic which is just above the eastern horizon at dark. If Jupiter, Saturn, or Mars were seen at that point, or a very little north or south of it, the planet would be at opposition. The earth would be passing between the planet and the sun, which would be just below the western horizon. This would be evi- dent to an observer who represented in imagination the posi- tions of the three bodies. Let the student imagine Saturn shining just above the eastern horizon. Next, let him trace the ecliptic by stars. Then he should imagine the earth's orbit extending a semicircle in space between this traced ecliptic and the sun. He can represent to himself the earth traveling from west to east around this half of her orbit during the past six months, and also during the corresponding six months of pre- vious years. He knows that during these same six months of every year the earth passes once between the sun and every point in succession of the ecliptic as visible. Now Saturn seems to advance east alongside of the ecliptic ; and, therefore, when Saturn is seen at dark just above the east- ern horizon, the portion of the ecliptic visible between the east- ern and western sides of the horizon would be very nearly that along which Saturn had traveled in the last half of his sidereal revolution, just completed. The student, standing under the stars at dark, should consider all this. Then let him imagine that another observer were present, who had been watching the heavens for fifteen years. This person could say, " I see four- teen points at regular intervals along the ecliptic, beginning in the west, where, in successive years, I have seen Saturn when the earth was passing nearly in line between the planet and the sun, just as she is doing now." The student who had studied the earth's motions a little in nature could accept this statement with some intelligence, and it would be evident, if this testimony of an eye-witness were true, that Saturn would, in fourteen and a half years, have traveled nearly half-way round the sun. Let the observer, still standing under the stars, consider the case of Jupiter. If Jupiter were the planet seen in the east at dark, the earth would be passing in line between Jupiter and the sun, and nearly the half of Jupiter's path last traveled would lie alongside the ecliptic, between the eastern and western hori- zons. The same eye-witness could say, " I see six points along the ecliptic, one in each zodiacal constellation, beginning in the west, in which I have seen Jupiter at times during the last five and a half years, when the earth was passing nearly in line be- between the planet and the sun." If this testimony were true, it would be evident that, in the last five and a half years, Jupiter had traveled nearly half-way round the sun. By following these directions the student's knowledge of the motion of superior planets is, to some extent, a knowledge of nature, and not mere book-study. But this is not enough. There is a vividness in seeing the thing itself, and then going over the reasoning, which is the only perfect security against confusion of mind about it. The student should, therefore, at once learn when the next oppositions of Jupiter and Saturn occur, and, bearing them in mind, should go out and repeat this study of nature at the exact dates, or, if it is cloudy, then on the very first clear night afterward. He should follow this up by watching the proper motion of the planet seen at opposition until it ceases to be evening star. If he sees it again at a second opposition, he will have gained a very reasonable and firmly fixed knowledge of the planet's revolution round the sun. Whenever he saw the planet afterward, he would at a glance recognize, from its change of position, its progress in its sidereal revolution. During some part of the school year Saturn and Jupiter can and will always be seen at dark. For many years to come the opposition of one or the other will come into the school year, but will not always do so. Saturn was at opposition February 25, 1889, and will be at the same about thirteen days later in every year. Jupiter was at opposition June 24, 1889, and will again be about a month later every year. Students who see the planets at opposition should draw them with the constellation in which they are found. They should give their teacher a copy to keep for the benefit of his future classes. No evidence of any eye-witness will give your teacher's future pupils such a convincing sense of reality as that coming from students of their own school or teacher. (Note on Art. 135.) That the apparent retrograde move- ment of superior planets is due to the earth's motion only will be made clearer if the experiment of Art. 135 is repeated, with the slight variation of making the person on the outer circle stand still while the other walks. The latter will see the retro- grade movement repeated as before. The following easily per- formed experiment illustrates the statement in Art. 136. To understand what it proves, the performer must remember that our eyes are our observers, and when we shut them alternately the observer changes place. Let the reader hold up a finger and close his eyes by turns, and he will find that when the left eye becomes the observer the finger seems to move over the background to the right, and it moves to the left when the right eye is used. Let him then try this experiment first with the finger held an inch from the nose, then with the finger about a foot from the nose, and finally with the finger at arm's-length from the nose. Careful observation during the experiment will show that the movement of an observer makes another body appear to retrograde over a smaller extent of background the farther it is removed from him. The theory of Copernicus, that the earth revolves round the sun, makes it necessary to suppose that the fixed stars are at an enormous distance from us. When Copernicus (a German contemporary with Columbus) published his theory, this made for a time a great difficulty in its accept- ance. But the wonderfully refined instruments used by astron- omers in this century have shown a very minute movement of the fixed stars, which must be caused by the earth's motion, since it is always in contrary direction to it. Thus, this is a demonstration of the earth's motion. It is called the " annual parallax " of the fixed stars. APPENDIX A. DESCRIPTION OF CONSTELLATIONS. A