Part 29

The mean distance of Mars from the earth when in conjunction is a hundred and forty million miles plus ninety-two million miles, or two hundred and thirty-two million miles. It will therefore be seen that Mars is nearly five times as far off at conjunction as at opposition.

237. The Apparent Size of Mars.--Owing to the varying distance of Mars from the earth, the apparent size of the planet varies almost as much as that of Venus. Fig. 264 shows the apparent size of Mars at its extreme and mean distances from the earth. The apparent diameter varies from about four seconds to about thirty seconds.

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.