Another lapse of seven days, and the moon is at D, in the phase called third quarter, while the earth, viewed from the cross on the moon, which is still pointed directly at it, appears again in the shape of a huge half-moon.

During the next seven days the moon returns to its original position at A, and becomes once more new moon, with "full earth" shining upon it.

Now it is evident that in consequence of the peculiar law of the moon's rotation its days and nights are each about two of our weeks, or fourteen days, in length. That hemisphere of the moon which is in the full sunlight at A, for instance, is buried in the middle of night at C. The result is different than in the case of Mercury, because the body toward which the moon always keeps the same face directed is not the luminous sun, but the non-luminous earth.

It is believed that the moon acquired this manner of rotation in consequence of the tidal friction exercised upon it by the earth. The tidal attraction of the earth exceeds that of the sun upon the moon because the earth is so much nearer than the sun is, and tidal attraction varies inversely as the cube of the distance. In fact, the braking effect of tidal friction varies inversely as the sixth power of the distance, so that the ability of the earth to stop the rotation of the moon on its axis is immensely greater than that of the sun. This power was effectively applied while the moon was yet a molten mass, so that it is probable that the moon has rotated just as it does now for millions of years.

As was remarked a little while ago, the moon traveling in an elliptical orbit about the earth has a libratory movement which, if represented in our picture, would cause the cross to swing now a little one way and now a little the other, and thus produce an apparent pendulum motion of the earth in the sky, similar to that of the sun as seen from Mercury. But it is not necessary to go into the details of this phenomenon. The reader, if he chooses, can deduce them for himself.

But we may inquire a little into the effects of the long days and nights of the moon. In consequence of the extreme rarity of the lunar atmosphere, it is believed that the heat of the sun falling upon it during a day two weeks in length, is radiated away so rapidly that the surface of the lunar rocks never rises above the freezing temperature of water. On the night side, with no warm atmospheric blanket such as the earth enjoys, the temperature may fall far toward absolute zero, the most merciful figure that has been suggested for it being 200 deg. below the zero of our ordinary thermometers! But there is much uncertainty about the actual temperature on the moon, and different experiments, in the attempt to make a direct measurement of it, have yielded discordant results. At one time, for instance, Lord Rosse believed he had demonstrated that at lunar noon the temperature of the rocks rose above the boiling-point of water. But afterward he changed his mind and favored the theory of a low temperature.

In this and in other respects much remains to be discovered concerning our interesting satellite, and there is plenty of room, and an abundance of original occupation, for new observers of the lunar world.



CHAPTER IX

HOW TO FIND THE PLANETS

There is no reason why everybody should not know the principal planets at sight nearly as well as everybody knows the moon. It only requires a little intelligent application to become acquainted with the other worlds that have been discussed in the foregoing chapters, and to be able to follow their courses through the sky and recognize them wherever they appear. No telescope, or any other instrument whatever, is required for the purpose. There is but one preliminary requirement, just as every branch of human knowledge presupposes its A B C. This is an acquaintance with the constellations and the principal stars—not a difficult thing to obtain.

Almost everybody knows the "Great Dipper" from childhood's days, except, perhaps, those who have had the misfortune to spend their youth under the glare of city lights. Some know Orion when he shines gloriously in the winter heavens. Many are able to point out the north star, or pole star, as everybody should be able to do. All this forms a good beginning, and may serve as the basis for the rapid acquirement of a general knowledge of the geography of the heavens.

If you are fortunate enough to number an astronomer among your acquaintance—an amateur will do as well as a professor—you may, with his aid, make a short cut to a knowledge of the stars. Otherwise you must depend upon books and charts. My Astronomy with an Opera-Glass was prepared for this very purpose. For simply learning the constellations and the chief stars you need no opera-glass or other instrument. With the aid of the charts, familiarize yourself with the appearance of the constellations by noticing the characteristic arrangements of their chief stars. You need pay no attention to any except the bright stars, and those that are conspicuous enough to thrust themselves upon your attention.