LPHA BE TIC A LLY A RRA NGED. (These are intended merely for reference, to aid students in learning the constellations from nature.) ABBREVIATIONS. I. 1st m., 2d m., etc., are applied to stars, to designate first magnitude, second magnitude, etc. II. E., W., N., S., N. E., S. E., N. W., S. W., are used to designate the various points of the compass. III. Z. C. These letters are affixed to all Constellations of the Zodiac. ANDROMEDA. ARIES. Z. C. This constellation is best learned after Pegasus. The N. E. star in the figure called the Great Square of Pegasus, belongs to An- dromeda. This and two other ad m. stars extend N. E. from Pegasus, in a line not quite straight. There are, also, two 3d m. stars on the map. The Great Nebula of Andromeda is one of the most remarkable in the heavens. Aries is best learned after Androm- eda, Pisces, or Taurus. There is a small irregular triangle, containing one 2d m., two 3d m. stars. This must not be confused with another triangle, which is the constellation Triangula. Triangula is a slender, nearly isosceles triangle, which contains no ad m. star. Both are S. E. from Andromeda. All other stars of Aries, except the three in the triangle, are very faint. The ecliptic runs a little more than 8 south of the triangle, which, therefore, is not in the Zodiac, though part of the constellation is there. No star visible to the unaided eye marks the ecliptic. The Water-Bearer. Also, Fomalhaut of Piscis Australis. Aquarius con- tains a small Y, formed of 4th m. stars, which is quite distinct and easy to find, since it is not far S. W. from the Great Square of Pegasus. There is also a figure which, in shape, resembles somewhat the continent of South America. S. A. has 3d m. stars at the angles, but the others are faint. There is risk of getting students to call it South America, by which name it is of course not known to astronomers ; but when attention is called to the resemblance, it makes the constellation much easier to find. A line from the little Y, through S. A., reaches the ist m. star, Fomalhaut, in the eye of the Southern Fish. Fomalhaut is farther south than any other ist m. star except Canopus, which is not seen north of Tennessee. To the east of S. A. there are a number of small stars in little clusters of twos and threes. Aquarius is a man pouring water from a cup. The little Y is on the cup, and the water is pouring from it. The little clusters of twos and threes are in the stream, and so are Fomalhaut and the Fish. The ecliptic crosses S. A., and AQUARIUS. Z. C. east of S. A. are two 4th m. stars joined together by a line on the map, which mark its course. The two southwest stars in S. A. belong to the constellation Capricornus. This should be impressed upon students after they find S. A. It is almost im- possible to have Aquarius learned without using the distin- guishable figure S. A., and with care there will be no confusion. The author has taught Aquarius to many children aged twelve, and always without going out with them at night. . or the Eagle. Aquila is easily distin- " ' guished by three stars in a short line, lying nearly across the Milky-Way. One is the ist m. star Altair. Aquila comes in summer, when the brightest part of the Milky-Way is visible. The figure of A. on the map contains 3d M. stars, and is easily recognized. It lies in the Milky-Way, nearly overhead when on the meridian. One of the stars, Eta, is on the equinoctial. A. can be learned by its position in the Milky- Way, without knowing any adjacent constellation ; or A. may be learned after Sagittarius. Altair can also be known by being the brightest star of the Milky- Way as seen in summer. ATTPTPA or the Wagoner. This can be learned AURIGA, after orion, Perseus, or Gemini. It is very conspicuous, and easily found. It is an irregular five-sided figure, containing one ist m. star, Capella, and two 2d m. stars. There is a small, slender triangle, called " The Kids," which is easy to find. The Ship Argo. This may be learned after Canis Major, from which it is S. E. There are a number of bright stars on the southern horizon, but they are not combined into any figure on the map. They are easily found. One ad m. star, Naos, is clearly seen ; and in the Southern States, another ad m. can be recognized on clear nights. Bootes can be easily found after the Great Dipper is known, from which it is S. E. In early spring, when the Great Dipper is in the east, a brilliant ist m. star can be seen in the northeast, long before the whole constellation Bootes can be found. This star is ARGO NAVIS. BOOTES. APPENDIX A. DESCRIPTION OF CONSTELLATIONS. NORTH EAST WEST CONSTELLATION ORION. SOUTH o Ul _1 (- < 111 ' ^ *\ 8 Ul " o Su o U J -* ; 0; 1-1 ..a. ui O U \ 111 < ee u < fe Ul (/) O Ul ou o OU 86 ASTRONOMY BY OBSERVATION. CANCER. Z. C. Arcturus, and in looking for Bootes it is bast to find Arcturus first. Three stars are in line with Arcturus, and two of these, with others, form a figure resembling a kite. There is also a small triangle S. W. of A., with one 3d m. star. Also, S. E. from A., there are three stars nearly in line, of which one is 3d m. Cancer is a very inconspicuous constellation, but not at all diffi- cult to find after Gemini or Leo are well known. It is between them, a little S. E. from Gemini, a little N. E. from the Sickle in Leo. There is a small triangle of 4th and 5th m. stars, made more evident by having in its center a patch of cloudy light; which is Praesepe, or the Bee-hive nebula. In the telescope, it is a cluster of stars. The triangle is important, because one of the most southern stars is on the ecliptic. PAKTTq MATnR The Greater D S' sometimes CAJNlb MAJUK. called siriuS) from the name of its ist m. star Sirius, which is the brightest of all the fixed stars. Sirius is sometimes called the "dog-star," and the dog- days of summer owe their name to the fact that Sirius is on the meridian above us at noon, during the dog-days. Of course, we can not see him there, but his position is known by calcu- lation. The Greater Dog is best learned after Orion, from which it is S. E. After drawing, there is no possible chance to mistake it. When Sirius is south of us, there can be seen in Florida, and States in the latitude of Georgia, a fine ist m. star, Canopus, just above the southern horizon. It is so far south that it makes but a small arc in crossing the sky, and is above the horizon but a few hours. As