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.