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EMBRACING ELEMENTARY GERMAN G-RA3MMIAR, COMPLETE OERIVLAIN' C on the ecliptic, in Gemini ; but at D, it appears to us to be at G, in Taurus. So that the planet has retrograded through "an entire sign on the ecliptic, while its course all the while has been directly for-s 78 THE SOLAR SYSTEM. ward in the order of the signs ; and to an observer at the sun, such would have been its motion. Phases of an inferior planet An inferior planet presents all the phases of the moon. At superior conjunction, the whole illumined disk is turned to- ward us ; but the planet is lost in the sun's rays : therefore neither Mercury nor Venus ever presents a full circular appearance, like the full moon. A little before or after superior conjunction, an inferior Fig. 21. >** JT\ O (V. ^^O^o^-^ 4) .ft, ^^0^ PHASES OF AN INFERIOR PLANET. planet may be seen with a telescope ; but the whole of the light side is not turned toward us, and so the planet appears gibbous, like the moon between first quarter and full. In quadrature, the planet shows us only one-half its illumined disk ; this decreases, becoming more and more crescent toward inferior conjunction, at which time the unillumined side is toward us. MOTIONS OF A SUPEKIOB PLANET. The superior planet moves in an orbit which entirely surrounds THE PLANETS. 79 that of the earth. When the earth is at E (Fig. 22), the planet at L is said to be in opposition to the sun. It is then at its greatest distance from him 180. The planet is on the meridian at midnight while the sun is on the corresponding meridian on the opposite side of the earth ; or the planet may be rising when the sun is just setting. When the planet is at N, it is in conjunction, and being lost in the sun's rays is invisible to us. Retrograde ' 'motion of a superior planet. Suppose the earth to be at E and the planet at L, and that we move on to G while the planet passes on to O the distance EG being longer than LO (just the reverse of what takes place in the movements of the inferior planets) ; at E, we should locate the planet at P on the ecliptic, in the sign Cancer ; but at G, it would appear to us at Q, in the sign Gemini, having apparently retrograded on the ecliptic the distance PQ, while it was all the while moving on in the direct order of the signs. Now, suppose the earth moves on to I and the planet to U, we should then see it at the point W, further on in the ecliptic than Q, which indicates direct motion again, and at some point near Q the planet must have appeared without motion. After this, it will continue direct until the earth has completed a large portion of her orbit, as we shall easily see by imagining various positions of the earth and planet, and then drawing lines as we have just done, noticing whether they indicate direct or retrograde motion. The greater 80 THE SOLAR SYSTEM. the distance of a planet the less it will retrograde, as we shall perceive by drawing another orbit out- side the one represented in the cut, and making the same suppositions concerning it as those we have already explained. RETROGRADE MOTION OF A SUPERIOR PLANET. SIDEREAL AND SYNODIC KEVOLUTION. The interval of time required by a planet to perform a revolution from one fixed star back to it again, is termed a sidereal revolution (sidus, a star). 1. The interval of time between two similar con- THE PLANETS. 81 junctions of an inferior planet with the earth and sun is termed a synodic revolution. Were the earth at rest, there would be no difference between a sidereal and a synodic revolution, and the planet would come into conjunction twice in each revolution. Since, however, the earth is in motion, it follows that after the planet has completed its sidereal revolution, it must then overtake the earth before they can both come again into the same position with regard to the sun. The faster a planet moves, the sooner it can do this. Mercury, travelling at the greater speed and on an inner orbit, accom- plishes it much quicker than Yenus. The synodic period always exceeds the sidereal. 2. The interval between two successive conjunc- tions or oppositions of a superior planet is termed a synodic revolution. Since the earth moves so much faster than any superior planet, it follows that after it has completed a sidereal revolution it must then overtake the planet before they can come again into the same position with regard to the sun. The slower the planet moves, the sooner it can do this. Uranus, making a sidereal revolution in eighty-four years, can be overtaken more quickly than Mars, which makes one in less than two years. It conse- quently requires over a second revolution to catch up with Mars, ^ of one to overtake Jupiter, and but little over y^ of one to come up with Uranus. In- deed, the earth repasses Neptune in two days after it has finished a sidereal revolution. 82 THE SOLAR SYSTEM. PLANETS AS EVENING AND MORNING STARS. The in- ferior planets are evening stars from superior to inferior conjunction, and the superior planets from opposition to conjunction. During the other half of their revolutions they are morning stars. Mercury, evening star 2 months. Venus, " " 9J '" Mars, " " 13 Jupiter, " " 6J Saturn, " " 6 Uranus, " " 6 " To avoid filling the text with a multiplicity of figures, many interesting items are condensed in tables at the close of the volume. VULCAN. SUPPOSED DISCOVERY. Le Verrier, having detected an error in the assumed motion of Mercury, sug- gested, in the fall of 1859, that there may be an interior planet, which is the cause of this disturb- ance. On this being made public, M. Lescarbault, a French physician, and an amateur astronomer, stated that on March 26 of that year he had seen a dark body pass across the sun's disk, and that this might have been the unknown planet. Le Verrier visited him, and found his instruments rough and home-made, but singularly accurate. His clock was a simple pendulum, consisting of an ivory ball hang- MERCURY. 83 ing from. a nail by a silk thread. His observations were on prescription paper, covered with grease and laudanum. His calculations were chalked on a board, which he planed off to make room for fresh ones. Le Verrier became satisfied that a new planet had been really discovered by this enthusiastic ob- server, and congratulated him upon his deserved success. On March 20, 1862, Mr. Lummis, of Man- chester, England, noticed a rapidly-moving, dark spot, apparently the transit of an inner planet. Many other instances are given of a somewhat sim- ilar character. As yet, however, the existence of the planet is not generally conceded. The name Vulcan and the sign of a hammer have been given to it. Its distance from the sun has been estimated at 13,000,000 miles, and its periodic time (its year) at 20 days. MEECUEY. The fleetest of the gods. Sign, , his wand. DESCRIPTION. Mercury is nearest to the sun of any of the definitely known planets. , When the sky is very clear, we may sometimes see it, just after the setting of the sun, as a bright sparkling star, near the western horizon. Its elevation increases evening by evening, but never exceeds 30.* If we watch it closely, we shall find that it again ap- * This distance varies much, owing to the eccentricity of Mcr cury's orbit. 84 THE SOLAR SYSTEM. proaclies the sun and becomes lost in his rays. Some days afterward, just before sunrise, we can see the same star in the east, rising higher each morn- ing, until its greatest elevation equals that which it before attained in the west. Thus the planet appears to slowly but steadily oscillate like a pendulum, to and fro from one side to the other of the sun. The ancients, deceived by this, failed to discover the iden- tity of the two stars, and called the morning star Apollo, the god of day, and the evening star Mer- cury, the god of thieves, who walk to and fro in the night-time seeking plunder. The Greeks gave to Mercury the additional name of "The Sparkling One." The astrologists looked upon it as the malig- nant planet. The chemists, because of its extreme swiftness, applied the name to quicksilver. The most ancient account that we have of this planet is given by Ptolemy, in his Almagest ; he states its location on the 15th of November, 265 B. c. The Chinese also state that on June 9, 118 A. D., it was near the Beehive, a cluster of stars in Cancer. Astronomers tell us that, according to the best calculations, it was at that date within less than 1 of that group. On account of the nearness of Mercury to the sun, it is difficult to be detected.* It is said that Coper- nicus, an old man of seventy, lamented in his last moments that, much as he had tried, he had never * An old English writer by the name of Goad, in 1686, humor- ously termed this planet, " A squinting lacquey of the sun, who seldom shows his head in these parts, as if he were in debt" MERCURY. 85 been able to see it. In our latitude and climate, we can generally easily detect it if we watch for it at the time of its greatest elongation or quadrature, as given in the almanac. MOTION IN SPACE. It revolves about the sun at a mean distance of 35,000,000 miles. Its orbit is the most eccentric (flattened) of any among the eight principal planets, so that although when in peri- helion it approaches to within 28,000,000 miles, in aphelion it speeds away 15,000,000 miles farther, or to the distance of 43,000,000 miles. Being so near the sun, its motion in its orbit is correspondingly rapid viz., 30 miles per second. At this rate. of speed, we could cross the Atlantic Ocean in two minutes. The Mercurial year comprises only about 88 days, or nearly three of our months. Mercury revolves upon its axis in about the same time as the earth, so that the length of the Mercurial day is nearly the same as that of the terrestrial one. Though Mercury thus completes a sidereal revolu- tion around the sun in 88 days, yet to pass from one inferior or superior conjunction to the same again (a synodic revolution) requires 116 days. The reason of this is, as already explained, that when Mercury comes around to the same spot in its orbit again, the earth has gone forward, and it requires 28 days for the planet to overtake us. DISTANCE FROM THE EARTH. This varies still more than its distance from the sun. At inferior conjunc- tion it is between the earth and the sun, and its dis- 86 THE SOLAR SYSTEM. tance from us is the difference between the distance of the earth and the planet from the sun : at supe- rior conjunction it is the sum of these distances. Its apparent diameter in these different positions varies in the same proportion as the distances, or as three to one. The greatest and least distances vary ac- cording as either planet may happen to be in aphe- lion or perihelion. If at inferior conjunction Mer- cury is in aphelion and the earth in perihelion, its distance from us is only 90,000,000 - 43,000,000 = 47,000,000 miles. If at superior conjunction Mer- cury is in aphelion and the earth in aphelion also, its distance from us is 93,000,000 + 43,000,000 = 136,000,000 miles. DIMENSIONS. Mercury is about 3,000 miles in di- ameter. Its volume is about -fa that of the earth i. e., it would require twenty globes as large as Mer- cury to make one the size of the earth, or 25,000,000 to equal the sun. Yet as it is | denser than the earth, its weight is nearly ^ that of the earth, and a stone let drop upon its surface would fall 7J feet the first second. Its specific gravity is about that of tin. A pound weight removed to Mercury would weigh only about seven ounces. SEASONS. As Mercury's axis is much inclined from a perpendicular, its seasons are peculiar. There are no distinct frigid zones; but large re- gions near the poles have six weeks continuous day and torrid heat, alternating with a night of equal length and arctic cold. The sun shines perpendic- MERCUKY. 87 ularly upon the torrid zone only at the equinoxes, while he sinks far toward the southern horizon at one solstice, and as far toward the northern hori- zon at the other. The equatorial regions, there- fore, modify their temperature during each rev- Fig. 29 ORBIT AND SEASONS OF MERCURY. olution from torrid to temperate, and the tropical heat is experienced alternately toward the north and south of what we call the temperate zones. There is no marked distinction of zones as with us, but each zone changes its character twice during the Mercurial year, or eight times during the terrestrial one. An inhabitant of Mercury 88 THE SOLAR SYSTEM. must be accustomed to the most sudden and vio- lent vicissitudes of temperature. At one time the sun not only thus pours down its vertical rays, and in a few weeks after sinks far down toward the horizon, but, on account of Mercury's elliptical orbit, when in perihelion the planet approaches so near the sun that the heat and light are ten times as great as that we receive, while in aphelion it recedes so as to reduce the amount to four and a half times (the average, however, is seven times), a temperature sufficient to turn water to steam, and even to melt many of the metals. This entire round of transitions is swept through four times during one terrestrial year. The relative length of the days and nights is much more variable than with us. The sun, apparently seven times as large as it seems to us, must be a magnifi- cent spectacle, and illumine every object with insuf- ferable brilliancy. The evening sky is, however, lighted by no moon. TELESCOPIC FEATURES. Under the telescope, Mer- cury presents all the phases of the moon, from a slender crescent to gibbous, when its light is lost in that of the sun. These phases prove that Mer- cury is spherical, and shines by the light reflected from the sun. When in quadrature, it can some- times be detected with a telescope in daylight. Being an inferior planet, we can never see it when full, and hence the brightest, nor when nearest the earth, as then its dark side is turned toward us. Owing to the dazzling light, and the vapors almost VENUS. 