it is so near the horizon, it is liable to be blotted out by smoke or fog except on very clear evenings. But the observer in the States mentioned should look for it. It can be seen in the latitude of Tennessee by a person familiar with it ; but it is very near the horizon, and its luster is somewhat obscured. The Lesser Dog. This can best be learned after Gemini, Orion, or Leo. It is S. or a little S. E. from Gemini, nearly E. from the northern part of Orion, and S. W. from Leo. There are two stars on the map; one a ist m. star, Procyon, the other 3d m. Procyon and the 3d m. star are about as far from each other as Castor and Pollux, and, if they were more nearly equal in magnitude, would look a good deal like Castor and Pollux. The Hunting Dogs. These form a very inconspicuous constella- tion between the Great Dipper and Bootes, and contain only a single 3d m. star midway between the two, which is not at all difficult to find. They are put in this " Description " because the constellation shows in the telescope a very remark- able spiral nebula, mentioned in another part of this book. This is best learned after Sa ' gittarius O r Aquarius. There are two 3d m. stars near each other, and forming a short line nearly at right angles with the line of the ecliptic. It is mid- way between Sagittarius and Aquarius, and is easily found. The ecliptic is a little farther from the most southern star than CANIS MINOR. CANES VENATICI. TAPRTrnRKTTTc: 7 r CAPRICORNUS. Z.C. CASSIOPEIA. CENTAURUS. CEPHEUS. they are from each other. In the figure in Aquarius resembling South America, the two 3d m. stars in the S. W. angle belong to Capricornus. Cassiopeia is best learned after the two Dippers. If a line is drawn connecting the stars in the handle-ends of the two Dip- pers, and it is prolonged on the side of Polaris, it will pass through part of the figure of Cassiopeia, given on our map. In late summer and early autumn Cassiopeia is to the right of the northern sky at dark ; in late autumn and early winter she is on or near the meridian at dark ; in late winter and early spring she is to the left of the northern sky at dark. The figure made by the bright stars and one faint one resembles a chair, and is often called Cassiopeia's Chair. The faint star is in the front of the seat of the chair. Without this star, the figure of Cassio- peia somewhat resembles a very irregular W. The Centaur. Centaurus is only par- tially seen in the United States, and is on the southern horizon, S. W. of Scorpio. Its most brilliant stars (ist m.) are below the horizon in the U. S. There are, however, some fine 3d m. stars. There is a figure of a large Y ; a very small and not bright triangle above it; a slender, nearly isosceles triangle east of the Y ; and, to the west of the other figures, two stars near the horizon. Centaurus is visible in the late spring. Cepheus is an inconspicuous con- stellation near the pole. The figure resembles an irregular K. It lies between Cassiopeia, the head of Draco, and the Little Dipper, and is best learned in summer, late spring, or early autumn. It is N. of the Cross in Cygnus. or the Whale. This constellation is ' best learned after Pegasus, Aquarius, and Pisces. It consists, as can be seen on the map, of a quadrilateral of 3d m. stars, E. of Aquarius, and easily dis- tinguished. One of the stars of the quadrilateral has another near it. N. E. from this quadrilateral there is a triangle containing a zd m. star. On the map, the triangle and the quadrilateral are joined, forming a figure resembling the mantis insect, popularly called the " dn one important figure, the Little Dipper, and that is fully described in Chapter I. Polaris is zd m. ; the Guardians are 3d m. \Ttiyr n T ^ e v ' r m - Thi s can best ^ e l earne d VIRGO. Z. C. after Leo> Libra> or Bootes It is nearly E. (a little S. E.) from Leo, nearly W. (a little N. W.) from Libra, and it is S. W. from Bootes. The ist m star Spica helps to identify it. After drawing, the figure is very easily identified. There is a figure resembling somewhat a chair turned back, only the seat of the chair projects too much. The stars of the figure are 3d m., except Spica and a faint 4th m., through which the ecliptic passes and gives it importance. There is another faint sth m. star in Virgo (but not in this figure) through which the ecliptic passes. It is, on the map, between the figure and Libra. This star is said to be in the feet of the Virgin. The ecliptic passes just S. of three of the 3d m. stars, which therefore mark its course. The point where the ecliptic crosses the equinoctial is the point where the sun is found Sep- tember 2 ist. This is therefore the point of the Autumnal Equi- nox, and it should be marked in all copies of this map of Virgo, A. E. NOTE ON ART. 154. We do not actually see Venus at her conjunctions, but, if we could see sun and planet at once, we should see her on or near the horizon. The observer at the sun would see her on our celestial horizon. A careful observer of Venus is almost sure to notice that as evening star she rises more slowly from the horizon than she descends. As morning star the reverse is true. The reason is as follows : Since the earth's annual revo- lution makes the stars seem to rise in the east and set in the west, it must move our horizon down in the east and up in the west. When Venus is evening star, this constantly lessens her d stance from the horizon, and thus her rise is retarded, her descent hastened. When she is morning star, the effect is reversed. The more rapid rise of Venus as morning star gives some advantages to the morning study. NOTE ON ART. 97 The difference between a synodical and a sidereal revolution of the moon can perhaps best be seen by watching her motion from full to full. When she returns to full, we find that she has traveled round the zodiac, or 360, and has also passed through one zodiacal constel- lation twice. When she has passed once round the zodiac, she has made a sidereal revolution ; when she returns to full, she has completed a synodical revolution. ASTRONOMY BY OBSERVATION. APPENDIX B. THE TELESCOPE. ( Taken cliicfly from Lockyers " Astronomy.") Construction. The telescope is a combination of lenses. The principle involved in its construction is simply an extension FIG. 108. Construction of the Astronomical 'lelescope. of that exhibited in the structure of the eye. In the eye nearly parallel rays