Learn by observation at what seasons particular constellations are on, or near, the meridian—i.e., the north and south line through the middle of the heavens. Make yourself especially familiar with the so-called zodiacal constellations, which are, in their order, running around the heavens from west to east: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces. The importance of these particular constellations arises from the fact that it is across them that the tracks of the planets lie, and when you are familiar with the fixed stars belonging to them you will be able immediately to recognize a stranger appearing among them, and will correctly conclude that it is one of the planets.[21] How to tell which planet it may be, it is the object of this chapter to show you. As an indispensable aid—unless you happen already to possess a complete star atlas on a larger scale—I have drawn the six charts of the zodiacal constellations and their neighbors that are included in this chapter.

[Footnote 21: In our latitudes, planets are never seen in the northern quarter of the sky. When on the meridian, they are always somewhere between the zenith and the southern horizon.]



Having learned to recognize the constellations and their chief stars on sight, one other step, an extremely easy one, remains to be taken before beginning your search for the planets—buy the American Ephemeris and Nautical Almanac for the current year. It is published under the direction of the United States Naval Observatory at Washington, and can be purchased for one dollar.

This book, which may appear to you rather bulky and formidable for an almanac, contains hundreds of pages and scores of tables to which you need pay no attention. They are for navigators and astronomers, and are much more innocent than they look. The plain citizen, seeking only an introduction to the planets, can return their stare and pass by, without feeling in the least humiliated.



In the front part of the book, after the long calendar, and the tables relating to the sun and the moon, will be found about thirty pages of tables headed, in large black letters, with the names of the planets—Mercury, Venus, Mars, Jupiter, Saturn, etc. Two months are represented on each page, and opposite the number of each successive day of the month the position of the planet is given in hours, minutes, and seconds of right ascension, and degrees, minutes, and seconds of north and south declination, the sign + meaning north, and the sign - south. Do not trouble yourself with the seconds in either column, and take the minutes only when the number is large. The hours of right ascension and the degrees of declination are the main things to be noticed.

Right ascension, by the way, expresses the distance of a celestial body, such as a star or a planet, east of the vernal equinox, or the first point of Aries, which is an arbitrary point on the equator of the heavens, which serves, like the meridian of Greenwich on the earth, as a starting-place for reckoning longitude. The entire circuit of the heavens along the equator is divided into twenty-four hours of right ascension, each hour covering 15 deg. of space. If a planet then is in right ascension (usually printed for short R.A.) 0 h. 0 m. 0 s., it is on the meridian of the vernal equinox, or the celestial Greenwich; if it is in R.A. 1 h., it will be found 15 deg. east of the vernal equinox, and so on.



Declination (printed D. or Dec.) expresses the distance of a celestial body north or south of the equator of the heavens.

With these explanations we may proceed to find a planet by the aid of the Nautical Almanac and our charts. I take, for example, the ephemeris for the year 1901, and I look under the heading "Jupiter" on page 239, for the month of July. Opposite the 15th day of the month I find the right ascension to be 18 h. 27 m., neglecting the seconds. Now 27 minutes are so near to half an hour that, for our purposes, we may say Jupiter is in R.A. 18 h. 30 m. I set this down on a slip of paper, and then examine the declination column, where I find that on July 15 Jupiter is in south declination (the sign - meaning south, as before explained) 23 deg. 17 min. 52 sec., which is almost 23 deg. 18 min., and, for our purposes, we may call this 23 deg. 20 min., which is what I set down on my slip.



Next, I turn to Chart No. 5, in this chapter, where I find the meridian line of R.A. 18 h. running through the center of the chart. I know that Jupiter is to be looked for about 30 m. east, or to the left, of that line. At the bottom and top of the chart, every twenty minutes of R.A. is indicated, so that it is easy, with the eye, or with the aid of a ruler, to place the vertical line at some point of which Jupiter is to be found.



Then I consult my note of the declination of the planet. It is south 23 deg. 20 min. On the vertical borders of the chart I find the figures of the declination, and I observe that 0 deg. Dec., which represents the equator of the heavens, is near the top of the chart, while each parallel horizontal line across the chart indicates 10 deg. north or south of its next neighbor. Next to the bottom of the chart I find the parallel of 20 deg., and I see that every five degrees is indicated by the figures at the sides. By the eye, or with the aid of a ruler, I easily estimate where the horizontal line of 23 deg. would fall, and since 20 min. is the third of a degree I perceive that it is, for the rough purpose of merely finding a conspicuous planet, negligible, although it, too, can be included in the estimate, if thought desirable.