89 always hanging around our horizon, this planet has not received much attention of late ; the cuts here given, and the remarks concerning its physical fea- tures, are based upon the observations of the older astronomers. It is thought by some to have a dense atmosphere loaded with clouds, which would materially diminish the intensity of the sun, and perhaps make Mercury quite habitable. Sir W. Herschel, however, emphatically denies this, and asserts that the atmosphere is too insignificant to be detected. There are some dark bands about its equator. It has lofty mountains, which intercept the light of the sun, and deep valleys plunged in shade. One mountain has been ascertained to be about ten miles in height, which is -3^5- of the di- ameter of the planet. The height of the Dhawa- laghiri of the Himalayas is less than 29,000 feet, or y^Vir P ar ^ of the earth's diameter. VENUS. The Queen of Beauty. Sign ? , a looking-glass. DESCRIPTION. Venus, the next in order to Mer- cury, is the most brilliant of all the planets. When visible before sunrise, she was called by the ancients Phosphorus, Lucifer, or the Morning Star, and when she shone in the evening after sunset, Hesperus, Ves- per, or the Evening Star. She presents the same appearances as Mercury. Owing, however, to the greater diameter of her orbit, her apparent oscillations 90 THE SOLAB SYSTEM. are nearly 48 east and west of the sun,* or about 18 more than those of Mercury. She is therefore seen much earlier in the morning and much later at night. She is " morning star" from inferior to supe- rior conjunction, and " evening star" from superior to inferior conjunction. She is the most brilliant about five weeks before and after inferior conjunc- tion, at which time the planet is bright enough to cast a shadow at night. If, in addition, at this time of greatest brilliancy, Yenus is at or near her high- est north latitude, she may be seen with the naked eye in full daylight.t This occurs once in eight years, in which interval the earth and planet return to the same situation in their orbits ; eight complete revolutions of the earth about the sun occupying nearly the same time as thirteen of Venus. This happened last in February, 1862. A less degree of brilliancy is attained once in twenty-nine months, under somewhat the same circumstances. MOTION IN SPACE. Unlike Mercury, Venus has an orbit the most circular of any of the principal * This distance varies but little, owing to the slight eccentricity ^>f Venus's orbit. t Arago relates that Bonaparte, upon repairing to the Luxem- bourg, when the Directory was about to give him a fete, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. Upon inquiry, he learned that these curious persons were observ- ing with astonishment a star which they supposed to be that of the Conqueror of Italy. The emperor himself was not indifferent when his piercing eye caught the clear lustre of Venus smiling upon him at midday. VENUS. 91 planets. Her mean distance from the sun is about 66,000,000 miles, which varies at aphelion and peri- helion within the limits of a half million miles against 15,000,000 miles in the case of the former planet. She makes a complete revolution around the sun in about 225 da} r s, at the mean rate of 22 miles per second ; hence her year is equal to about seven and one half of our months. This is a sidereal revolu- tion, as it would appear to an observer at the sun, but a synodic revolution is 584 days. Mercury, we remember, catches up with the earth in 28 days after it reaches the point where it left the earth at the last inferior conjunction. But it takes Venus nearly two and a half revolutions to overtake the earth and come into the same conjunction again. This grows out of the fact that Yenus has a longer orbit to travel through, and moves only about one-fifth faster than the earth, while Mercury travels nearly twice as fast. The planet revolves upon its axis in about 24 hours ; so the day does not differ in length essen- tially from ours. DISTANCE FROM THE EARTH. The distance of Ve- nus from the earth, like that of Mercury, when in inferior conjunction, is the difference between the distances* of these two planets from the sun, and when in superior conjunction the sum of these dis- tances. * Let the pupil calculate the distances of the earth and Venus from each other, when in perihelion and aphelion, as hi the case ol Mercury, (See tables in Appendix.) 92 THE SOLAR SYSTEM. The figure represents its apparent dimensions at the extreme, mean, and least distances from us. The variation is nearly as the numbers 10, 18, and 65. It would be natural to think that the planet is the brightest when the nearest, and thus the largest, Fig. 24. EXTREME, MEAN, AND LEAST APPARENT SIZE OP VENUS. but we should remember that then the bright side is toward the sun, and the unillumined side toward us. Indeed, at the period of greatest brilliancy of which we have spoken, only about one-fourth of the light is visible. At this time, however, many observers have noticed the entire contour of the planet to be of a dull gray hue, as seen in the cut. DIMENSIONS. Venus is about 7,500 miles in diame- ter. The volume of the planet is about four-fifths that of the earth, while the density is about the same. A stone let fall upon its surface would fall 14 feet in VENUS. 93 the first second : a pound weight removed to its equator would weigh about five-sixths of a pound. From this we see that the force of gravity does not decrease exactly in proportion to the size of the planet, any more than it increases with the mass of the sun. The reason of this is, that the body is brought nearer the mass of the small planet, and so feels its attraction more fully than when far out upon the extreme circumference of a large body, the attraction increasing as the square of the dis- tance from the particles decreases. SEASONS. As the axis of Yenus is very much in- clined from a perpendicular, its seasons are similar to those of Mercury. The torrid and temperate Fig. 25 VENUS AT ITS SOLSTICE. zones overlap each other ; the polar regions having alternately at one solstice a torrid temperature, and at the other a prolonged arctic cold. The inequality 94 THE SOLAR SYSTEM. of the nights is very marked. The heat and light are double that of the earth, while the circular form of its orbit gives nearly an equal length to its four seasons. If the inclination of its axis is 75, as some as- tronomers hold, its tropics must be 75 from the equator, and its polar circles 75 from the poles. The torrid zone is, therefore, 150 in width. The torrid and frigid zones inteiiap through a space of 60, midway between the equator and poles. TELESCOPIC FEATURES. Venus, being an interior planet, presents, like Mercury, all the phases of the moon. This fact was discovered by Galileo, and was among the first achievements of his telescopic obser- vations. It had been argued against the Coperni- ;an system that, if true, Venus should wax and wane Aike the moon. Indeed, Copernicus himself boldly declared that if means of seeing the planets more distinctly were ever invented, Venus would be found to present such phases. Galileo, with his telescope, proved this fact, and, by overthrowing that objec- tion, again vindicated the Copernican theory. This planet is not sensibly flattened at the poles. It is thought to have a dense, cloudy atmosphere. This was established by the fact that at the transit of Venus over the sun in 1761 and 1769, a faint ring of light was observed to surround the black disk of the planet. The evidence of an atmosphere, as well as of mountains, rests very much upon the peculiar appearance attending its crescent s ape. VENUS. 95 (1.) The luminous part does not end abruptly ; on the contrary, its light diminishes gradually, which diminution may be entirely explained by the twi- light on the planet. The existence of an atmosphere Pig. 26. CRESCENT AND SPOTS OP VENU9. which diffuses the rays of light into regions where the sun has already set, has hence been inferred. Thus, on Venus, the evenings, like ours, are lighted by twilight, and the mornings by dawn. (2.) The edge of the enlightened portion 'of the planet is un- even and irregular. This appearance is doubtless the effect of shadows cast by mountains. Spots have been noticed on its disk which are considered to be traceable to clouds. Indeed, Herschel thinks that we never see the real body of the planet, but only its atmosphere loaded with vapors, which may mitigate the glare of the intense sunshine. SATELLITES. Venus is not known to have any moon. THE SOLAR SYSTEM. THE EAUTH. Sign, 0, a circle with Equator and Meridian. The Earth is the next planet we meet in passing outward from the sun. To the beginner, it seems strange enough to class our world among the heav- enly bodies. They are brilliant, while it is dark and opaque ; they appear light and airy, while it is solid and firm ; we see in it no motion, while they are constantly changing their position ; they seem mere points in the sky, while it is vast and extended. Yet at the very beginning we are to consider the earth as a planet shining brightly in the heavens, and appearing to other worlds as a star does to us : we are to learn that it is in motion, flying through its orbit with inconceivable velocity ; that it is not fixed, but hanging in space, held by an invisible power of gravitation which it cannot evade ; that it is small and insignificant beside the mighty globes that so gently shine upon us in the far-off sky; that our earth is only one atom in a universe of worlds, all firm and solid, and equally well fitted to be the abode of life. DIMENSIONS. The earth is not "round like a bah 1 ," but flattened at the poles. Its form is that of an oblate spheroid. Its polar diameter is about 7,899 miles, and its equatorial about 7,925^. The com- pression is, therefore, about 26J miles. (See table THE EARTH. 97 in Appendix.) If we represent the earth by a globe one yard in diameter, the polar diameter would be one-tenth of an inch too long. It has been recently Fig. 27. THE EARTH IN SPACE. shown that the equator itself is not a perfect circle, but is somewhat flattened, since the diameter which 98 THE SOLAR SYSTEM. pierces the meridian 14 east of Greenwich is two miles longer than the one at right angles to it. The circumference of the earth is about 25,000 miles. Its density is about 5J times that of water. Its weight is 6,069,000,000,000,000,000,000 tons. The inequalities of its surface, arising from build- ings, valleys, mountains, etc., have been likened to the roughness on the rind of an orange. This is not an exaggeration. On a globe sixteen inches in diameter, the land, to be in proportion, should be represented by the thinnest writing-paper, the hills by small grains of sand, and elevated ranges by thick drawing-paper. To represent the deepest wells or mines, a scratch might be made that would be invisible except with a glass. The water in the ocean could be shown by a brush dipped in color and lightly drawn over the bed of the sea. THE KOTUNDITY or THE EARTH. This is shown in various ways, among which are the following : (1) By the fact that vessels have sailed around the earth ;* * It is curious, in connection with this well-known fact, lo re- call the arguments urged by the Spanish philosophers against the reasoning of Columbus, when he assured them that he could arrive at Asia just as certainly by sailing west as east. "How," they asked, "can the earth be round? If it were, then on the opposite side the rain would fall upward, trees would grow with their branches down, and everything would be topsy-turvy. Eveiy object on its surface would cer- tainly fall off; and if a ship by siiVng west should get around THE EARTH. 9 ( J (2) when a ship is coming into port w.e see the masts first ; (3) the shadow of the earth on the moon is circular; (4) the polar star seems higher in the heavens as we pass north ; (5) the horizon expands as we ascend an eminence.* If we climb to the top of a hill, we can see further than when on the plain at its foot. Our eyesight is not improved ; it is only because ordinarily the curvature of the earth shuts off the view of distant objects, but when we ascend to a higher point, we can see farther over the side of the earth. The curvature is eight inches per mile, 2 2 x 8 in - = 32 inches for two miles, 3 2 x 8 in - for three miles, etc. An object of these respective heights would be just hidden at these distances. APPARENT AND REAL MOTION. In endeavoring to understand the various appearances of the heavenly bodies, it is well to remember how in daily life we transfer motion. On the cars, when in rapid move- ment, the fences and trees seem to glide by us, there, it would never be able to climb up the side of the earth and get back again. How can a ship sail up hill ?" * The histoiy of aeronautic adventure affords a curious illustra- tion of this same principle. The late Mr. Sadler, the celebrated aeronaut, ascended on one occasion in a balloon from Dublin, and was wafted across the Irish Channel, when, on his approach to the Welsh coast, the balloon descended nearly to the surface of the sea. By this time the sun was set, and the shades of even- ing began to close in. He threw out nearly all his ballast, and suddenly sprang upward to a great height, and by so doing brought his horizon to dip below the sun, producing the whole phenomenon of a western sunrise. Subsequently descending in Wales, he, of course, witnessed a second sunset on the same evening. 100 THE SOLAR SYSTEM. while we sit still and watch them pass. On a bridge, when we are at rest, we follow the undula- tions of the waves, until at last we come to think that they are stationary and we are sweeping down stream. "In the cabin of a large vessel going smoothly before the wind on still waterj or drawn along a canal, not the smallest indication acquaints us with the ' way it is making.' We read, sit, walk, as if we were on land. If we throw a ball into the air, it falls back into our hand ; if we drop it, it alights at our feet. Insects buzz around us as in the free air, and smoke ascends in the same manner as it would do in an apartment on shore. If, indeed, we come on deck, the case is in some respects different ; the air, not being carried along with us, drifts away smoke and other light bodies such as feathers cast upon it, apparently in the opposite direction to that of the ship's progress ; but in reality they remain at rest, and we leave them behind in the air."