fall on a lens, and this lens throws an image. In the telescope nearly parallel rays fall on a biconvex lens ; this lens throws an image, and then another lens enables the eye to form an image of the image by rendering the rays again par- allel. These parallel rays enter the eye just as they do in ordi- nary vision. In Fig. 108, for instance, let A represent the front lens, called the object-glass, because it is nearest to the object viewed ; let C represent the other, called the eye-piece, because it is nearest the eye ; and let B represent the image of a distant arrow, the rays from which are seen falling on the object-glass from the left. These rays are refracted, and we get an inverted image at the focus of the object-glass, which is also the focus of the eye-piece. The rays leave the eye-piece adapted for vis- ion as they are when they fall on the object-glass ; the eye can therefore use them as well as if no telescope had been there. The efficiency of the telescope depends on two things its illuminating power and its magnifying power. First, as to its illuminating power. The object-glass, being larger than the pupil of our eye, receives more rays than the pupil. If its sur- face be a thousand times greater than that of the pupil, for instance, it receives a thousand times more light ; and, conse- quently, the image of a star formed at its focus is nearly a thou- sand times brighter than that thrown by the lens of our eye on the retina. The magnifying power depends on two things : first, it de- pends on the focal length of the object-glass ; next, the magni- fying power of the eye-piece is to be taken into account. This varies according to the eye-piece used, the ratio of the focal length of the object-glass to the eye-piece giving its exact amount. Thus, if the focal length of the object-glass is one hundred inches and that of the eye-piece one inch, the tele- scope will magnify one hundred times. But, unless the illumi- FlG - nating power is good and a perfect image is formed, a high magnifying power is useless. If the object-glass does not per- form its part properly, the image will be blurred. The telescope-tube keeps the object-glass and the eye-piece in their proper positions ; and the eye-piece is furnished with a draw-tube, which allows its distance from the object-glass to be varied. Mountings. An astronomer uses the tele- scope for different kinds of work. Accordingly, he mounts, or arranges, his instrument in several different ways. When he desires to watch the heavenly bodies, the only essential is that the instrument should be so arranged as to command every portion of the sky. The best mounting for this purpose is shown in Fig. 109. With such an instrument, called an equatorial, a heavenly body may be followed from its rising to its setting, the proper motion being communicated by machinery. In this arrangement there is a strong iron pillar supporting a head-piece, in which is fixed the polar axis of the instrument par- allel to the axis of the earth. This po- lar axis is made to turn round once in twenty - four hours. The machinery turns the telescope in one direction just as fast as the earth moves in the other, and thus the instrument is kept all the times fixed on a heavenly body. It is incon- venient to fix the telescope on the polar axis, as its range is then limited ; it is fixed, therefore, to an axis at right angles to the polar axis. ; fo A \ ,;! NORTHERN HEMISPHERE K. V II II ""[[^inH I III ! ^^ IX TO _ '; TAURUS ** o a * ,IONITOWSKI CELESTA Showing the Position c the most Irr S + X O^ AQUARIUS' XI IU ?s B O p*T E S - // r i j0F\x r '\ />s, l " ^H V ' WCES ; "7 / V CN AT 1C I PEGASUS ,$ J2t \ ,>^' \-, T IvN * T I ON ^ * Xll .E 2 -^ln 93 x T* V I'A-T ^ .\ * u RS-A-TM AJ OR \ ./i. On,- *< V^ , ~j&l. ^h 3** ^ Jl TELESCOPIU1 < - kt HERSCHEL1I ", Lff \ v /^-<> m c? *C ^xj. / "^ ' . *V . 6 *V ^OEM/^ ' Tolb ' 2^"'JL^V- ^J" . XCANIS-frMINORS / 13 ^ .uo*^ ' S 1W/ tf' ' s2s ! .. *P: - "^ Q' . i A*.. ', r. A * ,^ V > <*o * -^e^o' v-;,^^-*- "V ,,^ d# j^ O 3d 4'h 5*h 6th Nel NOTE The astronom< to the names of stars des constellation in the order tudes were not quite cor an observer detects occasii stars also have varied in Pollux in Gemini, which ii while Castor is a The r can not easily be change SOUTHttfN HEMISPHERE CHART, ne Constellations and rtant Stars. XVIi he applied Greek letters ;d to apply them in each magnitude. But magni- y ascertained, and thus lack of accuracy. Some aient magnitude : thus, ghter than Castor, is j3, :\clature, once applied, APPENDIX C OBSERVATION OF METEORS AND COMETS. APPENDIX C. OBSERVATION OF METEORS AND COMETS. MR. E. E. BARNARD, of Vanderbilt Observatory, Nash- ville, Tennessee, a well-known observer of comets, gave to the author's pupils some simple directions for observation of mete- ors and comets, without instruments. They are of wider use, so the substance of them is here given. Meteors. On any evening when you see them, note care- fully at what point among the stars each appears, at what point it disappears. Trace these paths, as you learn each, on a chart or globe of the heavens. If you are fortunate enough to see a number, you will find that some of the paths intersect. The point of intersection is the radiant, and you will have seen a shower. If you become an accurate observer, some of the peri- odicals devoted to the stars may be glad to publish your report. The record should be about as follows : ist m. meteor, appeared 2 1 /! N., 3 W. of Vega ; disappeared 4 due S. of Altair; time of flight, i 1 /, second ; color reddish, faint train, permanent for 5 seconds, exploded with several red sparks. To such a report is added the date, the mean time, and the exact location of the observer. Any change of color in the meteor during flight, or at the time of bursting, should be noticed and recorded. If the student chances to see a large meteor, there is a special value in his report, for the same object may be seen in some other place, and the two observations will enable astronomers interested in the study of meteors to tell all about it. To this, the author adds a few words in regard to the obser- vation of the Leonids, or November meteors. If the observer sees at once the trains of a number of meteors, the radiant point or intersection is evident without tracing on a map. On November i4th, the constellation Leo is on the eastern horizon at midnight, and the sun, of