Part 14

238. _The Volume and Density of Mars._--Among the larger planets Mars is next in size to Mercury. Its real diameter is somewhat more than four thousand miles, and its bulk is about one-seventh of that of the earth. Its size, compared with that of the earth, is shown in Fig. 265.

The density of Mars is only about three-fourths of that of the earth.

239. _Sidereal and Synodical Periods of Mars._--The _sidereal_ period of Mars, or the time in which he makes a complete revolution around the sun, is about six hundred and eighty-seven days, or nearly twenty-three months; but he is about seven hundred and eighty days in passing from opposition to opposition again, or in performing a _synodical_ revolution. Mars moves in his orbit at the rate of about fifteen miles a second.

240. _Brilliancy of Mars._--When near his opposition, Mars is easily recognized with the naked eye by his fiery-red light. He is much more brilliant at some oppositions than at others, for reasons already explained (236), but always shines brighter than an ordinary star of the first magnitude.

241. _Telescopic Appearance of Mars._--When viewed with a good telescope (see Plate IV.), Mars is seen to be covered with dusky, dull-red patches, which are supposed to be continents, like those of our own globe. Other portions, of a greenish hue, are believed to be tracts of water. The ruddy color, which overpowers the green, and makes the whole planet seem red to the naked eye, was believed by Sir J. Herschel to be due to an ochrey tinge in the general soil, like that of the red sandstone districts on the earth. In a telescope, Mars appears less red, and the higher the power the less the intensity of the color. The disk, when well seen, is mapped out in a way which gives at once the impression of land and water. The bright part is red inclining to orange, sometimes dotted with brown and greenish points. The darker spaces, which vary greatly in depth of tone, are of a dull gray-green, having the aspect of a fluid which absorbs the solar rays. The proportion of land to water on the earth appears to be reversed on Mars. On the earth every continent is an island; on Mars all seas are lakes. Long, narrow straits are more common than on the earth; and wide expanses of water, like our Atlantic Ocean, are rare. (See Fig. 266.)

Fig. 267 represents a chart of the surface of Mars, which has been constructed from careful telescopic observation. The outlines, as seen in the telescope, are, however, much less distinct than they are represented here; and it is by no means certain that the light and dark portions are bodies of land and water.

In the vicinity of the poles brilliant white spots may be noticed, which are considered by many astronomers to be masses of snow. This conjecture is favored by the fact that they appear to diminish under the sun's influence at the beginning of the Martial summer, and to increase again on the approach of winter.

242. _Rotation of Mars._--On watching Mars with a telescope, the spots on the disk are found to move (as shown in Fig. 268) in a manner which indicates that the planet rotates in about twenty-four hours on an axis inclined about twenty-eight degrees from a perpendicular to the plane of its orbit. The inclination of the axis is shown in Fig. 269. It is evident from the figure that the variation in the length of day and night, and the change of seasons, are about the same on Mars as on the earth. The changes will, of course, be somewhat greater, and the seasons will be about twice as long.

243. _The Satellites of Mars._--In 1877 Professor Hall of the Washington Observatory discovered that Mars is accompanied by two small moons, whose orbits are shown in Fig. 270. The inner satellite has been named _Phobos_, and the outer one _Deimos_. It is estimated that the diameter of the outer moon is from five to ten miles, and that of the inner one from ten to forty miles.

Phobos is remarkable for its nearness to the planet and the rapidity of its revolution, which is performed in seven hours thirty-eight minutes. Its distance from the centre of the planet is about six thousand miles, and from the surface less than four thousand. Astronomers on Mars, with telescopes and eyes like ours, could readily find out whether this satellite is inhabited, the distance being less than one-sixtieth of that of our moon.

It will be seen that Phobos makes about three revolutions to one rotation of the planet. It will, of course, rise in the west; though the sun, the stars, and the other satellite rise in the east. Deimos makes a complete revolution in about thirty hours.

III. THE ASTEROIDS.

244. _Bode's Law of Planetary Distances._--There is a very remarkable law connecting the distances of the planets from the sun, which is generally known by the name of _Bode's Law_. Attention was drawn to it in 1778 by the astronomer Bode, but he was not really its author.

To express this law we write the following series of numbers:--

0, 3, 6, 12, 24, 48, 96;

each number, with the exception of the first, being double the one which precedes it. If we add 4 to each of these numbers, the series becomes--

4, 7, 10, 16, 28, 52, 100;

which series was known to Kepler. These numbers, with the exception of 28, are sensibly proportional to the distances of the principal planets from the sun, the actual distances being as follows:--

Mercury. Venus. Earth. Mars. ---- Jupiter. Saturn.