* DIURNAL EEVOLUTION OF THE EARTH AROUND ITS Axis. The earth, in constantly turning from west * " And what is the earth itself but the good ship we are sailing in through the universe, bound round the sun ; and as we sit here in one of the ' berths,' we are unconscious of there being any 'way' at all upon the vessel. On deck, too, out in the open air, it's all the same as long as we keep our eyes on the ship; but immediately we look over the sides and the horizon is but the 'gunwale' of our vessel we see the blue tide of the great ocean around us go drifting by the ship, and sparkling with its million stars as the waters of the sea itself sparkle at night be- tween the tropics " THE EARTH. '101 to east, elevates our horizon above the stars on the west, and depresses it below the stars on the east. As the horizon appears to us to be sta- tionary, we assign the motion to the stars, think- ing those on the west whichjit pajsses 6vei> and hides to have sunk belo^.it or those on the east it has. moved above it or risen. So, also, the horizon is depressed below the sun, and we call it sunrise; it is elevated above the sun, and we call it sunset. We thus see that the diurnal movement of the sun by day and stars by night is a mere optical delu- sion that here as elsewhere we simply transfer motion. This seems easy enough for us to under- stand, because the explanation makes it so simple ; but it was the " stone of stumbling" to ancient as- tronomers for two thousand years. Copernicus him- self, it is said, first thought of the true solution while riding on a vessel and noticing how he insensibly transferred the movement of the ship to the objects on the shore. How much grander the beautiful simplicity of this theory than the cumbersome com- plexity of the old Ptolemaic belief ! Diurnal motion of the Sun. The explanation just given illustrates the apparent motion of the sun, and the cause of day and night. Suppose S to be the sun. E, the earth, turning upon its axis EF from west to east, has half its surface only illu- minated at one time by the sun. To a person at D, the sun is in the horizon and day commences, 102 THE SOLAR SYSTEM. the luminary appearing to rise higher and higher in the heavens with a westerly motion, as the ob- server is carried forward easterly by the earth's diurnal rotation to A, where he has the sun in his i f>. t -Vjxrt DAILY MOTION OP THE BUN. meridian, and it is consequently noon. The sun then begins to decline in the sky until the specta- tor arrives at B, where it sets, or is again in the horizon on the west side, and night begins. He moves on to C, which marks his position at midnight, the sun being then on the meridian of places on the opposite part of the earth, and he is then brought round again to D, the point of sunrise, when another day commences. (Hind.) The unequal rate of diurnal motion. Different points upon .the surface of the earth revolve with different velocities. At the two poles the speed of rotation is nothing, while at the equator it is great- est, or over 1,000 miles per hour. At Quito, the circle of latitude is much longer than one at the mouth of the St. Lawrence, and the velocities vary in the same proportion. The former place moves THE EABTH. 103 at the rate of about 1,038 miles per hour ; the lat- ter, 450 miles. In our latitude (41) the speed is about 780 miles per hour. We do not perceive this wonderful velocity with which we are flying through the air, because the air moves with us.* Yet were the earth suddenly to stop its rotation, the terrible shock would, without doubt, destroy the entire race of man, and we, with houses, trees, rocks, and even the oceans, in one confused mass, would be hurled headlong into space. On the other hand, were the rate of rotation to increase, the length of the day would be proportionately short- ened, and the weight of all bodies decreased by the centrifugal force thus produced. Indeed, if the rotary movement should become swift enough to * An ingenious inventor once suggested that we should utilize the earth's rotation, as the most simple and economical, as well as rapid mode of locomotion that could be conceived. This was to be accomplished by rising in a balloon to a height inacces- sible to aerial currents. The balloon, remaining immovable in this calm region, would simply await the moment when the earth, rotating underneath, would present the place of destination to the eyes of travellers, who would then descend. A well- regulated watch and an exact knowledge of longitudes would thus render travelling possible from east to west, all voyages north or south naturally being interdicted. This suggestion has only one fault ; it supposes that the atmospheric strata do not revolve with the earth. Upon that hypothesis, since we rotate in our latitude with the velocity of 333 yards hi a second, there would result a wind in the contrary direction ten times more violent than the most terrible hurricane. Is not the absence of such a state of things a convincing proof of the participation of the atmospheric envelope in the general movement ? (Guillemin.) 104 THE SOLAR SYSTEM. reduce the day to 84 minutes, or about ^\ its pres- ent length, the force of gravity would be overcome, and, at the equator, all bodies would be without weight; if the speed were still further increased, loose bodies would fly off from the earth like water from a grindstone when swiftly turned, while we should be compelled to constantly "hold on" to avoid sharing the same fate. But against such a catastrophe we are assured by the immutability of God's laws. " He is the same yesterday, to-day, and forever." The earth has not varied in its revo- lution T -jj of a second in 2,000 years. Unequal diurnal orbits of the stars. Let O repre- sent our position on the earth's surface, E Z B our meridian ; E I B K our horizon ; P and P' the north THE EARTH. 105 and south poles, Z the zenith, Z' the nadir; and GICK the celestial equator. Now PB, it will be seen, is the elevation of the north pole above the horizon, or the latitude of the place. Suppose we should see a star at A, on the meridian below the pole. The earth revolves in the direction GIG; the star will therefore move along A L to Z, when it is on the meridian above the pole. It continues its course along the dotted line around to A again, when it is on the meridian below the pole, having made a complete circuit around the pole, but not having descended below our horizon. A star rising at B would just touch the horizon ; one at I would move on the celestial equator, and would be above the horizon as long time as it is below twelve hours in each case ; a star rising at M, would just come above the horizon and set again at N. Unequal diurnal velocities of the stars. The stars appear to us to be set in a concave shell which ro- tates daily about the earth. As different parts of the earth really revolve with varying velocities, so the stars appear to revolve at different rates of speed. Those near the pole, having a small orbit, revolve very slowly, while those near the celestial equator move at the greatest speed. Appearance of the stars' at different places on the earth. Were we placed at the north pole, Polaris would be directly overhead, and the stars would seem to pass around us in circles parallel to the horizon, and increasing in diameter from the upper 5* 106 THE SOLAR SYSTEM. to lower ones. Were we placed at the equator, the pole-star would be at the horizon, and the stars would move in circles exactly perpendicular to the horizon, and decreasing in diameter, north and south from those in the zenith, while we could see one half of the path of each star. Were we placed in the southern hemisphere, the circumpolar stars would rotate about the south pole, and the others in cir- cles resembling those in our sky, only the points of direction would be reversed to correspond with the pole. Were we placed at the south pole, the ap- pearance would be the same as at the north pole, except that there is no star to mark the direction of the earth's axis. MOTION OF THE EARTH IN SPACE ABOUT THE SUN. The earth revolves in an elliptical path about the sun at a mean distance of 91 \ million of miles. This path is called the ecliptic ; its eccentricity, which is greater than that of the orbit of Venus, changes about j-oo~f oTo P er cen tury, so that in time the orbit would become circular, were it not that after the lapse of some thousands of years, the eccentricity will begin to increase again, and will thus vary through all ages within definite, although yet un- determined limits. Its entire circumference is near- ly 600,000,000 miles, and the earth pursues this wonderful journey at the rate of 18 miles per second. This revolution of the earth about the sun gives rise to various phenomena, of which we shall now proceed to speak. THE EARTH. 107 1. Change in the appearance of the heavens in differ- ent months. This is the natural result of the revolu- tion of the earth about the sun. In Fig. 30, suppose Fig. 30. APPEARANCE OF THE HEAVENS IN DIFFERENT SEASONS. A B C D to be the orbit of the earth, and E F G H the sphere of the fixed stars, surrounding the sun in every direction. When our globe is at A, the stars about E are on the meridian at midnight. Being seen from the earth in the opposite quarter 108 THE SOLAR SYSTEM. to the sun, they are most favorably placed for obser- vation. The stars at G, on the contrary, will be invisible, for the sun intervenes between them and the earth : they are on the meridian of the spectator about the same time as the sun, and are always hidden in his rays. In three months the earth has passed over one-fourth of her orbit, and has arrived at B. Stars about F now appear on the meridian at midnight, while those at E, which previously occupied their places, have descended toward the west and are becoming lost in the sun's refulgence, while those about G are just coming into sight in the east. In three months more the earth is situated at C, and stars about G shine in the midnight sky, those at F having, in their turn, vanished in the west. Stars at E are on the meridian at noon, and consequently hidden in daylight ; and those about H are just escaping from the sun's rays, and commencing their appearance in the east. One revolution of the earth brings the same stars again on the meridian at midnight. Thus it is that the earth's motion round the sun as a centre explains the varied aspect of the heavens in the summer and winter skies. (Hind.) 2. Yearly path of the sun through the heavens. We have spoken of the diurnal motion of the sun. "We now speak of its second apparent motion its yearly path among the stars.* If we look at the accom- * This yearly movement of the sun among the fixed stars is not as apparent to us as his daily motion, because his superior THE EARTH. 109 panying plate (Fig. 31), we can see how the motion of the earth in its orbit is also transferred to the sun, and causes him to appear to us to travel in a fixed path through the heavens. When the earth is in any part of the ecliptic, the sun seems to us to be in the point directly opposite. For example, when the earth is in Libra (==)* autumnal equinox the sun is in Aries (T) vernal equinox; when the sun enters the next sign, Taurus (), the earth in fact has passed on to Scorpio (^). Thus as the earth moves through her orbit, the sun seems to pass through the same path along the opposite side of the ecliptic, making the entire circuit of the heavens in the year, and returning at the end of that time to the same place among the stars. If the earth could leave a shining line as it passes through its orbit about the sun, we should see the sun apparently moving along this same line on the opposite side of the circle. We therefore define the ecliptic as the real orbit of the earth about the sun, or the apparent path of the sun through the heavens. The ecliptic crosses the celes- tial equator at two points. These are called the light blots out the stars. But if we notice a star at the western horizon just at sunset, we can tell what constellation the sun is then hi : now wait two or three nights, and we shall find that star is set, and another has taken its place. Thus we can trace the sun through the year in his path among the fixed stars. * When we say " the earth is hi Libra," we mean that a spec- tator placed at the sun would see the earth hi that part of the heavens which is occupied by the sign Libra. 110 THE SOLAR SYSTEM. 3. An apparent movement of the sun, north and south. Having now spoken of the apparent diurnal and annual motions of the sun, there yet remains a third motion, which has doubtless oftentimes at- tracted our attention. In summer, at midday, the sun is high in the heavens ; in the winter, quite low, near the southern horizon. In summer he is a long time above the horizon ; in the winter, a short time. In summer he rises and sets north of the east and west points ; in winter, south of the east and west points. This subject is so intimately connected with the next, that we shall understand it best when taken in connection with it. 4. CHANGE OF THE SEASONS. VARIATION IN LENGTH OF DAY AND NIGHT. By closely studying the accom- panying illustration and imagining the various posi- tions of the earth in its orbit, let us try to under- stand the several points. I. Obliquity of the ecliptic. The axis of the earth is inclined 23J from a perpendicular to its orbit. This angle is called the obliquity of the ecliptic. II. Parallelism of the axis. In all parts of the orbit, the axis of the earth is parallel to itself and constantly points toward the North Star.