course, is on the meridian below the horizon. A line drawn from the observer to the sun, and another to the point where the ecliptic and eastern horizon intersect in Leo, would, it is evident, make a right angle. At midnight, the ellipse which is the earth's orbit is below us, except the point we are on (for the orbit must always lie on the same side of us as the sun). Thus it is plain that the line to the eastern horizon is a tangent to the earth's orbit. Now, the student must remember that the earth's motion, like that of a key revolved by the hand on a string, would at any moment carry it in the direction of a tangent to its orbit but for the attraction at the sun holding it fast and continually bending the straight line into a curve or ellipse. Therefore, at midnight, November i4th, the earth is moving directly toward Leo, and the meteors are coming from the quarter toward which the earth is moving at that time. As we only see them on or near November i4th, it is evident that the paths of the earth and the meteors are not identical, but intersecting paths. As they come to meet us, their motion is retrograde, or from east t3 west. When the earth's rotation on her axis makes Leo appear to move west, the radiant point seems to move with it. Comets. In order to observe a comet with intelligence, the comet's position among the stars must be noted, and also the position of the sun at the time. Then it is easy for the ob- server to note its motion toward and from the sun, and the changes it undergoes in approaching and receding from him, especially the changes in its tail. Its path in regard to the ecliptic should be noted, as we thus gain some idea of the plane in which it moves. Mr. Barnard says, " In the case of a large comet, naked-eye observations may be worth recording. The time of the observation should be given to the nearest minute. The limits and general position of the tail should be carefully sketched, and that of the head, with notes as to curvature of tail, brightest parts, etc. Any markings on the tail, such as dark streaks, etc., should be carefully traced. Such work, done accurately, will be valuable work. Above all things, students should be taught to be very careful, and have no uncertainties without fully stating them." FIG. no. NOTE ON ART. 139. A student observing the retrograde movement care- fully in nature would be almost sure to notice that at the end (and also at the beginning) of this movement a planet seems to cease moving among the stars altogether. The almanac notes the times by an entry such as " $ stationary," or " $ stat'y." The explanation is simple. If a diagram is made showing the positions in their or- bits, of the earth and planet at the beginning and end of the stationary period, it is found that all lines running through the centers of the earth and a planet while it is stationary are parallels. (See Fig. no.) It has been shown (Arts. 44, 45) that all parallels passing through any points of the earth's orbit appear to converge to a point upon the celestial sphere. When a planet crosses such parallels, it is all the time seen against the same point of the sphere, and so appears to have stopped moving. NOTE ON ART. 212. The relation of a great planet to such a "comet- family " is expressed by saying that the planet's attraction has captured the comets. The probable history of the Lexell-Brooks comet, lately traced back in connection with that of Jupiter, confirms the " capture theory." Lexell's comet, which had a period of five years and a half, was seen in 1770. It went finally beyond observation after being near Jupiter, and there is some evidence that it was changed to a twenty-seven-year comet with a new ellipti- cal orbit, which would bring it again near Jupiler in 1886, where its orbit might again be changed. Now, a seven-year comet, discovered by Brooks in 1889, can be traced back to this neighborhood in 1886 ; and this leads to the belief that it is the transformed Lexell comet. The orbits of such comets were probably at first parabolas, but were changed to ellipses by planetary attraction. INDEX. (The numbers refer to articles, not to pages.) NOTE. In making this index, the author designed that it should both answer the object of an index and serve as a list of topics for review. The order in which it is wise to treat a subject for beginners, necessarily separates subjects with some connections. It is nearly always best to review in a different order where it can conveniently be managed. It brings out relations which have been neglected, and excites the minds of students, to whom review is tire- some. The author, in teaching other books, has found a good index useful for review, and has employed this experience in making this index. Teachers who have never tried the plan of adopting a different order for review are recommended to make trial. Where the plan seems to make too much repetition, there may be omissions. Absorption of light, by vapors, 174 ; by the at- mosphere of Mars, 198 ; by atmospheres of the fixed stars, 223. Aerolites, defined, 205 ; composition of, 206 ; me- teoric iron and meteoric stones, 206 ; origin of, 207. Aldebaran, 1st m. star, 5 ; color and composition of, 223. Algol, or ft Persei, variable star, 228. Alpha, or a Centauri, annual parallax and distance of, 224. Alphabet, Greek, 6. Altair, 1st m. star, 5 ; color and composition of, 223. Altitude. The angular distance of a heavenly body above the horizon, 19. Andromeda, nebula in, 232 ; new star in, note, page 80. Antares, 1st m. star, 5 ; color and composition of, 223. Aphelion. The point which is farthest from the sun on the orbit of a heavenly body revolving round the sun, 46. Apogee. The position or the sun or moon when at its greatest distance from the earth, 46. Apsides, line of. A line joining the perihelion and aphelion points of the earth's orbit. It is the major axis of an ellipse, 74. Aquarius, star-cluster in, 230. Ara, a southern circumpolar constellation, 235. Arctic and Antarctic circles, 62. Arcturus, 1st m. star, 5 ; motion of, 222 ; color and composition, 223. Aries, 1st point 0^79. Asteroids, minor planets, 188, 192, 193. Axis, celestial, 40. Azimuth. This is angular distance, measured hori- zontally, from the meridian. It is measured by the arc of the horizon intercepted between the meridian and a vertical circle passing through the body whose azimuth is sought, 19. Beta, or ft Persei, variable