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.