376 A Short History of Astronomy [CH. xm 



it. This is again an irreversible tendency for which we 

 know of no compensation. 



In fact, from the point of view which Lagrange and 

 Laplace occupied, the solar system appeared like a clock 

 which, though not going quite regularly, but occasionally 

 gaining and occasionally losing, nevertheless required no 

 winding up ; whereas modern research emphasises the 

 analogy to a clock which after all is running down, though 

 at an excessively slow rate. Modern study of the sun's 

 heat ( 319) also indicates an irreversible tendency towards 

 the " running down " of the solar system in another way. 



294. Our account of modern descriptive astronomy may 

 conveniently begin with planetary discoveries. 



The first day of the i9th century was marked by the 

 discovery of a new planet, known as Ceres. It was seen 

 by Giuseppe Piazzi (1746-1826) as a strange star in a 

 region of the sky which he was engaged in mapping, and 

 soon recognised by its motion as a planet. Its orbit 

 first calculated by Gauss ( 276) shewed it to belong 

 to the space between Mars and Jupiter, which had been 

 noted since the time of Kepler as abnormally large. That 

 a planet should be found in this region was therefore 

 no great surprise ; but the discovery by Heinrich Olbers 

 (1758-1840), scarcely a year later (March 1802), of a second 

 body (Pallas\ revolving at nearly the same distance from 

 the sun, was wholly unexpected, and revealed an entirely 

 new planetary arrangement. It was an obvious con- 

 jecture that if there was room for two planets there was 

 room for more, and two fresh discoveries (Juno in 1804, 

 Vesta in 1807) soon followed. 



The new bodies were very much smaller than any of 

 the other planets, and, so far from readily shewing a 

 planetary disc like their neighbours Mars and Jupiter, 

 were barely distinguishable in appearance from fixed stars, 

 except in the most powerful telescopes of the time ; hence 

 the name asteroid (suggested by William Herschel) or 

 minor planet has been generally employed to distinguish 

 them from the other planets. Herschel attempted to 

 measure their size, and estimated the diameter of the largest 

 at under 200 miles (that of Mercury, the smallest of the 

 ordinary planets, being 3000), but the problem was in reality 