star called Algol, 228. Betelguese, 1st m. star, 5 ; motion of, 222. Calendar. See " Time," 81-91. Cancer, Praesepe or Beehive Nebula in, 232. Canopus, 1st m. star in Centaurus, 5, 235. Capella, 1st m. star, 5 ; color and composition of, 223. Cassiopeia, described, 7 ; new star in, 229. Centaurus, cluster of stars in, 230. Ceres, an asteroid, 188. Cetus, variable star Mira in, 228. Circles. Great circles bisect each other, 52 ; diur- nal circles, 18, 22 ; small circles, 52 ; circles of perpetual apparition and disparition, 21 ; equi- noctial system of circles, 18, 49 ; horizon system, 19 ; ecliptic system, 48. Clusters of stars, 230. Colures. Great circles passing through the celes- tial poles and the equinoctial or solstitial points of the ecliptic, 49. Comets, description of, 209 ; the parts of comets, comets of Donati and Coggia, vaporous enve- lopes of comets, 210, 211 ; orbits and origin of comets, periodical comets, 212 ; return of com- ets, Halley's comet, effect of large planets on comets, 213 ; telescopic comets, Biela's comet, comet of 1843, 214 ; spectra of comets, comet of 1881, 215 ; numbers of the comets, Newton and the comet of 1680, comet of 1811, 216. Conjunction. A planet or the moon is in conjunc- tion with the sun, when the sun and earth are in line with it, and the earth is not in the middle, r4, 127 ; superior and inferior conjunctions, 148. Constellation, a, defined, 2 ; for account of all the important constellations of the northern hemi- sphere, see Appendix A, page 84. Corona, the sun's, 185. Corona Borealis, new star in, 229. Day and night, unequal, 54-61 ; uses of word day, 83, note ; sidereal and solar days, 26, 83 ; calen- dar days, 84. Declination. This is angular distance north or south from the equinoctial, 18. Deimos, moon of Mars, 198. Density, of sun, 169 ; of planets, 191. Disks, of fixed stars, 225. Distances, of sun from earth, 193 ; of planets from the sun, 193 ; comparative distances from the earth at opposition or conjunction of Mars, Ju- piter, and Saturn, 127, 138 ; variation in a plan- et's distances from the earth, 127 ; distances of fixed stars from the earth, 41, 44, 224. Dorado (sword-fish), southern circumpolar con- stellation, 235. Earth, the, her diurnal rotation, 9. 10 ; plane and direction of diurnal motion, II, 12 ; annual mo- tion, 35, 36 ; inclination of axis, 39, 40 ; the earth and the ecliptic, 34, 47 ; the earth's orbit, 43, 46 ; the earth in aphelion and perihelion, 46 ; inequality of her days and nights, 54-62 ; 1 her seasons, 62-72 ; inequalities of her annual motion, 72, 73 ; revolution of line of apsides, 75 ; revolution of equinoctial points and of the poles, 76, 77 ; her form, 189 ; diameter and vol- ume, 190 ; density, 191 ; distance from the sun, 193- East and west, 12. Eclipses, obseivation and, 104; account of, 112- 118 ; eclipses of moon, 114; of sun, 115 ; of Jupiter's moons, 199. Ecliptic, the. A great circle of the celestial sphere the plane of which passes through the centers of the earth and sun and intersects the plane of the equinoctial at an angle of 23^. Ecliptic de- scribed, 32 ; sun's motion on it, 33, 47 ; relation to the equinoctial, 32 ; the ecliptic as seen at dark and the earth's annual motion, 42 ; aspects of the ecliptic, 50 ; proved a great circle, 52 ; how it is traced on the heavens, 53 ; the eclip- tic and the moon's path, 99-102 ; the ecliptic and the moon's crescent, 105 ; harvest moon and the ecliptic, 108, 109 ; eclipses and ecliptic, 117 ; superior planets and ecliptic, 128, 139, 193 ; Ve- nus and, 144, 150, 193 ; Mercury and, 155. Ecliptic system of circles, 48, 49. Elements, chemical, in the sun, 177 ; in fixed stars, 223 ; in meteors, 205, 206. Ellipses, 46, 192, 208, 212. Epsilon (or c) Lyra;, double star, 227. Equinoctial, the. A great circle of the celestial INDEX. sphere perpendicular to its axis, 18 ; angle with ecliptic, 32. Equinoctial system of circles, 18. Equinoxes, vernal and autumnal, 49. Eta (or n) Argus, variable star, 228. Experiments illustrating the diurnal motion of the stars, 8 ; to investigate the earth's annual revo- lution, 38, 39, 44 ; to illustrate the inclination of the celestial axis, 40 ; to show the plane of the earth's motions, 40 ; to show the effect of oblique heat-rays, 67 ; to illustrate the preces- sion of the equinoxes, 77 ; to illustrate the retro- grade motions of superior planets, 135 ; to illus- trate the motions of inferior planets, 147. Fixed stars. See Stars. Fomalhaut, 1st m. star, 5. Fraunhofer's lines. Name of the dark lines of the solar spectrum, taken from their discoverer, 174, 223. Galaxy, the, the Milky-Way, 6, 231. Globes, celestial, their uses, 17. Gravitation, attraction of, 160-163. Great Dipper, description of, 7 ; motion of the stars in, 221. See constellation Ursa Major in Appendix A. Gregorian Calendar, 87. Grus, the Crane, southern circumpolar constella- tion, 235. Guardians of the Pole, 7. Harvest moon, 109. Heat, obliquity of sun's rays and heat, 63, 68 : the degree of the sun's heat, 171. Hemispheres. The meridian of every place di- vides the celestial sphere into eastern and western hemispheres. Their area constantly changes, 56. The equinoctial divides the celes- tial sphere into northern and southern hemi- spheres. Their area does not change. See maps v and vi. Horizon, celestial. This is a great circle of the celestial sphere of which every point is 90 from the zenith, 19, 45 ; the horizon and the poles, 20. Horizon system of circles, 19. Hour circles. These are great circles passing through the celestial poles and perpendicular to the equinoctial, 18. Hydrus, southern circumpolar constellation, 235. Inclination of the earth's axis, 39, 40. Inferior conjunction, 148. Inferior planets, 140, 188. Illumination of earth, 62. Juno, a minor planet, 188. Jupiter, superior and major planet how to find him, 120-124 I motions as superior planet, 124- 140 ; his retrograde motion, 136 ; comparative distance from the earth at opposition, 127, 138 ; his form, 189 ; diameter and volume, 190 ; den- sity, 191 ; orbit, 192, 193 ; distance from sun, 193 ; period of revolutions, 194 ; telescopic as- pect and physical condition, moons, 199 ; Jupi- ter and comets, 212, 213. Kepler's laws, 159 ; his star, 229. Latitude, celestial, angular distance north or south from the ecliptic, 48, 80. Leonids, meteor train revolving round the sun, 208. Librations of the moon, 107. Light, of