* This is only an instance of what is very familiar to us all. Nature reveals to us nothing more permanent than the axis of rotation in anything that is rapidly turned. It is its rotation which keeps a boy's hoop * There is a slight variation from this, which we shall soon notice. THE EARTH. Fig. 31. Ill THB ORBIT OF THE EABTH- 112 THE SOLAR SYSTEM. from falling. For the same reason a quoit retains its direction when whirled, and it will keep in the same plane at whatever angle it may be thrown. A man slating a roof wishes to throw a slate to the ground ; he simply whirls it, and as it descends it will strike on the edge without breaking. As long as a top spins there is no danger of its falling, since its tendency to preserve parallel its axis of rotation is greater than the attraction of the earth. This wonderful law would lead us to think that the axis of the earth always points in the same direction, even if we did not know it from direct observation. III. TJie rays of the sun strike the various por- tions of the earth, when in any position, at different angles. Example. "When the earth is in Libra, and also when in Aries, the rays strike vertically at the equator, and more and more obliquely in the northern and southern hemispheres, as the distance from the equator increases, until at the poles they strike almost horizontally. This variation in the direction of the rays produces a corresponding variation in the intensity of the sun's heat and light at dif- ferent places, and accounts for the difference between the torrid and polar regions. IV. As the earth changes its position the angle at which the rays strike any portion is varied. Ex- ample. Take the earth as it enters Capricornus (\s) and the sun in Cancer () He is now over- head, 23J north of the equator. His rays strike THE EARTH. 113 less obliquely in tire northern hemisphere than when the earth was in Libra. Let six months elapse : the earth is now in Cancer and the sun in Capricornus; and he is overhead, 23J south of the equator. His rays strike less obliquely in the southern hemisphere than before, but in the northern hemisphere more obliquely. These six months have changed the direction of the sun's rays on every part of the earth's surface. This accounts for the dif- ference in temperature between summer and winter. V. The Equinoxes. At the equinoxes one half of each hemisphere is illuminated : hence the name Equinox (cequus, equal, and nox, night). At these points of the orbit the days and nights are equal over the entire earth,* each being twelve hours in length. VI. Northern and southern hemispheres unequally illuminated. While one half of the earth is con- stantly illuminated, at all points in the orbit except the equinoxes the proportion of the northern or southern hemisphere which is in daylight or dark- ness varies. When more than half of a hemi- sphere is in the light, its days are longer than the nights, and vice versa. VII. The seasons and the comparative length of days and nights in the South Temperate Zone, at any specified time, are the reverse of those in the North Temperate Zone, except at the Equinoxes, where the days and nights are of equal length. * Except a small space at each pole. 114 THE SOLAB SYSTEM. VIII. The earth at the Summer Solstice. When the earth is at the summer solstice, about the 21st of June, the sun is overhead 23J north of the equator, and if its vertical rays could leave a gold- en line on the surface of the earth as it revolves, they would mark the Tropic of Cancer. The sun is at its furthest northern declination, ascends the high- est it is ever seen above our horizon, and rises and sets 23 J north of the east and west points. It seems now to stand still in its northern and southern course, and hence the name Solstice (sol, the sun, sto, to stand). The days in the north temperate zone are longer than the nights. It is our summer, and the 21st of June is the longest day of the year. In the south temperate zone it is winter, and the shortest day of the year. The circle that sepa- rates day from night extends 23^ beyond the north pole, and if the sun's rays could in like manner leave a golden line on that day, they would trace on the earth the Arctic Circle. It is the noon of the long six months polar day. The reverse is true at the Antarctic Circle, and it is there the midnight of the long six months polar night. IX. The earth at the Autumnal Equinox. The earth crosses the aphelion point the 1st of July, when it is at its furthest distance from the sun, which is then said to be in apogee. The sun each day rising and setting a trifle farther toward the south, passes through a lower circuit in the heavens. We reach the autumnal equinox the 22d of Sep- THE EABTH. 115 tember. The sun being now on the equinoctial, if its vertical rays could leave a line of golden light, they would -mark on the earth the circle of the equator. It is autumn in the north temperate zone 'and spring in the south temperate zone. The days and nights are equal over the whole earth, the sun rising at 6 A. M. and setting at 6 p. M., exactly in the east and west where the equinoctial intersects the horizon. X. The earth at the Winter Solstice. The sun after passing the equinoctial "crossing the line," as it is called sinks lower toward the southern ho- rizon each day. We reach the winter solstice the 21st of December. The sun is now directly overhead 23J south of the equator, and if its rays could leave a line of golden light they would mark on the earth's surface the Tropic of Capricorn. It is at its furthest southern declination, and rises and sets 23J south of the east and west points. It is our winter, and the 21st of December is the short- est day of the year. In the south temperate zone it is summer, and the longest day of the year. The circle that separates day from night extends 23J beyond the south pole, and if the sun's rays in like manner could leave a line of golden light they would mark the Antarctic Circle. It is there the noon of the long six months polar day. At the Arctic Circle the reverse is true ; the rays fall 23| short of the north pole, and it is there the midnight of the long six months polar night. Here 116 THE SOLAR SYSTEM. again the sun appears to us to stand still a day or two before retracing its course, and it is there- fore called the Winter Solstice. XI. The earth at the Vernal Equinox. The earth reaches its perihelion about the 31st of December. It is then nearest the sun, which is therefore said to be in perigee. The sun rises and sets each day further and further north, and climbs up higher in the heavens at midday. Our days gradually increase in length, and our nights shorten in the same proportion. On the 21st of March* the sun reaches the equinoctial, at the vernal equinox. He is overhead at the equator, and the days and nights are again equal. It is our spring, but in the south temperate zone it is autumn. XII. The yearly path finished. The earth moves on in its orbit through the spring and summer months. The sun continues its northerly course, ascending each day higher in the heavens, and its rays becoming less and less oblique. On the 21st of June it again reaches its furthest northern decli- nation, and the earth is at the summer solstice. We have thus traced the yearly path, and noticed the course of the changing seasons, with the length of the days and nights. The same series has been repeated through all the ages of the past, and will be till time shall be no more. XIII. Distance of the earth from the sun varies. * The precise time of the equinoxes and solstices varies each year, but within a small limit. THE EARTH. 117 We notice, from what we have just seen, that we. are nearer the sun by 3,000,000 miles in winter than in summer. The obliqueness with which the rays strike the north temperate zone at that time pre- vents our receiving any special benefit from this favorable position of the earth. XIV. Southern summer. The inhabitants of the south temperate zone have then: summer while the earth is in perihelion, and the sun's rays are about ^warmer than when in aphelion, our summer-time. This will perhaps partly account for the extreme heat of their season. Herschel tells us that he has found the temperature of the surface soil of South Africa 159 F. Captain Sturt, in speaking of the extreme heat of Australia, says that matches accidentally dropped on the ground were immediately ignited. The southern winters, for a similar reason, are colder ; and this makes the average yearly tempera- ture about the same as ours. XV. Extremes of heat and cold not at the solstices. We notice that we do not have our greatest heat at the time of the summer solstice, nor our greatest cold at the winter solstice. After the 21st of June, the earth, already warmed by the genial spring days, continues to receive more heat from the sun by day than it radiates by night : thus its tempera- ture still increases. On the other hand, after the 21st of December the earth continues to become colder, because it loses more heat during the night than it receives during the day. 118 THE SOLAR SYSTEM. XYI. Summer longer than winter. As the sun is not in the centre of the earth's orbit, but at one of its foci, that portion of the orbit which the earth passes through in going from the vernal to the autumnal equinox comprises more than one-half the entire ecliptic. On this account the summer is longer than the winter. The difference is still fur- ther enhanced by the variation in the earth's ve- locity at aphelion and perihelion. The annexed table gives the mean duration of the seasons : Seasons. Days. Seasons. Days. Spring 92.9 Autumn 89.7 Summer .93.6 Winter 89.0 The difference of time in the earth's stay in the two portions of the ecliptic, as will be seen from the above, is 7.8 days. XVII. Varying velocity of the earth. We can see, by looking at the plate, that the velocity of the earth must vary in different portions of its orbit. When passing from the vernal equinox to aphelion, the attraction of the sun tends to check its speed ; from that point to the autumnal equinox, the at- traction is partly in the direction of its motion, and so increases its velocity. The same principle applies when going to and from perihelion. XVIII. Curious appearance of the sun at the north pole. " To a person standing at the north pole, the sun appears to sweep horizontally around the sky every twenty-four hours, without any perceptible THE EABTH. 119 variation in its distance from the horizon. It is, however, slowly rising, until, on the 21st of June, it is twenty-three degrees and twenty-eight minutes above the horizon, a little more than one-fourth of the distance to the zenith. This is the highest point it ever reaches. From this altitude it slowly de- scends, its track being represented by a spiral or screw with a very fine thread i and in the course of three months it worms its way down to the horizon, which it reaches on the 22d of September. On this day it slowly sweeps around the sky, with its face half hidden below the icy sea. It still continues to descend, and after it has entirely disappeared it is still so near the horizon that it carries a Bright twilight around the heavens in its daily circuit. As the sun sinks lower and lower, this twilight grows gradually fainter, till it fades away. December 21st, the sun is 23 28' below the horizon, and this is the midnight of the dark polar winter. From this date the sun begins to ascend, and after a time it is her- alded by a faint dawn, which circles slowly around the horizon, completing its circuit every twenty-four hours. This dawn grows gradually brighter, and on the 22d of March the peaks of ice are gilded with the first level rays of the six months day. The biinger of this long day continues to wind his spiral way upward, till he reaches his highest place on the 21st of June, and his annual course is completed." XIX. Results, if the axis of the earth were perpen- dicular to the ecliptic. The sun would then always 120 THE SOLAK SYSTEM. appear to move through the equinoctial. He would rise and set every day at the same points on the horizon, and pass through the same circle in the heavens, while the days and nights would be equal the year round. There would be near the equator a fierce torrid heat, while north and south the climate would melt away into temperate spring, and, lastly, into the rigors of a perpetual winter. XX. Results, if the equator of the earth were perpen- dicular to the ecliptic. Were this the case; to a spec- tator at the equator, as the earth leaves the vernal equinox, the sun would each day pass through a smaller circle, until at the summer solstice he would reach the north pole, when he would halt for a time and then slowly return in an inverse manner. In our own latitude, the sun would make his diurnal revolutions in the way we have just de- scribed, his rays shining past the north pole fur- ther and further, until we were included in the region of perpetual day, when he would seem to wind in a spiral course up to the north pole, and then return in a descending curve to the equator. PKECESSION OF THE EQUINOXES. We have spoken of the equinoxes as if they were stationary in the orbit of the earth. Over two thousand years ago, Hipparchus found that they were falling back along the ecliptic. Modern astronomers fix the rate at about 50" of space annually. If we mark either point in the ecliptic at which the days and nights are equal over the earth, which is where the plane of the earth's THE EARTH. 121 equator passes exactly through the centre of the sun, we shall find the earth the next year comes back to that position 50" (20 m. 20 s. of time) earlier. This remarkable effect is called the Precession of the Equinoxes, because the position of the equinoxes in any year precedes that which they occupied the year before. Since the circle of the ecliptic is divided into 360, it follows that the time occupied by the equinoctial points in making a complete revolution at the rate of 50.2" per year is 25,816 years. Results of tlie Precession of the Equinoxes. In Fig. 31, we see that the line of the equinoxes is not at right angles to the ecliptic. In order that the plane of the terrestrial equator should pass through the sun's centre 50" earlier, it is necessary that the plane itself should slightly change its place. The axis of the earth is always perpendicular to this plane, hence it follows that the axis is not rigorously parallel to itself. It varies in direction, so that the north pole describes a small circle in the starry vault twice 23 28' in diameter. To illustrate this, in the cut we suppose that after a series of years the position of the earth's equator has changed from efh to g K 1. The inclination of the axis of the earth, C P, to CQ, the pole of the ecliptic, remains unchanged ; but as it must turn with the equator, its position is moved from CP to OP', and it passes slowly around through a portion of a circle whose centre is C Q. The direc- tion of this motion is the same as that of the hands of a watch, or just the reverse of that of the revolution a 122 THE SOLAR SYSTEM. of the earth itself. The position of the north pole in the heavens is therefore gradually but almost insen- sibly changing. It is now distant from the north polar star about 1J. It will continue to approach CHANGE OF EARTH'S EQUATOR AND AXIS. it until they are not more than half a degree apart. In 12,000 years Lyra will be our polar star : 4,50C years ago the polar star was the bright star in the constellation Draco. As the right ascension of the stars is reckoned eastward from the vernal equinox along the equinoctial, the precession of the equinoxes increases the E. A. of the stars 50" per year. On this account, star maps must be accompanied by the date of their calculations, that they may be corrected to correspond with this annual variation. The con- stellations are fixed in the heavens, while the signs of THE EARTH. 