3·9 7·2 10 15·2 52·9 95·4

245. _The First Discovery of the Asteroids._--The great gap between Mars and Jupiter led astronomers, from the time of Kepler, to suspect the existence of an unknown planet in this region; but no such planet was discovered till the beginning of the present century. _Ceres_ was discovered Jan. 1, 1801, _Pallas_ in 1802, _Juno_ in 1804, and _Vesta_ in 1807. Then followed a long interval of thirty-eight years before _Astræa_, the fifth of these minor planets, was discovered in 1845.

246. _Olbers's Hypothesis._--After the discovery of Pallas, Olbers suggested his celebrated hypothesis, that the two bodies might be fragments of a single planet which had been shattered by some explosion. If such were the case, the orbits of all the fragments would at first intersect each other at the point where the explosion occurred. He therefore thought it likely that other fragments would be found, especially if a search were kept up near the intersection of the orbits of Ceres and Pallas.

Professor Newcomb makes the following observations concerning this hypothesis:--

"The question whether these bodies could ever have formed a single one has now become one of cosmogony rather than of astronomy. If a planet were shattered, the orbit of each fragment would at first pass through the point at which the explosion occurred, however widely they might be separated through the rest of their course; but, owing to the secular changes produced by the attractions of the other planets, this coincidence would not continue. The orbits would slowly move away, and after the lapse of a few thousand years no trace of a common intersection would be seen. It is therefore curious that Olbers and his contemporaries should have expected to find such a region of intersection, as it implied that the explosion had occurred within a few thousand years. The fact that the required conditions were not fulfilled was no argument against the hypothesis, because the explosion might have occurred millions of years ago; and in the mean time the perihelion and node of each orbit would have made many entire revolutions, so that the orbits would have been completely mixed up.... A different explanation of the group is given by the nebular hypothesis; so that Olbers's hypothesis is no longer considered by astronomers."

247. _Later Discoveries of Asteroids._--Since 1845 over two hundred asteroids have been discovered. All these are so small, that it requires a very good telescope to see them; and even in very powerful telescopes they appear as mere points of light, which can be distinguished from the stars only by their motions.

To facilitate the discovery of these bodies, very accurate maps have been constructed, including all the stars down to the thirteenth magnitude in the neighborhood of the ecliptic. A reduced copy of one of these maps is shown in Fig. 271.

Furnished with a map of this kind, and with a telescope powerful enough to show all the stars marked on it, the observer who is searching for these small planets will place in the field of view of his telescope six spider-lines at right angles to each other, and at equal distances apart, in such a manner that several small squares will be formed, embracing just as much of the heavens as do those shown in the map. He will then direct his telescope to the region of the sky he wishes to examine, represented by the map, so as to be able to compare successively each square with the corresponding portion of the sky. Fig. 272 shows at the right hand the squares in the telescopic field of view, and at the left hand the corresponding squares of the map.

He can then assure himself if the numbers and positions of the stars mapped, and of the stars observed, are identical. If he observes in the field of view a luminous point which is not marked in the map, it is evident that either the new body is a star of variable brightness which was not visible at the time the map was made, or it is a planet, or perhaps a comet. If the new body remains fixed at the same point, it is the former; but, if it changes its position with regard to the neighboring stars, it is the latter. The motion is generally so sensible, that in the course of one evening the change of position may be detected; and it can soon be determined, by the direction and rate of the motion, whether the body is a planet or a comet.

IV. OUTER GROUP OF PLANETS.

Jupiter.

248. _Orbit of Jupiter._--The orbit of Jupiter is inclined only a little over one degree to the ecliptic; and its eccentricity is only about half of that of Mars, being less than one-twentieth. The mean distance of Jupiter from the sun is about four hundred and eighty million miles; but, owing to the eccentricity of his orbit, his actual distance from the sun ranges from four hundred and fifty-seven to five hundred and three million miles.

249. _Distance of Jupiter from the Earth._--When Jupiter is in opposition, his mean distance from the earth is four hundred and eighty million miles _minus_ ninety-two million miles, or three hundred and eighty-eight million miles, and, when he is in conjunction, four hundred and eighty million miles _plus_ ninety-two million miles, or five hundred and seventy-two million miles. It will be seen that he is less than twice as far off in conjunction as in opposition, and that the ratio of his greatest to his least distance is very much less than in the case of Venus and Mars. This is owing to his very much greater distance from the sun. Owing to the eccentricities of the orbits of the earth and of Jupiter, the greatest and least distances of Jupiter from the earth vary somewhat from year to year.

250. _The Brightness and Apparent Size of Jupiter._--The apparent diameter of Jupiter varies from about fifty seconds to about thirty seconds. His apparent size at his extreme and mean distances from the earth is shown in Fig. 273.