Having already found the vertical line on which Jupiter is placed and having now found the horizontal line also, I have simply to regard their crossing point, which will be the situation of the planet among the stars. I note that it is in the constellation Sagittarius in a certain position with reference to a familiar group of stars in that constellation, and when I look at the heavens, there, in the place thus indicated, Jupiter stands revealed.



The reader will readily perceive that, in a precisely similar manner, any planet can be located, at any time of the year, and at any point in its course about the heavens. But it may turn out that the place occupied by the planet is too near the sun to render it easily, or at all, visible. Such a case can be recognized, either from a general knowledge of the location of the constellations at various seasons, or with the aid of the Nautical Almanac, where at the beginning of each set of monthly tables in the calendar the sun's right ascension and declination will be found. In locating the sun, if you find that its right ascension differs by less than an hour, one way or the other, from that of the planet sought, it is useless to look for the latter. If the planet is situated west of the sun—to the right on the chart—then it is to be looked for in the east before sunrise. But if it is east of the sun—to the left on the chart—then you must seek it in the west after sunset.

For instance, I look for the planet Mercury on October 12, 1901. I find its R.A. to be 14 h. 40 m. and its Dec. 18 deg. 36 min. Looking at the sun's place for October 12th, I find it to be R.A. 13 h. 8 m. and Dec. 7 deg. 14 min. Placing them both on Chart No. 4, I discover that Mercury is well to the east, or left hand of the sun, and will consequently be visible in the western sky after sundown.

Additional guidance will be found by noting the following facts about the charts:

The meridian (the north and south line) runs through the middle of Chart No. 1 between 11 and 12 o'clock P.M. on November 1st, between 9 and 10 o'clock P.M. on December 1st, and between 7 and 8 o'clock P.M. on January 1st.

The meridian runs through the middle of Chart No. 2 between 11 and 12 o'clock P.M. on January 1st, between 9 and 10 o'clock P.M. on February 1st, and between 7 and 8 o'clock P.M. on March 1st.

The meridian runs through the middle of Chart No. 3 between 11 and 12 o'clock P.M. on March 1st, between 9 and 10 o'clock P.M. on April 1st, and between 7 and 8 o'clock P.M. on May 1st.

The meridian runs through the middle of Chart No. 4 between 11 and 12 o'clock P.M. on May 1st, between 9 and 10 o'clock P.M. on June 1st, and between 7 and 8 o'clock P.M. on July 1st.

The meridian runs through the middle of Chart No. 5 between 11 and 12 o'clock P.M. on July 1st, between 9 and 10 o'clock P.M. on August 1st, and between 7 and 8 o'clock P.M. on September 1st.

The meridian runs through the middle of Chart No. 6 between 11 and 12 o'clock P.M. on September 1st, between 9 and 10 o'clock P.M. on October 1st, and between 7 and 8 o'clock P.M. on November 1st.

Note well, also, these particulars about the charts: Chart No. 1 includes the first four hours of right ascension, from 0 h. to 4 h. inclusive; Chart No. 2 includes 4 h. to 8 h.; Chart No. 3, 8 h. to 12 h.; Chart No. 4, 12 h. to 16 h.; Chart No. 5, 16 h. to 20 h.; and Chart No. 6, 20 h. to 24 h., which completes the circuit. In the first three charts the line of 0 deg., or the equator, is found near the bottom, and in the last three near the top. This is a matter of convenience in arrangement, based upon the fact that the ecliptic, which, and not the equator, marks the center of the zodiac, indicates the position of the tracks of the planets among the stars; and the ecliptic, being inclined 23 deg. to the plane of the equator, lies half to the north and half to the south of the latter.

Those who, after all, may not care to consult the ephemeris in order to find the planets, may be able to locate them, simply from a knowledge of their situation among the constellations. Some ordinary almanacs tell in what constellations the principal planets are to be found at various times of the year. Having once found them in this way, it is comparatively easy to keep track of them thereafter through a general knowledge of their movements. Jupiter, for instance, requiring a period of nearly twelve years to make a single journey around the sun, moves about 30 deg. eastward among the stars every year. The zodiacal constellations are roughly about 30 deg. in length, and as Jupiter was in Sagittarius in 1901, he will be in Capricornus in 1902. Saturn, requiring nearly thirty years for a revolution around the sun, moves only between 12 deg. and 13 deg. eastward every year, and, being in conjunction with Jupiter in Sagittarius in 1901, does not get beyond the border of that constellation in 1902.