sun, 170 ; of fixed stars, 224. Little Dipper, 7. See Ursa Minor, in Appendix A, page 89. Longitude, celestial. Angular distance measured on the ecliptic east from the first point of Aries, 48, 80. Magnitudes of fixed stars (comparative), 3, 5 ; (absolute), 225. Major planets, 186, 187. Mars, superior and major planet how to find him, 120-124 J motions as superior planet, 124-140 ; variation in apparent diameter and distance from the earth, 127 ; comparative distance from the earth at opposition, 138 ; retrograde move- ment, 136 ; orbit and parallax, 167 ; phases, note on page 47 ; form, 189 ; diameter and vol- ume, 190 ; density, 191 ; orbit, 192, 193 ; plane of motion and distance from the sun, 193 ; pe- riod of revolutions, 194 ; telescopic appearance, physical condition, moons, 198. Measurement, celestial, 165. Mercury, inferior major planet how to find him, 155 ; motions as inferior planet, 140-156 ; phases, 155; form, 189; volume and diameter, 190; density, 191 ; orbit, 155, 192 ; plane of motion and distance from the sun, 193 ; period of revo- lutions, 194 ; telescopic aspect, 196. Meridian, the, 19. Meteoroids, defined, 207 ; account of, 203-209 ; meteors, 204 ; meteoric stones and iron, 206 ; numbers of, 207 ; showers of, 208 ; Leonids and Perseids, 208. Milky-Way, 6, 231. Minor planets, 186, 188. Mira, variable star in Cetus, 228. Moon, the, opportunities to observe, 91 ; apparent diurnal revolution, 92 ; real or proper motion, 93 ; revolution round the earth, opposition and conjunction, 94 ; moon's orbit, and her varia- tion in apparent size, 96 ; sidereal and synodical revolution, 97 ; motion round sun, 98 ; moon's path among the stars, 99 ; nodes, 102 ; phases and quarters, 103 ; eclipses and observation, 104 ; position of crescent, 105 ; rotation on axis, 106 ; librations, 107 ; times of rising, 108 ; harvest moon, 109 ; north and south motion, no ; revo- lution of nodes, in ; eclipses, 112-118 ; dis- tance from the earth, 193 ; telescopic aspect and physical condition, 202. Nadir. The nadir is the point of the celestial sphere under our feet. It is one pole of the celestial horizon, 19. Nebulae ; definition ; nebuUe in Andromeda, Can- cer, and Orion ; planetary, ring-shaped, oval and spiral nebula 1 ; spectra of nebuke, 232. Nebular theory, 234. Neptune, major and superior planet ; its discov- ery, 161 ; motions as a superior planet described, 123-140 ; form, 189 ; diameter and volume, 190 ; density, 191 ; orbit, 192 ; plane of orbit and dis- tance from the sun, 193 ; period of revolutions, 194 ; physical condition and telescopic aspect, moons, 201, 234. New style, 88, 89. Newton, Sir Isaac, 160, 162. Nodes. These are points where the orbits of bod- ies revolving round the sun cross the plane of the ecliptic. They cross, going south, at the descend- ing node ; going north, at the ascending node. When we see them apparently crossing the eclip- tic on the celestial sphere, this is the effect of projection, but they are then crossing the plane of the ecliptic, 102. Nomenclature of the fixed stars, 4. Nubecula Major and Nubecula Minor, 233. Obliquity of the sun's rays, 63-68. Occultation of stars by the moon, 202 ; of Jupiter's moons by Jupiter, 199. Old style, 88, 89. Ophiuchus, new star in, 229. Opposition. A planet or the moon is in opposi- tion with the sun when sun, earth, and planet are in line with the earth in the middle, 95, 125, 128. Orbits. These are the paths of bodies revolving around the sun or some planet, 39, 43, 46, 96, 192, 193. Orion, nebula in, 232. Pallas, minor planet, 188. Parallactic motions, the sun's motion north and south, 28 ; retrograde motion of the superior planets, 133-139 ; secular motion of fixed stars toward a point in Hercules, 219. Parallax. An observer's change of place causes a displacement of bodies on the background against which he sees them. This is called parallax, 166 ; horizontal parallax, the parallax of Mars, Venus, and the moon, 166, 167 ; annual parallax of some fixed stars, 224. Parallels. Celestial parallels are small circles par- allel to the ecliptic, 48 ; parallel lines appear convergent, 44. Pavo, the Peacock, southern circumpolar constel- lation, 235. Penumbra, H2. Perigee. The position of the sun or moon when at their least distance from the earth, 46. Perihelion. This is the point nearest the sun's center, on the orbit of a heavenly body revolv- ing round the sun, 46. Perseids, a stream of revolving meteors, 208. Perseus, cluster of stars in, 230. Perturbations of planetary bodies, 160. Phases, of the moon, 103 ; of Venus, 149 ; of Mars, 149, note; of Mercury, 155; of Saturn's rings, 200. Phoenix, a southern circumpolar constellation, 235- 94 ASTRONOMY BY OBSERVATION. Photosphere, the sun's, 178-182. Planes, of ecliptic, 39, 40 ; of planetary orbits, 139, 144, 193- Planets, defined and distinguished, I, 118, 168 ; superior and inferior, 127, 140, 188 ; major and minor planets, 187, 188 ; primary and secondary, 218 ; motion of superior planets discussed, 120- 140 ; their synodical revolutions, oppositions, and conjunctions, 125 ; their proper motions, 126 ; variation in apparent size, variation distance fiom opposition to conjunction, 127 ; sidereal revolutions, 128 ; period of revolutions, how learned, 130; appearances discussed, 131, 132 ; their retrograde motions, 133-138 ; sum- mary of observations of superior planets, 139 ; motions of inferior planets discussed, 140-156 ; their apparent daily revolutions, 142 ; proper motions, 143 ; paths among the stars, and the ecliptic, 144 ; elongations, 145 ; theory of mo- tions, 147 ; their conjunctions, 148 ; their phases, 149 ; variations in apparent size, 146, 155 ; tran- sits, 150, movements as morning stars, 151 ; re- trograde motions, 152 ; how we learn their sy- nodical and sidereal periods, 153, 154 ; the planets of all classes, 186-203 ; their forms, 189 ; diam- eters and volumes, 190 ; densities, 191 ; orbits, 192 ; planes of orbits and distances 'from the sun, 193 ; periods of revolutions, 194 ; rotations, 195 ; telescopic appearances and physical con- ditions, 196-203. Pleiades, the, cluster of stars in Taurus, 230. Pointers, the, stars in Great Dipper, 7. Poles, the celestial, 13 ; elevated and depressed poles, 21 ; revolution of, 77. Polhymnia, a minor planet, its orbit, 192. Pollux., 1st m. star, 5 ; its color, 223. Precession of equinoxes, 76-79. Procyon, 1st m. star, 5 ; color and physical