Jupiter shines with a brilliant white light, which exceeds that of every other planet except Venus. The planet is, of course, brightest when near opposition.

251. _The Volume and Density of Jupiter._--Jupiter is the "giant planet" of our system, his mass largely exceeding that of all the other planets combined. His mean diameter is about eighty-five thousand miles; but the equatorial exceeds the polar diameter by five thousand miles. In volume he exceeds our earth about thirteen hundred times, but in mass only about two hundred and thirteen times. His specific gravity is, therefore, far less than that of the earth, and even less than that of water. The comparative size of Jupiter and the earth is shown in Fig. 274.

252. _The Sidereal and Synodical Periods of Jupiter._--It takes Jupiter nearly twelve years to make a _sidereal_ revolution, or a complete revolution around the sun, his orbital motion being at the rate of about eight miles a second. His _synodical_ period, or the time of his passage from opposition to opposition again, is three hundred and ninety-eight days.

253. _The Telescopic Aspect of Jupiter._--There are no really permanent markings on the disk of Jupiter; but his surface presents a very diversified appearance. The earlier telescopic observers descried dark belts across it, one north of the equator, and the other south of it. With the increase of telescopic power, it was seen that these bands were of a more complex structure than had been supposed, and consisted of stratified, cloud-like appearances, varying greatly in form and number. These change so rapidly, that the face of the planet rarely presents the same appearance on two successive nights. They are most strongly marked at some distance on each side of the planet's equator, and thus appear as two belts under a low magnifying power.

Both the outlines of the belts, and the color of portions of the planet, are subject to considerable changes. The equatorial regions, and the spaces between the belts generally, are often of a rosy tinge. This color is sometimes strongly marked, while at other times hardly a trace of it can be seen. A general telescopic view of Jupiter is given in Plate V.

254. _The Physical Constitution of Jupiter._--From the changeability of the belts, and of nearly all the visible features of Jupiter, it is clear that what we see on that planet is not the solid nucleus, but cloud-like formations, which cover the entire surface to a great depth. The planet appears to be covered with a deep and dense atmosphere, filled with thick masses of clouds and vapor. Until recently this cloud-laden atmosphere was supposed to be somewhat like that of our globe; but at present the physical constitution of Jupiter is believed to resemble that of the sun rather than that of the earth. Like the sun, he is brighter in the centre than near the edges, as is shown in the transits of the satellites over his disk. When the satellite first enters on the disk, it commonly seems like a bright spot on a dark background; but, as it approaches the centre, it appears like a dark spot on the bright surface of the planet. The centre is probably two or three times brighter than the edges. This may be, as in the case of the sun, because the light near the edge passes through a greater depth of atmosphere, and is diminished by absorption.

It has also been suspected that Jupiter shines partly by his own light, and not wholly by reflected sunlight. The planet cannot, however, emit any great amount of light; for, if it did, the satellites would shine by this light when they are in the shadow of the planet, whereas they totally disappear. It is possible that the brighter portions of the surface are from time to time slightly self-luminous.

Again: the interior of Jupiter seems to be the seat of an activity so enormous that it can be ascribed only to intense heat. Rapid movements are always occurring on his surface, often changing its aspect in a few hours. It is therefore probable that Jupiter is not yet covered by a solid crust, and that the fiery interior, whether liquid or gaseous, is surrounded by the dense vapors which cease to be luminous on rising into the higher and cooler regions of the atmosphere. Figs. 275 and 276 show the disk of Jupiter as it appeared in December, 1881.

255. _Rotation of Jupiter_.--Spots are sometimes visible which are much more permanent than the ordinary markings on the belts. The most remarkable of these is "the great red spot," which was first observed in July, 1878, and is still to be seen in February, 1882. It is shown just above the centre of the disk in Fig. 275. By watching these spots from day to day, the time of Jupiter's axial rotation has been found to be about nine hours and fifty minutes.

The axis of Jupiter deviates but slightly from a perpendicular to the plane of its orbit, as is shown in Fig. 277.

THE SATELLITES OF JUPITER.

256. _Jupiter's Four Moons._--Jupiter is accompanied by four moons, as shown in Fig. 278. The diameters of these moons range from about twenty-two hundred to thirty-seven hundred miles. The second from the planet is the smallest, and the third the largest. The smallest is about the size of our moon; the largest considerably exceeds Mercury, and almost rivals Mars, in bulk. The sizes of these moons, compared with those of the earth and its moon, are shown in Fig. 279.

The names of these satellites, in the order of their distance from the planet, are _Io_, _Europa_, _Ganymede_, and _Callisto_. Their times of revolution range from about a day and three-fourths up to about sixteen days and a half. Their orbits are shown in Fig. 280.

