| THE PLANET EROS. |
By Professor SOLON I. BAILEY,
HARVARD COLLEGE OBSERVATORY.
EROS is the name of a small planet discovered in 1898, by Witt, of Berlin. It does not appear to be altogether certain that it really belongs to the group of minor planets, usually known as planetoids or asteroids. With the exception of Eros, all known asteroids move in orbits whose mean distances are greater than that of Mars and less than that of Jupiter. The mean distance from the sun of Mars is 141 million miles, and that of Jupiter is 483 million miles, while the distances of the asteroids vary in round numbers between 200 and 400 million miles. The mean distance of Eros, however, is only 135 million miles, which is less than that of Mars. In spite of this very important difference, Eros has been placed among the great band of asteroids, among whom he numbers 433. To belong to the celestial 400 is perhaps more of misfortune than of honor, for the number of this plebeian band has already waxed so great that they have become a care which threatens in the future to balance the benefits which they bring to astronomy. Nevertheless, the history of this numerous family is sufficiently full of interest, and throws light upon the way in which we should regard them.
In 1772, Bode announced the so-called law which bears his name. The law may be stated as follows: If to a series of 4's, beginning at the second, the numbers 3, 6, 12, 24, etc., be added, the resulting numbers divided by 10 will approximately express the distance of the planets from the sun in terms of the distance of our earth taken as unity. The law gave fairly well the distances of all the planets known at that time, except that it called for a planet between Mars and Jupiter, where nothing was then known to exist. When, a few years later, in 1784, Uranus was discovered and was found to conform closely to the law, the impression was deepened that the missing member of the solar system must somehow be supplied or explained, and an association of astronomers was formed to hunt for it. At that time the discovery of a small body, such as one of the asteroids, was no easy matter, and the honor of finding the first did not fall to one of the associates, but to Piazzi, a Sicilian astronomer, who discovered it while making a star catalogue. It was perhaps fitting that a century which was to be signalized by the discovery of some 450 new but small worlds, where one had been sought, should be properly