Jupiter having been in opposition to the sun June 30, 1901, will be similarly placed early in August, 1902, the time from one opposition of Jupiter to the next being 399 days.

Saturn passes from one opposition to the next in 378 days, so that having been in that position July 5, 1901, it reaches it again about July 18, 1902.

Mars requires about 687 days to complete a revolution, and comes into conjunction with the earth, or opposition to the sun—the best position for observation—on the average once every 780 days. Mars was in opposition near the end of February, 1901, and some of its future oppositions will be in March, 1903; May, 1905; July, 1907; and September, 1909. The oppositions of 1907 and 1909 will be unusually favorable ones, for they will occur when the planet is comparatively near the earth. When a planet is in opposition to the sun it is on the meridian, the north and south line, at midnight.

Mercury and Venus being nearer the sun than the earth is, can never be seen very far from the place of the sun itself. Venus recedes much farther from the solar orb than Mercury does, but both are visible only in the sunset or the sunrise sky. All almanacs tell at what times these planets play their respective roles as morning or as evening stars. In the case of Mercury about 116 days on the average elapse between its reappearances; in the case of Venus, about 584 days. The latter, for instance, having become an evening star at the end of April, 1901, will become an evening star again in December, 1902.

With the aid of the Nautical Almanac and the charts the amateur will find no difficulty, after a little practise, in keeping track of any of the planets.

In the back part of the Nautical Almanac will be found two pages headed "Phenomena: Planetary Configurations." With the aid of these the student can determine the position of the planets with respect to the sun and the moon, and with respect to one another. The meaning of the various symbols used in the tables will be found explained on a page facing the calendar at the beginning of the book. From these tables, among other things, the times of greatest elongation from the sun of the planets Mercury and Venus can be found.

It may be added that only bright stars, and stars easily seen, are included in the charts, and there will be no danger of mistaking any of these stars for a planet, if the observer first carefully learns to recognize their configurations. Neither Mars, Jupiter, nor Saturn ever appears as faint as any of the stars, except those of the first magnitude, included in the charts. Uranus and Neptune being invisible to the naked eye—Uranus can occasionally be just glimpsed by a keen eye—are too faint to be found without the aid of more effective appliances.



INDEX

Agassiz, Alexander, on deep-sea animals, 63.

Asteroids, the, 16, 129. brightness of, 130. imaginary adventures on, 146. life on, 144. number of, known, 129. orbits of, 132. origin of, 138, 143. size of, 129.

Aristarchus, lunar crater, 226.

Atmosphere, importance of, 20.

Bailey, Solon I., on oppositions of Eros, 134.

Barnard, E.E., discovers fifth satellite of Jupiter, 181. measures asteroids, 129. on Saturn's rings, 205.

Belopolski, on rotation of Venus, 79.

Ceres, an asteroid, 129, 130.

Clefts in the moon, 226.

Copernicus, lunar crater, 223, 242.

Darwin, George H., on Jupiter and Saturn, 206. on origin of moon, 235. theory of tidal friction, 32.

Davy, Sir Humphry, on Saturn, 190.

Dawes sees canals on Mars, 93.

Deimos, satellite of Mars, 125.

Denning, W.F., description of Jupiter, 175.

De Vico on rotation of Venus, 76.

Dewar, James, discovers free hydrogen in air, 232.

De Witt discovers Eros, 133.

Dick, Thomas, on Saturn, 201.

Douglass, A.E., sees Mars's canals, 92. sees clouds in Mars, 119.

Doppler's principle, 79, 200.

Earth and moon's orbit, 217. birth of moon from, 236. change of distance from sun, 27. less advanced than Mars, 89. older than Venus, 58. seen from Mercury, 41. seen from Venus, 69-71, 75. seen from moon, 214.

Earth, similarity to Venus, 46. supposed signals to and from Mars, 110.

Elger, T.G., on cracks in moon, 227. on Tycho's rays, 246.

Ephemeris, how to use, 260, 264.