123 the zodiac are not ; they are simply abstract divisions of the ecliptic which move with the equinox. When named, the sun was in both the sign and constellation Aries, at the time of the vernal equinox ; but since then the equinoxes have retrograded nearly a whole sign, so that now while the sun is in the sign Aries on the ecliptic, it corresponds to the constellation Pisces in the heavens. Pisces is therefore the first constellation in the zodiac. (See Fig. 72.) Causes of the Precession of the Equinoxes. Before commencing the explanation of this phenomenon, it is necessary to impress upon the mind a few facts. 1. The earth is not a perfect sphere, but is swollen at the equator. It is like a perfect sphere covered with padding, which increases constantly in thick- ness from the poles to the equator ; this gives it a turnip-like shape. 2. The attraction of the sun is INFLUENCE OF THE SUN ON A MOUNTAIN NEAR THE EQUATOR. greater the nearer a body is to it. 3. The attraction is not for the earth as a mass, but for each particle separately. In the figure, the position of the earth 124 THE SOLAR SYSTEM. at the time of the winter solstice is represented, P is the north pole, a b the ecliptic, C the centre of the earth, C Q a line perpendicular to the eclip- tic, so that the angle QCP equals the obliquity of the ecliptic. In this position the equatorial pad- ding we have spoken of the ring of matter about the equator is turned, not exactly toward the sun, but is elevated above it. Now the attraction of the sun pulls the part D more strongly than the centre ; the tendency of this is to bring D down to a. In the same way the attraction for C is greater than for I, so it tends to draw C away from I, and as at the same time D tends toward a, to pull I up toward b. The tendency of this, one would think, would be to change the inclination of the axis C P toward C Q, and make it more nearly perpendic- ular to the ecliptic. This would be the result if the earth were not revolving upon its axis. Let us con- sider the case of a mountain near the equator. This, if the sun did not act upon it, would pass through the curve H D E in the course of a semi-revolution of the earth. It is nearer the sun than the centre C is ; the attraction therefore tends to pull the mountain downward and tilt the earth over, as we have just described; so the mountain will pass through the curve H/V/, and instead of crossing the ecliptic at E it will cross at g a little- sooner than it otherwise would. The same influence, though in a less degree, obtains on the opposite side of the earth. The mountain passes around the earth in a curve nearer THE EARTH. 125 to b, and crosses the ecliptic a little earlier. The same reasoning will apply to each mountain and tc all the protuberant mass near the equatorial regions. The final effect is to turn slightly the earth's equator so that it intersects the ecliptic sooner than it would were it not for this attraction. At the summer sol- stice the same tilting motion is produced. At the equinoxes the earth's equator passes directly through the centre of the sun, and therefore there is no ten- dency to change of position. As the axis C P must move with the equator, it slowly revolves, keeping its inclination unchanged, around C Q, the pole of the ecliptic, describing, in about 26,000 years, a small circle twice 23 28' in diameter. Precession illustrated in the spinning of a top. This motion of the earth's axis is most singularly illus- trated in the spinning of a top, and the more remarkably because there the forces are of an opposite character to those which act on the earth, and so produce an opposite effect. "We have seen that if the earth had no rotation, the sun's attraction on the " padding" at the equator would bring C P nearer to C Q, but that in consequence of this rotation tho effect really produced is that CP, the earth's axis, SPINNING OP A TOP. 126 THE SOLAR SYSTEM. slowly revolves around C Q, the pole of the heavens, in a direction opposite to that of rotation. In Fig. 34, let C P be the axis of a spinning top, and C Q the vertical line. The direct tendency of the earth's attraction is to bring C P further, from C Q (or to make the top fall), and if the top were not spinning this would be the result; but in consequence of the rotary motion the inclination does not sensibly alter (until the spinning is retarded by friction), and so C P slowly revolves around C Q in the same direction as that of rotation. NUTATION (nutatio, a nodding). "We have noticed the sun as producing precession ; the moon has, however, treble its influence ; for although her mass is not s-ff.info.TnnF P art tnat f tne sun > J et sne * s 400 times nearer and her effect correspondingly greater. (See p. 168.) The moon's orbit does not He par- allel to the ecliptic, but is inclined to it. Now the sun attracts the moon, and disturbs it as he would the path of the mountain we have just sup- posed, and the effect is the same viz., the intersec- tions of the moon's orbit with the ecliptic travel backward, completing a revolution in about 18 years. During half of this time the moon's orbit is inclined to the ecliptic in the same way as the earth's equator ; during the other half it is inclined in the opposite way. In the former state, the moon's attractive tendency to tilt the earth is very small, and the precession is slow ; in the latter, the tendency is great, and precession goes on rapidly. PATH OF THE NORTH POLE THE EARTH. 127 The consequence of this is, that the pole of the earth is irregularly shifted, so that it travels in a slightly curved line, giving it a kinti of "wabbling" or " nodding" mo- tion, as shown though greatly exaggerated in Fig. 35. The obliquity of the ecliptic, which we consider 23 28', is the mean of the irregularly curved line IN THB HEAVENS. and is represented by the dotted circle. Periodical change in the obliquity of the ecliptic. Although it is sufficiently near for all general pur- poses to consider the obliquity of the ecliptic invari- able, yet this is not strictly the case. It is subject to a small but appreciable variation of about 46" per century. This is caused by a slow change of the position of the earth's orbit, due to the attraction of the planets. The effect of this movement is to gradually diminish the inclination of the earth's equator to the ecliptic (the obliquity of the ecliptic). This will continue for a time, when the angle will as gradually increase ; the extreme limit of change being only 1 21'. The orbit of the earth thus vibrates backward and forward, each oscillation requiring a period of 10,000 years. This change is so intimately blended, in its effect upon the obliquity of the ecliptic, with that caused by pre- cession and nutation, that they are only separable in theory ; in point of fact, they all combine to 128 THE SOLAR SISTEM. produce the waving motion we have already de- scribed. As a consequence of this variation in the obliquity of the ecliptic, the sun does not come as far north nor decline as far south as at the Creation, while the position of all the terrestrial circles Tropic of Cancer, Capricorn, Arctic, etc. is con- stantly but slowly changing. Besides this, it tends to vary slightly the comparative length of the days and nights, and, as the obliquity is now dimin- ishing, to equalize them. As the result of this vari- ation in the position of the orbit, some stars which were formerly just south of the ecliptic are now north of it, and others that were just north are now a little further north ; thus the latitude of these stars is gradually changing. Change in tlie major axis (line of apsides) of the earth's orbit. Besides all the changes in the posi- tion of the earth in its orbit due to precession, the line connecting the aphelion and perihelion points of the orbit itself is slowly moving. The conse- quence of this is a variation in the length of the seasons at different periods of time. In the year 4089 B. c., about the supposed epoch of the crea- tion, the earth was in perihelion at the autumnal equinox, so that the summer and autumn seasons were of equal length, but shorter than the winter and spring seasons, which were also equal.* In the * There is much discrepancy in the views held concerning the Great Year of the astronomers, as it is often called. (See 14 Weeks in Geology, pp. 272-3, note.) The statement given in the text is that held by Lockyer, Hind and others. The terms, it THE EAETH. 129 year 1250 A. D., the earth was in perihelion when it was at the winter solstice, December 21, instead of January 1st, as now ; the spring quarter was there- fore equal to the summer one, and the autumn quarter to the winter one, the former being the longer. In the year 6589 A. D., the earth will be in perihelion when it is at the vernal equinox ; summer will then be equal to autumn and winter to spring, the former seasons being the longer. In the year 11928 A. D., the earth will be in perihelion when it is at the summer solstice : finally, in 17267 A. D., the cycle will be completed, and for the first time since the creation of man the autumnal equinox will co- incide with the earth's perihelion. PEBMANENCE IN THE MIDST OF CHANGE. "We thus see that the ecliptic is constantly modifying its ellip- tical shape ; that the orbit of the earth slowly oscil- lates upward and downward ; that the north pole steadily turns its long index-finger over a dial that marks 26,000 years ; that the earth, accurately poised in space, yet gently nods and bows to the attraction of sun and moon. Thus changes are con- tinually taking place that would ultimately entirely reverse the order of nature. But each of these has its bounds, beyond which it cannot pass. The promise made to man after the Deluge, is that " while the earth remaineth, seed-time and harvest, and cold and heat, and summer and winter, and should be noticed, are applied to the real position of the earth and not the apparent position of the sun. The dates are those given by Chambers in his Descriptive Astronomy. 130 THE SOLAR SYSTEM. day and night shall not cease." The modern dis- coveries of astronomy prove conclusively that the seasons are to be permanent ; that the Creator, amid all these transitions, has ordained the means of carrying out His promise through all time. EEFEACTION. The atmosphere extends above the earth about 500 miles. Near the surface it is dense, while in the upper regions it is exceedingly rare. The rays of light from the heavenly bodies Fig. 36. REFRACTION. passing through these different layers are turned downward toward a perpendicular more and more as the density increases. According to a well- known law of optics, if the ray of light from a star were bent in fifty directions before entering the eye, the star would nevertheless appear to be in the line of the one nearest the eye. The effect of this is, that the apparent place of a heavenly body is higher THE EARTH. 131 than the true place. This is illustrated in Fig. 36. The sun at S, were it not for the atmosphere, would send a direct ray to L. Instead, the ray at A is refracted downward, and would then enter the eye at N ; passing, however, through a layer of a differ- ent density, at B it is again bent, and meets the eye of the observer at C. He sees the sun, not in the direction of the curved line C B A S, but that of the straight line CBS. The amount of refraction varies with the tempera- ture, moisture, and other conditions of the atmos- phere. It is zero for a body in the zenith, and increases gradually toward the horizon (as the thick- ness of the intervening atmosphere increases), where it is about 33'. Fig. 37. Change of place and appearance of the sun and moan. The sun may be really below the horizon, and yet 132 THE SOLAR SYSTEM. seem to be above it. For example, on April 20, 1837, the moon was eclipsed before the sun had set. The mean diameter of both the sun and moon being rather less than 33', it follows that when we see the lower edge of either of these lumina- ries apparently just touching the horizon, in reality the whole disk is completely beloiu it, and would be altogether hidden were it not for the effect of refraction. The day is consequently materially lengthened. The sun and moon often appear flattened when near the horizon. This is easily accounted for on the principle just stated. The rays from the lower edge pass through a denser layer of the atmosphere, and are therefore refracted about 4' more than those from the upper edge : the effect of this is to make the vertical diameter appear about 4' less than the horizontal, and so distort the figure of the disk into an oval shape. The sun and moon often appear larger when near the horizon than when high in the sky. This is not caused by refraction, but is a mere error of judg- ment. At the horizon we compare them with va- rious terrestrial objects which lie between them and us, while aloft we have no association to guide us, and we are led to underrate their size. On looking at them through a tube, the illusion disappears. The moon should naturally appear largest when at a great altitude, as it is then at a less distance from us. THE EARTH. 