consti- tution, 223. Prominences, the solar, 183-185 ; similar phe- nomena of fixed stars, 229. Quadrature. A planet or the moon is in quadra- ture with the sun when lines drawn from the earth to the sun and planet make a right angle. At quadrature the heavenly bodies are on the meridian at sunrise and sunset. The time of quadrature is half-way between opposition and conjunction. The symbol of quadrature is D, 202. Radiant point, 208. Radius vector, 159. Refraction, 164. Regulus, 1st m. star, 5 ; motion of, 222. Retrograde motion of superior planets, 133-139 ; of inferior planets, 152; of meteors, 208. Revolutions. Apparent diurnal revolution of stars, 8; of sun, 9; of moon, 92; of planets, 125, 142 ; apparent annual revolution of stars, 24 ; of sun on the ecliptic, 47 ; synodical revolution of the moon, 94-98 ; of the superior planets, 125, 130, !53> J 94 J diurnal rotation of earth on her axis, 10, n; annual revolution of earth round the sun, 35, 36 ; sidereal revolution of moon, 97 ; of the planets, 128, 154, 194 ; revolution of the moon's nodes, in; of the equinoctial points upon the ecliptic, 76 ; of perihelion and aphe- lion Ifjoints of the earth's orbit, 75 ; of celestial poles, 76-79 ; apparent revolution of the earth 'cliptic (as seen from the sun), 47 ; revo- double stars around a center, 227. Rigei, -tar, 5 ; motion of, 222. Right as, . This is angular distance meas- ured east on the ecliptic from the first point of Aries, that is fro-n the intersection of the eclip- tic and equinoctial in Pisces, 1 8. Satellites, a name applied to moons, 198-200. Saturn, a superior and major planet, how to find it, 120-124 I comparative distance from the earth at conjunction, 127 ; at opposition, 138 ; Saturn's motions as a superior planet, 120-140 ; his form, 189; volume and diameter, 190; density, 191; orbit, 192 ; plane of orbit, and distance from the sun, 193 ; periods of revolution, 194 ; rotation, 195 ; rings, moons, telescopic aspect, and physi- cal condition, 200 ; attraction for comets, 212, 213. Seasons, the, 63-71. Sirius, 1st m. star, 5 ; motion of, 222 ; physical constitution, 223. Solar system, the, 168. Solstices, summer and winter, 49. Southern Cross, southern circumpolar constella- tion, 235. Spectri, bright-lined, 173 ; reversed and continu- ous, 174 ; solar spectrum, 177 ; spectra of chro- mosphere and solar prominences, 182-184 \ of the solar corona, 185 ; of comets, 215 ; of fixed stars, 223 ; of nebulae, 232. Spectroscope, description of, 175 ; spectroscope and motion, 176. Spica, 1st m. star, 5 ; color and physical constitu- tion, 223. Stars, fixed, definition, i ; constellations of, 2 ; magnitudes, 3 ; nomenclature, 4 ; apparent diur- nal revolution, 8 ; paths of diurnal motion, 22, 23 ; enormous distances of, 41, 44 ; secular ap- parent motion of the stars, 219 ; secular proper motion of, 220 ; star-drift, 221 ; motion of prin- cipal stars toward or from us, 222 ; physical con- stitution of the stars and resemblance to the sun, 223 ; annual parallax and their distances from us, 224 ; fixed stars in telescope, 225 ; numbers of the fixed stars, 226 ; double and multiple stars, 227 ; variable stars, 228 ; new stars, 229 ; star-clusters, 230 ; nebulous stars, 232. Sun, the, apparent diurnal motion of, 9 ; does not make annual revolution, 25, 38 ; solar days, 26 ; apparent motion of the sun north and south, 28 ; sun stationary, 28, 50; revolution on the eclip- tic, 47 ; measurement of the sun's annual mo- tion on the ecliptic, 51 ; variation in distance and apparent size, 43, 46; unequal motion on the ecliptic, 72 ; eclipses of, 115 ; measurement of distance from the earth, 167, 193; diameter, volume, mass, 169 ; light of, 170; heat of, 171 ; telescopic appearance, 178-186; solar spec- trum, 177 ; chemical elements in the sun, 177; elements in chromosphere, 183, 184; corona, 185 ; faculae and granules, 180 ; sun-spots, 181 ; rotation and inclination of axis to the plane of the ecliptic, 181 ; secular motion of the sun, 219 ; comparison of the sun and the fixed stars, 223. Talks with Observers, page 81. Telescope, the, Appendix B, page 86. Temperature, annual change of, 6372 ; tempera- ture and length of days, 70. Terminator. The line separating the dark and illumined portions of the moon, 2O2. Tides, the, 163. Time, or the calendar, 81-91. Titan, moon of Saturn, 200. Toucan, a southern circumpolar constellation, 230, 235- Transits of Venus, 150, 167 ; of the moons of Ju- piter, 199. Tropics of Cancer and Capricorn, 62, 80. Tycho Brahe, 159. Tycho, a crater on the moon, 202. Umbra, 112. Uranus, a superior and major planet, discovery of, 187 ; for explanation of his motions, see account of superior plants, 120-140 ; volume and diame- ter, 190 ; density, 191 ; orbit, 192 ; plane of or- bit and distance from the sun, 193 ; period of his sidereal and synodical revolutions, 194 ; ro- tation, 195 ; physical condition and telescopic aspect, moons, 201. Vega, 1st m. star, 5 ; motion of, 222 ; color and physical constitution of, 223. Venus, an inferior and major planet, 140, 187 ; how to find her, 141 ; apparent diurnal revolution, 142 ; proper motion, 143 ; comparative rate of angular motion, 143 ; Venus and the ecliptic, 144 ; her elongations, 145 ; variation in brill- iancy, 146 ; explanation of her motions, 147 ; her conjunctions, 148 ; her phases, 149 ; her transits, 150, 167 ; Venus, morning star, 151 ; her diameter and volume, 190 ; density, 191 ; orbit, 192 ; plane of o'rbit and distance from the sun, 193 ; time of revolution, 194 ; rotation, 195 ; atmosphere and physical condition, 197. Volumes of planets, 190. Year, the, tropical and sidereal, 8 1 ; calendar year, 86-90 ; leap-year, 86. Zenith. This is the point of the celestial sphere directly over the observer's head, 19. T HE END YE 13142 * \ t f APPLETONS' SCIENCE TEXT-BOOKS. RECENTLY ISSUED. Appletons' School Physics. BY JOHN D. QUACKENBOS (Literary Editor), ALFRED M. MAYER, FRANCIS E. NIPHER, SILAS W. HOLMAN, FRANCIS B. CROCKER. PREVIOUSLY PUBLISHED. The Elements of Chemistry. BY F. W. CLARKE. The Essentials, of Anatomy, Physiology, and Hygiene. BY ROGER S. TRACY, M. D. A Compend of Geology. B i r JOSEPH 'LE CONTE. Elementary Zoology. BY C. F. HOLDER AND y. B. If OLDER, M. L\ Descriptive Botany. Applied Geology. BY ELIZA A. YOUUAKS. BY SAMUEL G. WILLIAMS. Physiological Botany. BY DR. ROBERT BE NT LEY, EDITED BY ELIZA A. YOUMANS. i i i 5 i i i i I I i i *