Eros, an asteroid, 131-134, 136, 137.

Flammarion, C., observes Venus's atmosphere, 56. on plurality of worlds, 8.

Forbes, Prof. George, on ultra-Neptunian planet, 210.

Galileo on lunar world, 215.

Gravity, as affecting life on planets, 20, 46.

Hall, Asaph, discovers Mars's moons, 90.

Herodotus, lunar crater, 227.

Herschel, Sir John, on Saturn, 185.

Holden, E.S., on photograph of lunar crater, 242.

Huggins on Mercury's atmosphere, 21.

Inhabitants of foreign planets, 1, 4, 5.

Interplanetary communication, 1, 3, 72, 110, 112.

Juno, an asteroid, 129.

Jupiter, cloudy aspect of, 165. density of, 162. distance of, 161. equatorial belts on, 165. future of, 180. gravity on, 162. great red spot on, 169. markings outside the belts, 168. and the nebular theory, 178. once a companion star, 179. polar compression of, 161. possibly yet incandescent, 177. question of a denser core, 176. resemblance of, to sun, 174. rotation of, 161, 173. satellites of, 166, 181. seen from satellites, 182. size of, 160. solar light and heat on, 182. south belt of, 172. surface conditions of, 163. theories about the red spot, 170. trade-winds and the belts of, 167. various rates of rotation of, 173. visibility of rotation of, 166.

Keeler, J.E., on Saturn's rings, 200.

Kepler, lunar crater, 223.

Kinetic theory of gases, 116.

Kirkwood, Daniel, on asteroids, 131.

Lagrange on Olbers's theory, 139.

Lick Observatory and Mars's canals, 92.

Life, a planetary phenomenon, 10. in sea depths, 62. on planets, 62, 63. prime requisites of, 64. resisting extreme cold, 123. universality of, 9.

Loewy and Puiseux, on lunar atmosphere, 248. on lunar "seas," 234.

Lowell, Percival, description of Mars, 108. on markings of Venus, 60. on Mercury's rotation, 33. on rotation of Venus, 77. sees Mars's canals, 92. theory of Martian canals, 101.

Lucian, on appearance of earth from moon, 213.

Lyman, C.S., observes Venus's atmosphere, 55.

Mars, age of, 89. atmosphere of, 86, 115, 117. bands of life on, 104. canals on, 90. described by Schiaparelli, 93. gemination of, 91, 105. have builders of, disappeared? 107. and irrigation, 101. and lines of vegetation, 102. and seasonal changes, 99. and water circulation, 100. carbon dioxide on, 118. circular spots or "oases" on, 103. colors of, 89. dimensions of, 86. distance of, 85, 86. enigmatical lights on, 111. gravity on, 86. inclination of axis, 86. length of year, 86. Lowell's theory of, 101. light and heat on, 85. moonlight on, 128. orbit of, 85. polar caps of, 87, 118. possible size of inhabitants, 106. satellites of, 90, 124, 126. seasons on, 87. supposed signals from, 110, 112. temperature of, 120, 122. water vapor on, 117.

Mercury, atmosphere of, 21, 28, 43, 44. day and night on, 34, 38, 40. dimensions, 18. earth seen from, 41. habitability of, 33, 40, 44. heavens seen from, 41, 42. heat and light on, 25, 28. holds place of honor, 19. length of year, 24. mass of, 19. moon visible from, 41. resemblances to moon, 43. rotation of, 30. shape of orbit, 23. sun as seen from, 37. velocity in orbit, 23. Venus seen from, 41. virtual fall toward sun, 24. visibility of, 21. water on, 43.

Moon, the area of surface, 219. atmosphere of, 7, 215, 231, 247, 248. clouds on, 6, 245. constitution of, 236. craters, 221. day and night on, 254. distance of, 212, 215. density of, 219. former cataclysm on, 237. former life on, 241, 243. giantism on, 228, 229. gravity on, 219, 228, 229. libration of, 249. meteorites and, 230. mountains on, 220. the older world in, 242. origin of, 235. phases and motions of, 250. rotation of, 249. seas of, 234. size of, 218. snow on, 246. speculation about, 212. temperature of, 255. vegetation on, 6, 244, 247. visibility of features of, 213.

Nasmyth and Carpenter on lunar craters, 224.