133 The dim and hazy appearance of the heavenly bodies when near the horizon is caused not only by the rays of light having to pass through a larger space in the atmosphere, but also by their travers- ing the lower and denser part. The intensity of the solar light is so greatly diminished by passing through the lower strata, that we are enabled to look upon the sun at that time without being daz- zled by his brilliant beams. Twiliylit. The glow of light after sunset and before sunrise, which we term ttvilight, is caused by the refraction and reflection of the sun's rays by the atmosphere. For a time after the sun has truly set, the refracted rays continue to reach the earth ; but when these have ceased, he still continues to illumi- nate the clouds and upper strata of the air, just as he may be seen shining on the summits of lofty mountains long after he has disappeared from the view of the inhabitants of the plains below. The air and clouds thus illuminated reflect back part of the light to the earth. As the sun sinks lower, less light reaches us until reflection ceases and night ensues. The same thing occurs before sun- rise, only in reverse order. The duration of twilight is usually reckoned to last until the sun's depres- sion below the horizon amounts to 18 ; this, how- ever, varies with the latitude, seasons, and condi- tion of the atmosphere. Strictly speaking, in the latitude of Greenwich there is no true night for a month before and after the summer solstice, but 134 THE SOLAR SYSTEM. constant twilight from sunset to sunrise. The sun is then near the Tropic of Cancer, and does not descend so much as 18 below the horizon during the entire night. The twilight is shortest at the equator and longest toward the poles, where the night of six months is shortened by an evening twilight of about fifty days and a morning one of equal length. Diffused light. The diffused light of day is pro- duced in the same manner as that of twilight. The atmosphere reflects and scatters the sunlight in every direction. "Were it not for this, no object would be visible to us out of direct sunshine ; every shadow of a passing cloud would be pitchy dark- ness ; the stars would be visible all day ; no window would admit light except as the sun shone directly through it, and a man would require a lantern to go around his house at noon. This is illustrated very clearly in the rarified atmosphere of elevated re- gions, as on Mont Blanc, where it is said the glare of the direct sunlight is almost insupportable ; the darkness of the shadows is deeper and denser ; all nice shading and coloring disappear; the sky has a blackish hue, and the stars are seen at midday. The blue light reflected to our eyes from the atmos- phere above us, or more probably from the vapor in the air, produces the optical delusion we call the sky. Were it not for this, every time we cast our eyes upward we should feel like one gazing over a dizzy precipice ; while now the crystal dome of blue THE EARTH. 135 smiles down upon us so lovingly and beautifully that we call it heaven. ABERRATION OF LIGHT. "We have seen that the places of the heavenly bodies are apparently changed by refraction. Besides this, there is another change due to the motion of light, combined with the mo- tion of the earth in its orbit. For example : the mean distance of the earth from the sun is ninety- one and a half millions of miles, and since light travels 183,000 miles per second, it follows that the time occupied by a ray of light in reaching us from the sun is about 8-J min. ; so that, in point of fact, when we look at the sun (1), we do not see it as it is, but as it was SJmin. since. If our globe were at rest, this would be well enough, but since the earth is in motion, when the ray enters our eye we are at some distance in advance of the position we occupied when it started. During the SJmin. the earth has moved in its orbit nearly 20^", so that (2) we never see that luminary in the place it occu- pies at the time of observation. Illustration. Suppose a ball let fall from a point P, above the horizontal line A B, and a tube, of which A is the lower extremity, placed to receive it. If the tube were fixed, the ball would strike it on the lower side ; but if the tube were carried forward in the direction A B, with a velocity properly ad- justed at every instant to that of the ball, while pre- serving its inclination to the horizon, so that when the ball, in its natural descent, reached B, the tube 136 THE SOLAR SYSTEM. would have been carried into the position BQ, it is evident that the ball throughout its whole descent would be found in the tube ; and a spectator refer- ring to the tube the motion of the ball, and carried Fig. 38. ABERRATION OP LIGHT. along with the former, unconscious of its motion, would fancy that the ball had been moving in an inclined direction, and had come from Q. A very common illustration may be seen almost any rainy day. Choose a time when the air is still and the drops large. Then, if you stand still, you will .see that the drops fall vertically ; but if you walk for- ward, you will see the drops fall as if they were meeting you. If, however, you walk backward, you will observe that the drops fall as if they were com- ing from behind you. We thus see that the drops have an apparent as well as real motion THE EAKTH. 137 The general effect of aberration of light is to cause each star to apparently describe a minute ellipse in the course of a year, the central point of which is the place the star would actually occupy were our globe at rest. PARALLAX. This is tlie difference in the direction of an object as seen from two different places. For a simple illustration of it, hold your finger before you Fig. 39. PARALLAX. in front of the window. Upon looking at it with the left eye only, you will locate your linger at some point on the window ; on looking with the right eye only, you will locate it at an entirely different point. Use your eyes alternately and quickly, and you will 138 THE SOLAR SYSTEM. be astonished at the rate with which your finger will seem to change its place. Now, the difference in the direction of your finger as seen from the two eyes is parallax. In astronomical calculations, the position of a body as seen from the earth's surface is called its apparent place, while that in which it would be seen from the centre of the earth is called its true place. Thus, in the cut, a star is seen by the ob- server at O in the direction OP ; if it could be viewed from the centre R, its direction would be in the line EQ. It is therefore seen from O at a point in the heavens beloiu its position in reference to R. From looking at the cut, we can see (1), that the parallax of a star near the horizon is greatest, while it decreases gradually until it disappears alto- gether at the zenith, since an observer at O, as wel] as one at R, would see the star Z directly overhead ; and (2), that the nearer a body is to the earth the greater its parallax becomes. It has been agreed by astronomers, for the sake of uniformity in their calculations, to correct all observations so as to refer them to their true places as seen from the centre of the earth. Tables of parallax are constructed for this purpose. The question of parallax is also one of very great importance, because as soon as the parallax of a body is once accurately known, its dis- tance, diameter, etc., can be readily determined. (See Celestial Measurements.) Horizontal Parallax. This is the parallax of THE MOON. 139 a body when at the horizon. It is, in fact, the earth's semi-diameter as seen from the body. In the figure, the parallax of the star S is the angle OSR, which is measured by the line OK the semi-diam- eter of the earth. The sun's horizontal parallax (8.94") is the angle subtended (measured) by the earth's semi-diameter as seen from that luminary. As the moon is nearest the earth, its horizontal par- allax is the greatest of any of the heavenly bodies. Annual Parallax. The fixed stars are so distant from the earth that they exhibit no change of place when seen from different parts of the earth. The lines OS and US are so long that they are ap- parently parallel, and it becomes impossible to discover the slightest inclination. Astronomers, therefore, instead of taking the earth's semi-diam- eter, or 4,000 miles, as the measuring tape, have adopted the plan of observing the position of the fixed stars at opposite points in the earth's orbit. This gives a change in place of 183,000,000 miles. The variation of position which the stars under- go at these remote points is called their annual parallax. THE MOON. New Moon, . First Quarter, . Full Moon, . Last Quarter, >. ITS MOTION IN SPACE. The orbit of the moon, con- sidering the earth as fixed, is an ellipse of which our planet occupies one of the foci. Its distance from 140 THE SOLAR SYSTEM. the earth therefore, varies incessantly. At perigee it is 26,000 miles nearer than in apogee : the mean distance is about 238,000 miles. It would require a chain of thirty globes equal in size to the earth to reach the moon. An express-train would take about a year to accomplish the journey. The moon com pletes its revolution (sidereal) around the earth in about 27i days ; but, as the earth is constantly pass- Fig. 40. PATH OP MOON. ing on in its own orbit around the sun, it requires over two days longer before it comes into the same position with respect to the sun and earth, thus com- pleting its synodic revolution. THE MOON. 141 The real path of the moon is the result of its own proper motion and the onward movement of the earth. The two combined produce a wave-like curve that crosses the earth's path twice each month ; this, owing to its small diameter com- pared with that of the ecliptic, is always concave toward the sun. As the moon constantly keeps the same side turned toward us, it follows that it must turn on its axis once each month. DIMENSIONS. Its diameter is about 2,160 miles. It would require fifty globes the size of the moon to equal the earth. Its apparent size varies with its distance ; the mean is, however, about one half a Pig. 41. THE SIZE OP MOON AT HORIZON AND ZENITH. degree, the same as that of the sun. It always ap- pears larger than it really is, on account of its brightness. This is the effect of what is termed in optics Irradiation. To illustrate this principle, cut two circular pieces of the same size, one of black THE 80LAE SYSTEM. /the other of white paper. The white circle, en held in a bright light, will appear much larger than the black one. For the same reason it is often noticed that the crescent moon seems to be a part of a larger circle than the rest of the moon. As we have already said, the moon appears larger on the ho- rizon than when high up in the sky. By an examina- tion of the cut, it is easily seen that it is 4,000 miles nearer when on the zenith than when at the horizon. Besides these general variations in size, the moon varies in apparent size to different observers. Much amusement may be had in a large party or class by a comparison of its apparent magnitude. The esti- mates will differ from a small saucer to a wash-tub. LIBKATIONS (librans, swinging). "While the moon presents the same hemisphere to us, there are three causes which enable us to see about 576 out of the 1,000 parts of its entire surface. (1.) The axis of the moon is inclined a little to its orbit, as also its orbit is inclined to the earth's orbit; so when its north pole leans alternately toward and from the earth, we see sometimes past its north, and some- times past its south pole. This is called libration in latitude. (2.) The moon's rotation on its axis is al- ways performed in the same time, while its move- ment along its orbit is variable ; hence it happens that we occasionally see a little further around each limb (outer edge) than at other times. This is called libration in longitude. (3.) The size of the earth is so much greater than that of the moon, that an ob- THE MOON. 143 server, by the rotation of the earth, or by going north or south, can see further around the limbs. LIGHT AND HEAT. If the whole sky were covered with full moons, they would scarcely make daylight, since the brilliancy of the moon does not exceed sir^Tnnj- tnat f tne sun - That portion of the moon's surface which is exposed to the sun is supposed to be highly heated, possibly to the degree of boil- ing water, yet its rays impart no heat to us ; indeed Prof. Tyndall considers them rays of cold. This is probably caused by the fact that our dense atmos- phere absorbs all the heat, which in the higher re- gions produces the effect of scattering the clouds. It is a well-known fact that the nights are oftenest clear at full moon. (Herschel.) CENTEE OF GRAVITY. It is thought that the centre of gravity of the moon is not exactly at its centre of magnitude, but nearly thirty-three miles beyond, and that the lighter half is toward us. If that be so, this side is equivalent to a mountain of that enormous height. We can easily see that if water and air exist upon the moon, they cannot remain on this hemisphere, but must be confined to the side which is forever hidden from our view. ATMOSPHERE OF THE MOON. The existence of an atmosphere upon our satellite is at present an open question. If there be any, it must be extremely rarefied, perhaps as much so as that which is found in the vacuum obtained in the receiver of our best air-pumps. 