257. _The Variability of Jupiter's Satellites._--Remarkable variations in the light of these moons have led to the supposition that violent changes are taking place on their surfaces. It was formerly believed, that, like our moon, they always present the same face to the planet, and that the changes in their brilliancy are due to differences in the luminosity of parts of their surface which are successively turned towards us during a revolution; but careful measurements of their light show that this hypothesis does not account for the changes, which are sometimes very sudden. The satellites are too distant for examination of their surfaces with the telescope: hence it is impossible to give any certain explanation of these phenomena.

258. _Eclipses of Jupiter's Satellites._--Jupiter, like the earth, casts a shadow away from the sun, as shown in Fig. 281; and, whenever one of his moons passes into this shadow, it becomes eclipsed. On the other hand, whenever one of the moons throws its shadow on Jupiter, the sun is eclipsed to that part of the planet which lies within the shadow.

To the inhabitants of Jupiter (if there are any, and if they can see through the clouds) these eclipses must be very familiar affairs; for in consequence of the small inclinations of the orbits of the satellites to the planet's equator, and the small inclination of the latter to the plane of Jupiter's orbit, all the satellites, except the most distant one, are eclipsed in every revolution. A spectator on Jupiter might therefore witness during the planetary year forty-five hundred eclipses of the moons, and about the same number of the sun.

259. _Transits of Jupiter's Satellites._--Whenever one of Jupiter's moons passes in front of the planet, it is said to make a _transit_ across his disk. When a moon is making a transit, it presents its bright hemisphere towards the earth, as will be seen from Fig. 282: hence it is usually seen as a bright spot on the planet's disk; though sometimes, on the brighter central portions of the disk, it appears dark.

It will be seen from Fig. 282 that the shadow of a moon does not fall upon the part of the planet's disk that is covered by the moon: hence we may observe the transit of both the moon and its shadow. The shadow appears as a small black spot, which will precede or follow the moon according to the position of the earth in its orbit. Fig. 283 shows two moons of Jupiter in transit.

260. _Occultations of Jupiter's Satellites._--The eclipse of a moon of Jupiter must be carefully distinguished from the _occultation_ of a moon by the planet. In the case of an eclipse, the moon ceases to be visible, because the mass of Jupiter is interposed between the sun and the moon, which ceases to be luminous, because the sun's light is cut off; but, in the case of an occupation, the moon gets into such a position that the body of Jupiter is interposed between it and the earth, thus rendering the moon invisible to us. The third satellite, _m''_ (Fig. 282), is invisible from the earth _E_, having become _occulted_ when it passed behind the planet's disk; but it will not be _eclipsed_ until it passes into the shadow of Jupiter.

261. _Jupiter without Satellites._--It occasionally happens that every one of Jupiter's satellites will disappear at the same time, either by being eclipsed or occulted, or by being in transit. In this event, Jupiter will appear without satellites. This occurred on the 21st of August, 1867. The position of Jupiter's satellites at this time is shown in Fig. 284.

Saturn.

THE PLANET AND HIS MOONS.

262. _The Orbit of Saturn._--The orbit of Saturn is rather more eccentric than that of Jupiter, its eccentricity being somewhat more than one-twentieth. Its inclination to the ecliptic is about two degrees and a half. The mean distance of Saturn from the sun is about eight hundred and eighty million miles. It is about a hundred million miles nearer the sun at perihelion than at aphelion.

263. _Distance of Saturn from the Earth._--The mean distance of Saturn from the earth at opposition is eight hundred and eighty million miles _minus_ ninety-two million miles, or seven hundred and eighty-eight million; and at conjunction, eight hundred and eighty million miles _plus_ ninety-two million, or nine hundred and seventy-two million. Owing to the eccentricity of the orbit of Saturn, his distance from the earth at opposition and at conjunction varies by about a hundred million miles at different times; but he is so immensely far away, that this is only a small fraction of his mean distance.

264. _Apparent Size and Brightness of Saturn._--The apparent diameter of Saturn varies from about twenty seconds to about fourteen seconds. His apparent size at his extreme and mean distances from the earth is shown in Fig. 285.

The planet generally shines with the brilliancy of a moderate first-magnitude star, and with a dingy, reddish light, as if seen through a smoky atmosphere.

265. _Volume and Density of Saturn._--The real diameter of Saturn is about seventy thousand miles, and its volume over seven hundred times that of the earth. The comparative size of the earth and Saturn is shown in Fig. 286. This planet is a little more than half as dense as Jupiter.

266. _The Sidereal and Synodical Periods of Saturn._--Saturn makes a complete revolution round the sun in a period of about twenty-nine years and a half, moving in his orbit at the rate of about six miles a second. The planet passes from opposition to opposition again in a period of three hundred and seventy-eight days, or thirteen days over a year.