Olbers's discovery, on the 28th of March, 1802, of a second planet, Pallas, revolving around the sun at the same mean distance as Ceres, presented the question under a new aspect. Gauss's calculation showed that Ceres and Pallas might, in time, come to pass very near each other on the line of intersection described as A B of the planes of their several orbits. Olbers was thus led to think that the two little bodies might be fragments of a greater planet which had been broken up by an internal commotion. If this were the case, there would probably be other fragments, the orbits of which would pass the line A B, so that by watching the two points A and B, where this line strikes the celestial sphere, chances might occur of seeing the fragments of the primitive planet pass. Eventually Harding found Juno near A in 1804, and Olbers discovered Vesta near B in 1807.
Further researches, carried on by Olbers till 1816, brought no result; and it was not till 1845 that a fifth body, still smaller than the other four, was discovered by Encke. After this time, discoveries became frequent and regular, till now 299 are known.[1] But the size of the new stars keeps getting smaller: the first four were between the sixth and the eighth magnitudes; the two discovered by Encke were of the ninth; and those which are now discovered from time to time seldom exceed the thirteenth magnitude. William Herschel, in consideration of the small size of these bodies, and hardly regarding them as sufficient to fill the place of a planet, thought it more fitting to call them asteroids than planets.
A survey of the orbits of the asteroids as a whole will help us to gain clearer ideas respecting them, and may bring out a few simple relations that will cast some light on the origin of the bodies. The supposition of Olbers is not sustained. Prof. Newcomb, having studied the orbits of the first forty asteroids, found that, as they move to-day, they are far from passing in the same line. Even the hypothesis that the geometrical condition supposed may once have existed, but has been changed by the perturbations caused by the attractions of the other planets, is contradicted by the calculations.
But this hypothesis, though it must be abandoned, has the credit of having provoked the discovery of Juno and Vesta, and of having suggested Lagrange's theory of the origin of comets, to which M. Faye has added a number of curious speculations, and by the aid of which we may, perhaps, some day find an explanation of the origin of meteors. The smallest mean distance from the sun of the known asteroids, 149, is 2·13 (times the mean distance of the earth); the greatest, that of 279, 4·26; the corre-
- ↑ Since increased to 308.
