Asteroids, a ring of small planets travelling between the orbits of Mars and Jupiter. It had long been noticed that no empirical law of planetary distances would give an account of the wide disparity between the distance separating the orbits of the earth and Mars and that which separates the paths of Mars and Jupiter. When Sir W. HerscheFs discovery of Uranus in 1781 had confirmed Bode's empirical law, astronomers were led to search for a planet travelling in the orbit which, according to that law, should lie between the paths of Mars and Jupiter. On Jan. 1, 1801, such a planet was discovered by Piazzi, who called it Ceres. In March, 1802, while looking for the new planet, Olbers discovered another, travelling at about the same distance from the sun. He called it Pallas. Two others discovered before 1808 were called Juno and Vesta. In 1845 Hencke of Prussia discovered a fifth. Since then the progress of discovery has scarcely been interrupted by a single barren year. Luther in Germany, Goldschmidt in France, Watson in America, Hind in England, and De Gasparis in Italy were until 1873 the most successful asteroid seekers.
Recently Prof. Peters of the Litchfield observatory, Clinton, N. Y., has shared their honors, having thus far discovered more asteroids than any other astromomer save Luther. He discovered three new asteroids in July and August, 1872, and two more in February, 1873, raising the known number to 130. - Olbers endeavored to explain the existence of the zone of asteroids by the theory that a planet which had once travelled between the paths of Mars and Jupiter had exploded, and that the asteroids are its fragments. But Prof. Newcomb has shown, by an elaborate investigation of the asteroidal motions, that "although there are some peculiarities which might favor Olbers's hypothesis, there are a far greater number of cases which undoubtedly negative the assumption." Prof. Ivirkwood has shown that when the mean distances of the asteroids are arranged in order, certain gaps can be recognized; that in fact "there are no asteroids having mean distances lying near certain definite values." He shows how these gaps by their position indicate the probability that the asteroidal zone was formed from scattered cosmical matter travelling around the sun under the perturbing influence of the planet Jupiter. Leverrier, from an analysis of the motions of Mars, has shown that the combined mass of all the asteroids probably falls far short of one fourth of the earth's mass.