144 THE SOLAR SYSTEM. Pig. 42. APPEARANCE OF THE EARTH TO LUNARIANS. If tlieie be any lunar inhabitants on the side toward us, the earth must present to them all the phases which their world exhibits to us, only in a reverse order. When we have a new moon, they have a futt earth, a bright full-orbed moon fourteen times as large as ours. The lunar inhabitants upon the side opposite to us of course never see our earth, unless they take a journey to the re- gions from whence it is visible, to behold this wonderful spec- tacle. Those living near the limbs of the disk might, however, on ac- count of the librations, get occasional glimpses of it near their horizon. THE EARTH-SHINE. For a few days before and after new moon, we may distinguish the outline of the unillumined portion of the moon. In England, it is popularly known as " the old moon in the new moon's arms." This reflection of the earth's rays must serve to keep the lunar nights quite light, even in new earth. PHASES OF THE MOON. The phases of the moon show conclusively that it is a dark body, which shines only by reflecting the light it receives from APPEARANCE OP EARTH AS SEEN FROM MOON. THE MOON. 145 the sun. Let us compare its various appearances with the positions indicated in the figure. Fig. 43. PHASES OF MOON. We see it (1) as a delicate crescent in the west just after sunset, as it first emerges from the sun's 7 146 THE SOLAR SYSTEM. rays at conjunction. It soon sets below the horizon Half of its surface is illumined, but only a slender edge with its horns turned from the sun is visible to us. Each night the crescent broadens, the moon recedes about 13 further from the sun, and sets cor- respondingly later, until at quadrature half of the enlightened hemisphere is turned toward us, and the moon is said to be in her first quarter. Continuing her eastern progress round the earth, the moon (2) becomes gibbous* in form, and, about the fifteenth day from new moon, reaches the point in the heavens directly opposite to that which the sun occupies. She is then in opposition, the whole of the illumined side is turned toward us, and we have a full moon. She is on the meridian at midnight, and so rises in the east as the sun sets in the west, and vice versa. The moon (3) passing on in her orbit from oppo- sition, presents phases reversed from those of the sec- ond quarter. The proportion of the illumined side visible to us gradually decreases ; she becomes gibbous again ; rises nearly an hour later each evening, and in the morning lingers high in the western sky after sunrise. She now comes into quadrature, and is in her third quarter. From the third quarter the moon (4) turns her en- lightened side from us and decreases to the crescent form again; as, however, the bright hemisphere * (jitibw* means less than a half and more than a quarter circle. THE MOON. 147 constantly faces the sun, the horns are pointed toward the west. She is now seen as a bright cres- cent in the eastern sky just before sunrise. At last the illumined side is completely turned from us, and the moon herself, coming into conjunction with the sun, is lost in his rays. To accomplish this journey through her orbit from new moon to new moon, has required 29J days a lunar month. Moon runs high or low. All have, doubtless, no- ticed that, in the long nights of winter, the full moon is high in the heavens, and continues a long time above the horizon; while in midsummer it is low, and remains a much shorter time above the horizon. This is a wise provision of Providence, which is seen yet more clearly in the arctic regions. There the moon; during the long summer day of six months, is above the horizon only for her first and fourth quar- ters, when her light is least ; but during the tedious winter night of equal length, she is continually above the horizon for her second and third quarters. Thus in polar regions the moon is never full by day, but is always full every month in the night. We can easily understand these phenomena when we remem- ber that the new moon is in the same quarter and the full moon in the opposite quarter of the heavens from the sun. Consequently, the moon always be- comes full in the other solstice from that in which the sun is. When, therefore, the sun sinks very low in the southern sky the full moon rises high, and when the sun rises high the full moon sinks low. 148 THE SOLAR SYSTEM. HARVEST MOON. While the moon rises on the average 50 m. later each night, the exact time va- ries from less than half an hour to a full hour. Near the time of autumnal equinox the moon, at her full, rises about sunset a number of nights in succession. This gives rise to a series of brilliant moonlight evenings. It is the time of harvest in England, and hence has received the name of the Harvest Moon. Its return is celebrated as a festi- val among the peasantry. In the following month (October) the same occurrence takes place, and it is then termed the Hunter's Moon. The cause of this phenomenon lies in the fact that the moon's path is variously inclined to the horizon at different seasons of the year. When the equinoxes are in the hori- zon, it makes a very small angle with the horizon ; whereas, when the solstitial points are in the horizon, the angle is far greater. In the former case, the moon moving eastward each day about 13, will de- scend but little below the horizon, and so for sev- eral successive evenings will rise at about the same hour. In the latter, glie will descend much further each day and thus will rise much later each night. The least possible variation in the hour of rising is 17 minutes the greatest is 1 hour, 16 minutes. In the figure, S represents the sun, E the earth, M the moon ; C F the moon's path around the earth \vhen the solstitial points are in the horizon E D when the equinoxes are in the horizon ; A M B S the THE MOON. 149 Fig. 44. horizon ; M.d = M.b =13, the distance the moon moves each day. When passing along the path G F, the moon sinks below the horizon the distance al, and when mov- ing along the path E D, only the distance cd. It is ob- vious that be- fore the moon j\ can rise in the former case, the horizon must be de- pressed the distance a I, and in the lat- ter only cd; and the moon will rise correspondingly later in the one and earlier in the other. NODES. The orbit of the moon is inclined to the ecliptic about 5, the points where her path crosses it being termed nodes. The ascending node (&) is the place where the moon crosses in coming above the ecliptic or toward the north star ; the descending node (8) is where it passes below the ecliptic. The imaginary line connecting these two points is called the " line of the nodes." OCCULTATION. The moon, in the course of her monthly journey round the earth, frequently passes in front of the stars or planets, which disappear on HARVEST MOON. 150 THE SOLAR SYSTEM. one side of her disk and reappear on the other, This is termed an occultation, and is of practical use in determining the difference of longitude between various places on the earth. LUNAR SEASONS; DAY AND NIGHT, ETC. As the moon's axis is so nearly perpendicular to her orbit, she cannot properly be said to have any change of seasons. During nearly fifteen of our days, the sun pours down its rays unmitigated by any atmosphere to temper them. To this long, torrid day succeeds a night of equal length and polar cold. How strange the lunar ' appearance would be to us ! The disk of the sun seems sharp and distinct. The sky is black and overspread with stars even at midday. There is no twilight, for the sun bursts instantly into day, and after a fortnight's glare, as suddenly gives place to night; no air to conduct sound, no clouds, no winds, no rainbow, no blue sky, no gor- geous tinting of the heavens at sunrise and sunset, no delicate shading, no soft blending of colors, but only sharp outlines of sun and shade. What a bleak waste ! A barren, voiceless desert ! The nights, however, of the visible hemisphere must be brilliantly illuminated by the earth, while its phases " serve well as a clock a dial all but fixed in the same part of the heavens, like an immense lamp, behind which the stars slowly defile along the black sky." TELESCOPIC FEATURES. The lunar landscape is yet more wonderful than its other physical features THE MOON. Pi*. 45. 151 EAT, LANDSCAPE OF THK MOON. 152 THE SOLAR SYSTEM. Even with the naked eye we see on its surface bright spots the summits of lofty mountains, gilded by the first rays of the sun and darker portions, low plains yet lying in comparative shadow. The tele- scope reveals to us a region torn and shattered by fearful, though now extinct* volcanic action. Every- where the crust is pierced by craters, whose irregu lar edges and rents testify to the convulsions our satellite has undergone at some past time. Mountains. The heights of more than 1,000 of these lunar mountains have been measured, some of which exceed 20,000 feet. The shadows of the mountains, as the sun's rays strike them obliquely, are as distinctly perceived as that of an upright staff when placed opposite the sun. Some of these are insulated peaks that shoot up solitary and alone from the centre of circular plains ; others are moun- tain ranges extending hundreds of miles. Most of the lunar elevations have received names of men distinguished in science. Thus we find jPlato, Aris- tarchus, Copernicus, Kepler, and Newton, associated however with the Apennines, Carpathians, etc. Gray plains or seas. These are analogous to our prairies. They were formerly supposed to be sheets of water, but have more recently been found to ex- * Several distinguished astronomers assert, however, that the crater Linnaeus has undergone of late certain marked changes. Its sides seem to have fallen in, and the interior to have become filled up, as if by a new eruption. It is said to present an ap- pearance similar to that of the Sea of Serenity. 154 THE SOLAR SYSTEM. hibit the uneven appearances of a plain, instead of the regular curve of bodies of water. The former names have been retained, and we find on lunar maps the " Sea of Tranquillity," the " Sea of Nee- tar," " Sea of Serenity," etc. Rills, luminous bands. The latter are long bright streaks, irregular in outline and extent, which radi- ate in every direction from Tycho, Kepler, and other mountains ; the former are similar, but are sunken, and have sloping sides, and were at first thought to be ancient river-beds. Their exact nature is yet a mystery. Craters. These constitute by far the most curious feature of the lunar landscape. They are of volcanic origin, and usually consist of a cup-like basin, with a conical elevation in the centre. Some of the craters have a diameter of over 100 miles. They are great walled plains, sunk so far behind huge volcanic ram- parts, that the lofty wall which surrounds an ob- server at the centre would be beyond his horizon. Other craters are deep and narrow, as Newton, which is said to be about four miles in depth, so that neither earth nor sun is ever visible from a great part of the bottom. The appearance of these craters is strikingly shown in the accompanying view of the region to the southeast of Tycho. (Fig. 46.) ECLIPSES. 155 ECLIPSES. ECLIPSE OF THE SUN. If the moon should pass through either node at or near the time of conjunc- tion or neiv moon, she would necessarily come be- tween the earth and the sun, for the three bodies are then in the same straight line. This would cause Fig. 47. of &* ECLIPSE OP SUN. an eclipse of the sun. If the moon's orbit were in the same plane as the ecliptic, an eclipse of the sun would occur at every new moon ; but as the orbit is inclined, it can occur only at or near a node. The eclipse may be partial, total, or annular. In Fig. 48, we see where the dark shadow (umbra) of Fig. 48 UMBRA AND PENUMBRA. the moon falls on the earth and obscures the entire body of the sun. To the persons within that region 156 THE SOLAR SYSTEM. there is a total eclipse; the breadth of this space is not large, averaging only 140 miles. Beyond this umbra there is a lighter shadow, penumbra (pene, almost - umbra, a shadow), where only a portion of the sun's disk is obscured. Within this region there is a partial eclipse. To those persons liv- ing north of the equator and of the umbra, the eclipse passes over the lower limb of the sun ; to those south of the umbra, it passes over the upper limb.* When the eclipse occurs exactly at the node, it is said to be central. If the eclipse takes place when the moon is at apogee, or furthest from the earth, her apparent diameter is less than that of the sun ; as a conse- quence, her disk does not cover the disk of the sun, and the visible portions of that luminary appear in the form of a ring (annulus) ; hence there is an an- nular eclipse in all those places comprised within the limits of the cone of shadow prolonged to the earth. General facts concerning a solar eclipse. The fol- lowing data may perhaps guide in better under- . standing the phenomena of solar eclipses. (1.) The moon must be new. (2.) She must be at or near a node. (3.) When her distance from the earth is less than the length of her shadow, the eclipse will be total or partial. (4.) When her distance is greater than the length of her shadow, the eclipse will be annular or partial. (5.) There can be no eclipse at those places where the sun himself is invisible during the time. * South of the equator the reverse of these phenomena would happen. ECLIPSES. 15V (6.) An eclipse is not visible over the whole illu- mined side of the earth. As the moon's diameter is so much less than that of the earth, her cone of shadow is too small to enshroud the entire globe, so that the region in which it is total cannot exceed 180 miles in breadth. As, however, the earth is con- stantly revolving on its axis during the duration of the eclipse, the shadow may travel over a large sur- face of territory. (7.) If the moon's shadow fall upon the earth when she is just nearing her ascending node, it will Fiff. 49 SOLAR ECLIPTIC LIMIT (17). only sweep across the south polar regions : if when nearing her descending node, it will graze the earth near the north pole. The nearer a node the con- junction occurs, the nearer the equatorial regions the shadow will strike. (8.) At the equator, the longest possible duration of a total solar eclipse is only about eight minutes, and of an annular, twelve minutes. One reason of the greater length of the latter is, that then the moon is in apogee, when it always moves slower than when in perigee. The duration of total obscuration is greatest when the moon is in perigee and the sun in apogee ; for then the apparent size of the moon is greatest and that of the sun least. We see from 158 TilE SOLAR SYSTEM. this that the relative situation of the moon and sun decides the length and kind of the eclipse. (9.) There cannot be more than five nor less than two solar eclipses per year. A total or an an- nular eclipse is exceedingly rare. For instance, there has not been a total eclipse visible at London since 1715, and previous to that, there had been none visible for five and a half centuries. (10.) A solar eclipse comes on the western limb or edge of the sun and passes off on the eastern. (11.) The disk of the sun and moon is divided into twelve digits, and the amount of the eclipse is esti- mated by the number of digits which it covers. Thus an eclipse of six digits is one in which half the di- ameter of the disk is concealed. Curious phenomena. Various singular appearances always attend a total eclipse. Around the sun is seen a beautiful Fig. 50. corona or halo of light, like that which paint- ers give to the head of the Virgin Mary. Flames of a blood-red color play around the disk of the moon, and when only a mere crescent of the sun is BottpsE OF isss ECLIPSES. 