Neptune, description of, 208-210.

Newcomb, Simon, on Olbers's theory, 141.

Newton, lunar crater, 222.

Olbers's theory of planetary explosion, 138. on Vesta's light, 138.

Pallas, an asteroid, 129.

Perrotin sees canals on Mars, 92.

Phobos, satellite of Mars, 125.

Pickering, E.C., discovers ninth moon of Saturn, 195. finds Eros on Harvard plates, 133. on shape of Eros, 136. on light of Eros, 137.

Pickering, W.H., on lunar atmosphere, 247. observes changes in moon, 244. sees Mars's canals, 92. theory of Tycho's rays, 246. on Venus's atmosphere, 54.

Planets, classification of, 15. how to find, 256, 273. resemblances among, 12.

Plato, lunar ring plain, 225.

Plurality of worlds in literature, 2. subject ignored, 8.

Proctor, R.A., on Jupiter's moons, 180. on other worlds, 8.

Roche's limit, 201.

Rosse, Lord, on temperature of moon, 255.

Saturn, age of, 189. composition of, 190. density of, 188. distance of, 186. the gauze ring, 199-202. gravity on, 188. inclination of axis, 187. interior of, 206. length of year, 186. popular telescopic object, 185. rings of, 185, 196. gaps in, 197. origin of, 200. periodic disappearance of, 198. seen from planet, 207. shadow of, 198. rotation of, 187. satellites of, 195. size of, 187.

Schiaparelli discovers canals on Mars, 90. describes Martian canals, 93. discovers Mercury's rotation, 30, 32. on rotation of Venus, 76.

Solar system, shape and size of, 14. unity of, 9. viewed from space, 11.

Stoney, Johnstone, on atmospheres of planets, 116. on escape of gases from moon, 231.

Sun, the, isolation in space, 13. no life on, 10. resemblances with Jupiter, 174.

Swedenborg, on Saturn's rings, 204.

Tidal friction, 80, 81, 236, 253.

Tycho, lunar crater, 222.

Ultra-Neptunian planet, 210.

Uranus, description of, 208-210.

Venus, age of, 58. atmosphere of, 53, 55, 59, 61, 68. absence of seasons on, 51. density of, 47. distance of, 47, 50. gravity on, 46, 47. inclination of axis, 50. life on, 57, 58, 61, 65, 67, 68, 82, 117. light and heat on, 50-57. orbit of, 50. phases of, 49. resemblances of, to earth, 46. rotation of, 76, 79, 80. size of, 46. twilight on, 83. visibility of, 47.

Vesta, an asteroid, 129, 130, 138.

Vogel on Mercury's atmosphere, 21.

Wireless telegraphy, 1, 112.

Young, C.A., on Olbers's theory of asteroids, 142. on temperature of Mars, 122. on Venus's atmosphere, 53.

Zodiac, the, 258.

THE END



A NEW BOOK BY PROF. GROOS.

The Play of Man.

By KARL GROOS, Professor of Philosophy in the University of Basel, and author of "The Play of Animals." Translated, with the author's cooperation, by Elizabeth L. Baldwin, and edited, with a Preface and Appendix, by Prof. J. Mark Baldwin, of Princeton University. 12mo. Cloth, $1.50 net; postage, 12 cents additional.

The results of Professor Groos's original and acute investigations are of peculiar value to those who are interested in psychology and sociology, and they are of great importance to educators. He presents the anthropological aspects of the subject treated in his psychological study of the Play of Animals, which has already become a classic. Professor Groos, who agrees with the followers of Weismann, develops the great importance of the child's play as tending to strengthen his inheritance in the acquisition of adaptations to his environment. The influence of play on character, and its relation to education, are suggestively indicated. The playful manifestations affecting the child himself and those affecting his relations to others have been carefully classified, and the reader is led from the simpler exercises of the sensory apparatus through a variety of divisions to inner imitations and social play. The biological, aesthetic, ethical, and pedagogical standpoints receive much attention from the investigator. While this book is an illuminating contribution to scientific literature, it is of eminently practical value. Its illustrations and lessons will be studied and applied by educators, and the importance of this original presentation of a most fertile subject will be appreciated by parents as well as by those who are interested as general students of sociological and psychological themes.

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