MINOR PLANET. The minor planets or asteroids are a vast host of very small planets that revolve round the sun in orbits that nearly all lie between those of Mars and Jupiter, though a few of them transgress these limits. The existence of a planet between Mars and Jupiter was suspected before any of them were found, on the ground of an empirical law of planetary distances which was first put forward by Titius, a contemporary of Kepler, though it attracted more notice when Bode restated it in 5772. The law assigned the following numbers as represent ing the distances very closely: Mercury 4, Venus 7, Earth JO, Mars 16, (blank) 28, Jupiter 52, Saturn zoo, (next planet) 196. It will be seen that, except in the first case, each interval is double the preceding one.
When in 1781 Sir W. Herschel discovered Uranus, which fitted exactly with the next term in the series after Saturn, conviction was strengthened of the existence of a planet in the gap, and a society of 24 astronomers, with Baron de Zach at its head, was formed to devote itself to the search. Giuseppe Piazzi, director of the Palermo Observatory, was not a member of this society, but on Jan. 1, 18oi, while examining a region in Taurus, he observed a small star which he had not seen in the place before ; he soon found that it was moving, and followed it up till Feb. 1 5. In the interval it ceased to retrograde, and commenced to advance. In the following autumn difficulty was experienced in finding the body again ; this served as an incentive to the mathematician Karl Friedrich Gauss to improve the existing methods of computing orbits from a few observed positions. As a result of his calcula tions Heinrich W. M. Olbers of Bremen recovered Piazzi's planet just a year after its discovery. (The three astronomers associated with the finding of the first asteroid have been commemorated by naming the planets numbered I,000, i,00i, 1,002 Piazzia, Gaussia and Olbersia.) The new planet was named Ceres; its distance agreed exactly with the value predicted by the Titius-Bode law, but it was very small (modern measures give a diameter of 480 miles, about one-fifth of the moon's), and its orbit was inclined to the ecliptic at the large angle of 10°37'. Perhaps these circumstances led Olbers to suspect that it was one of a group of small planets, for he continued to sweep the sky after he had recovered it, and three months later he found Pallas, whose distance from the sun proved to be almost the same as that of Ceres; its orbit was inclined at the very large angle of and the eccentricity was also large. A third and fourth member, Juno and Vesta, were
added to the family within six years by Harding and Olbers respectively. Vesta is the brightest of the whole family, sometimes attaining visibility to the naked eye ; hut its diameter is only half that of Ceres. It was natural that the discovery of these four little planets, revolving in closely adjacent orbits, should suggest the idea that a larger orb had been rent asunder by an explosion. This idea held the field for many years, and indeed has been revived of late in a modified form. It seems to have been assumed that four fragments completed the set, and search was abandoned till 1830; it was then renewed by M. Hencke of Driessen, who after fifteen years' search found Astraea, and commenced a chain of discoveries which has continued without intermission. By 1890 about 30o small planets had been found, all by visual search at the telescope, a very laborious method, necessitating the chart ing or memorizing of numbers of faint stars and searching for strangers among them. At the time Prof. Max Wolf of Konig stuhl, Heidelberg, introduced the photographic method of search, which had been suggested earlier by Dr. Isaac Roberts. An equatorial telescope with a photographic plate in the focus was made to follow the stars, which were registered as discs, while a planet appeared as a short trail owing to its motion during the exposure. This method produced a great acceleration in the rate of discovery, and nearly two thousand announcements of dis coveries were made between 1890 and 1927. Not all of these have been permanently numbered. When the number of planets grew large there was found to be a danger of mistaking a previously discovered planet for a new one, and it was decided to postpone the numbering of a planet till five good observations, extending over some weeks, were available, thus enabling a reliable orbit to be deduced. 1,055 planets have now (1927) been numbered, but several hundreds more are known to exist, and approximate orbits have been found for some of them.