159 Fig. 51. visible, it seems to resolve itself into bright spots interspersed with dark spaces, having the appear- ance of a string of bright beads (Baily's Beads.) Attendant cir- cumstances of a total eclipse. These are of a peculiarly im- pressive charac- ter. The dark- ness is so intense that the brighter stars and planets are seen, birds cease their songs and fly to their nests, flowers close, and the face of nature assumes an unearthly cadaverous hue, while a sudden fall of the temperature causes the air to feel damp, and the grass wet as if from excessive dew. Orange, yellow, and copper tints give every object a strange appearance, and startle even the most indifferent. The ancients regarded a total eclipse with feelings of indescribable terror, as an indication of the anger of an offended Deity, or the presage of some impending calamity. Even now, when the causes are fully understood, and the time of the eclipse can be predicted within the fraction of a second, the change from broad daylight to in- ANNULAR ECLIPSE OP 183& SHOWING BAILY'd BEADS. 1GO THE SOLAR SYSTEM. stantaneous gloom is overwhelming, and inspires with awe even the most careless observer. Curious custom among the Hindoos. Among the Hindoos a singular custom is said to exist. When, during a solar eclipse, the black disk of our satellite begins slowly to advance over the sun, the natives believe that some terrific monster is gradually de- vouring it. Thereupon they beat gongs, and rend the air with most discordant screams of terror and shouts of vengeance. For a time their frantic efforts seem futile and the eclipse still progresses. At length, however, the increasing uproar reaches the voracious monster ; he appears to pause, and then, like a fish rejecting a nearly swallowed bait, grad- ually disgorges the fiery mouthful. When the sun is quite clear of the great dragon's mouth, a shout of joy is raised, and the poor natives disperse, ex- tremely self-satisfied on account of having so suc- cessfully relieved their deity from his late peril. THE SAROS. The nodes of the moon's orbit are constantly moving backward. They complete a rev- olution around the ecliptic in about eighteen and a half years. Now the moon makes 223 synodic revolutions in 18 yr. 10 da. ; the sun makes 19 rev- olutions with regard to the lunar nodes in about the same time. Hence, in that period the sun and moon and the nodes will be in nearly the same rela- tive position. If, then, we reckon 18 yr. 10 da. from any eclipse, we shall find the time of its repetition. This method was discovered, it is said, by the dial- ECLIPSES. 161 deans. The ancients were enabled, by means of it, to predict eclipses, but it is considered too rough by modern astronomers : eclipses are now foretold cen- turies in advance, true to a second. In this manner historical incidents are verified, and their exact dates determined. METONIC CYCLE. The Metonic Cycle (sometimes confounded with the Saros) was not used for foretell- ing eclipses, but for the purpose of ascertaining the age of the moon at any given period. It consists of nineteen tropical years,* during which time there are exactly 235 new moons ; so that, at the end of this period, the new moons will recur at seasons of the year exactly corresponding to those of the pre- ceding cycle. By registering, therefore, the exact days of any cycle at which the new or full moons occur, such a calendar shows on what days these events will occur in succeeding cycles. Since the regulation of games, feasts, and fasts has been made very extensively, both in ancient and modern times, according to new or full moons, such a calen- dar becomes very convenient for finding the day on which the new or full moon required takes place. Thus if a festival were decreed to be held in any given year on the day of the first full moon after the vernal equinox : find what year it is of the lunar cycle, then refer to the corresponding year of * A tropical year is the interval between two successive retums of the sun to the vernal equinox. 162 THE SOLAlt SYSTEM. the preceding cycle, and the day will be the same aa it was then. The Golden Number, a term still used in our almanacs, denotes the year of the lunar cycle. Seven is the golden number for 1868. ECLIPSE OF THE MOON. This is caused by the passing of the moon into the shadow of the earth, Fig. 52. ECLIPSE OF THE MOON. and hence can take place only at full moon oppo- sition. As the moon's orbit is inclined to the ecliptic, her path is partly above and partly below the earth's shadow ; thus an eclipse of the moon can take place only at or near one of the nodes. In the figure, the umbra is represented by the space between the lines K c and I b ; outside of this is the penumbra, where the earth cuts off the light of only a portion of the sun. The moon enters the penumbra of the earth at a, this is termed her first contact with the penumbra ; next she encounters the dark shadow of the earth at b, this is called ike first contact with the umbra ; she then emerges from the umbra at c, which is called the second con- tact with the umbra ; finally, she touches the outer edge of the penumbra at d, t lie second contact with the penumbra. Since the earth is so much larger than ECLIPSES. 163 the moon, the eclipse can never be annular , as, however, the eclipse may occur a little above or be- low the node, the moon may only partly enter the earth's shadow, either on its upper or lower limb. From the first to last contact with the penumbra, five hours and a half may elapse. Total eclipses of the moon are rarer events than those of the sun, since the lunar ecliptic limit is only about 12 ; yet they are more frequently seen by us, (1) because each one is visible over the entire unillumined hemisphere of the earth, and also (2) because by the diurnal ro- tation during the long duration of the eclipse, large areas may be brought within its limits. So it will happen that while the inhabitants of one district wit- ness the eclipse throughout its continuance, those of other regions merely see its beginning, and others only its termination. The moon does not completely disappear even in total eclipses. The cause of this fact lies in the refraction of the solar rays in traversing the lower strata of the earth's atmos- phere ; they are analyzed, and purple our moon with the tints of sunset. The amount of refraction and the color depend upon the state of the air at the time. HISTORICAL ACCOUNTS OF ECLIPSES. The earliest account of an eclipse on record is in the Chinese annals ; it is thought to be the solar eclipse of Octo- ber 13, 2127 B. c. On May 28, 584 B. c., one oc- curred which was predicted by Thales, as wo have before mentioned. In the writings of the early Eng- 164 THE SOLAK SYSTEM. lish chroniclers are numerous passages relating to eclipses. William of Malmesbury thus refers to that of August 2, 1133, which was considered a presage of calamity to Henry I. : " The elements manifested their sorrows at this great man's last departure. For the sun on that day, at the 6th hour, shrouded his glorious face, as the poets say, in hideous darkness, agitating the hearts of men by an eclipse ; and on the 6th day of the week, early in the morn- ing, there was so great an earthquake, that the ground appeared absolutely to sink down ; an horrid noise being first heard beneath the surface." The same writer, speaking of the total eclipse of March 20, 1140, says : " During this year, in Lent, on the 13th of the kalends of April, at the 9th hour of the 4th day of the week, there was an eclipse, through- out England, as I have heard. With us, indeed, and with all our neighbours, the obscuration of the Sun also was so remarkable, that persons sitting at table, as it then happened almost every where, for it was Lent, at first feared that Chaos was come again : afterwards learning the cause, they went out and beheld the stars around the Sun. It was thought and said by many, not untruly, that the king [Ste- phen] would not continue a year in the govern- ment." Columbus made use of an eclipse of the moon, which took place March 1, 1504, to relieve his fleet, which was in great distress from want of sup- plies. As a punishment to the islanders of Jamaica, who refused to assist him, he threatened to deprive THE TIDES. 165 them of the light of the moon. At first they were indifferent to his threats, but " when the eclipse ac- tually commenced, the barbarians vied with eacli other in the production of the necessary supplies for the Spanish fleet." THE TIDES. DESCKIPTION. Twice a day, at intervals of about twelve hours and twenty-five minutes, the water be- gins to set in from the ocean, beating the pebbles and the foot of the rocky shore, and dashing its spray high in air. For about six hours it climbs far up on the beach, flooding the low lands and transforming simple creeks into respectable rivers. The instant of high-water or flood-tide being reached, it begins to descend, and the ebb succeeds the flow. The water, however, falls somewhat slower than it rose. CAUSE OF THE TIDES. The tides are caused by a great wave, which, raised by the moon's attraction, Fte. 53. Spring Tidts SPKINO TIDE. follows her in her course around the earth. The sun, also, aids somewhat in producing this effect; but as the moon is 400 tim^s nearer the earth, her 1S6 THE SOLAR SYSTEM. influence is far greater. As the waters are free to yield to the attraction of the moon, she draws them away from C and D and they become heaped up at A. The earth, being nearer the moon than the waters on the opposite side, is more strongly at- tracted, and so, being drawn away from them, they are left heaped up at B. As the result, high-water is produced at A by the water being pulled from the earth, and at B by the earth being pulled from the water. The influence of the moon is not instanta- neous, but requires a little time to produce its full effect ; hence high- water does not occur at any place when the moon is on the meridian, but a few hours after. As the moon rises about fifty minutes later each day, there is a corresponding difference in the time of high-water. While, however, the lunar tide- wave thus lags about fifty minutes every day, the solar tide occurs uniformly at the same time. They therefore steadily separate from each other. At one time they coincide, and high-water is the sum of lunar and solar tides ; at other times, high-water of the solar tide and low-water of the lunar tide occur simultaneously, and high-water is the difference between the lunar and solar tides. We should bear in mind tha philosophical truth, that the tide in the open sea ioes not consist of a progressive movement of the water itself, but only of the form of the wave. Causes that modify the tides. At new and full moon (the syzygies) the sun acts with the moon (as in Fig. THE TIDES. 167 53) in elevating the waters ; this produces the highest or Spring tide. In quadrature (as in Fig. 54), the sun tends to diminish the height of the water : this is called Neap-tide. When the moon is in perigee her attraction is stronger ; hence the flood-tide is higher and the ebb-tide lower than at other times. This re- Fig. 54. Bcap Tides NEAP-TIDE. mark applies also to the sun. The height of the tide also varies with the declination of the sun and moon, the highest or equinoctial tides taking place at the equinoxes, if, when the sun is over the equator, the moon also happens to be very near it : the lowest occur at the solstices. The force and direction of the winds, the shape of the coast, and the depth of the sea wonderfully complicate the explanation of local tides. Height of the tide at different places. In the open sea the tide is hardly noticeable, the water some- times rising not higher than a foot ; but where tho wave breaks on the shore, or is forced up into bays or narrow channels, it is very conspicuous. The difference between ebb and flood neap-tide at New York is over three feet, and that of spring tide over 168 THE SOLAR SYSTEM. fivo feet ; while at Boston it is nearly double this amount. A headland jutting out into the ocean will diminish the tide ; as, for instance, off Cape Florida, where the average height is only one and a half feet. A deep bay opening up into the land like a funnel, will converge the wave, as at the Bay of Fundy, where it rolls in, a great roaring wall of water sixty feet high, frequently overtaking and sweeping off men and animals. The tide sets up against the current of rivers, and often entirely changes their character ; for example, the Avon at Bristol is a mere shallow ditch, but at flood-tide it becomes a deep channel navigable by the largest Indiarnen. Differential effect. The whole attraction of the moon is only T J-g- that of the sun : yet her influence in producing the tides and precession is greater, because that depends not upon the entire attraction either exerts, but upon the difference between their attrac- tion upon the earth's centre and upon the earth's nearest surface. For the moon, on account of her nearness, the proportion of the distance of these parts is treble that of the sun, and hence her greater effect. MARS. The god of war. Sign, $ , shield and spear. DESCRIPTION. Passing outward in our survey of the solar system, we next meet with Mars. This is the first of the superior planets, and the one most like the earth. It appears to the naked eye as a bright MAES. 169 red star, rarely scintillating, and shining with a steady light, which distinguishes it from the fixed stars. Its ruddy appearance has led to its being celebrated among all nations. The Jews gave it the appellation of " blazing," and it bore in other lan- guages a similar name. At conjunction its apparent Fi