The astronomers of that day placed much reliance on what is known as Bode's law—not a law at all, but a mere arithmetical succession of numbers which represented very well the relative distances of all the planets from the sun. And the distance of the newly found Uranus fitted in so well with this law that the utter absence of a planet in the gap between Mars and Jupiter became very strongly marked.
Quite by accident a discovery of one of the guessed-at small planetary bodies was made, on January 1, 1801, in Palermo, Sicily, by Piazzi, who was regularly occupied in making an extensive catalogue of the stars. His observations soon showed that the new object he had seen could not be a fixed star, because it moved from night to night among the stars. He concluded that it was a planet, and named it Ceres (1), for the tutelary goddess of Sicily.
Other astronomers kept up the search, and another companion planet, Pallas (2) was found in the following year. Juno (3) was found in 1804, and Vesta (4), the largest and brightest of all the minor planets, in 1807. Vesta is sometimes bright enough when nearest the earth to be seen with the naked eye; but it was the last of the brighter ones, and no more discoveries of the kind were made till the fifth was found in 1845. Since then discoveries have been made in great abundance, more and more with every year till the number of little planets at present known is very near 1,000.
The early asteroid hunters found the search rather tedious, and the labor increased as it became necessary to examine the increasing thousands of fainter and fainter stars that must be observed in order to detect the undiscovered planets, which naturally grow fainter and fainter as the chase is prolonged. First a chart of the ecliptic sky had to be prepared containing all the stars that the telescope employed in the search would show. Some of the most detailed charts of the sky in existence were prepared in connection with this work, particularly by the late Dr. Peters of Hamilton College. Once such charts are complete, they are compared with the sky, night after night when the moon is absent. Thousands upon thousands of tedious hours are spent in this comparison, with no result whatever except that chart and sky are found to correspond exactly.
But now and then the planet hunter is rewarded by finding a new object in the sky that does not appear on his chart. Almost certainly this is a small planet, and only a few night's observation will be necessary to enable the discoverer to find out approximately the orbit it is traveling in, and whether it is out-and-out a new planet or only one that had been previously recognized, and then lost track of.
Nearly all the minor planets so far found have had names assigned to them principally legendary and mythological, and a nearly complete catalogue of them, containing the elements of their orbits (that is, all the mathematical data that tell us about their distance from the sun and the circumstances of their motion around him) is published each year in the "Annuaire du Bureau des Longitudes" at Paris. But these little planets require a great deal of care and attention, for some astronomers must accurately observe them every few years, and other astronomers must conduct intricate mathematical computations based on these observations; otherwise they get lost and have to be discovered all over again. Professor Watson, of the University of Michigan and later of the University of Wisconsin, endowed the 22 asteroids of his own discovery, leaving to the National Academy of Sciences a fund for prosecuting this work perpetually, and Leuschner is now ably conducting it.
Jupiter, Largest of the Planets. The irregular belts change their mutual relation and shapes because they do not represent land, but are part of the atmosphere. (Photo, Yerkes Observatory.)
