372 A Short History of Astronomy [CH. xm. 



ing month Dr. Galle of the Berlin Observatory received 

 from Leverrier a request to search for it, and on the same 

 evening found close to the position given by Leverrier a 

 strange body shewing a small planetary disc, which was 

 soon recognised as a new planet, known now as Neptune. 



It may be worth while noticing that the error in the 

 motion of Uranus which led to this remarkable discovery 

 never exceeded 2', a quantity imperceptible to the ordinary 

 eye ; so that if two stars were side by side in the sky, one 

 in the true position of Uranus and one in the calculated 

 position as given by Bouvard's tables, an observer of 

 ordinary eyesight would see one star only. 



290. The lunar tables of Hansen and Professor Newcomb, 

 and the planetary and solar tables of Leverrier, Pro- 

 fessor Newcomb, and Dr. Hill, represent the motions of 

 the bodies dealt with much more accurately than the corre- 

 sponding tables based on Laplace's work, just as these were 

 in turn much more accurate than those of Euler, Clairaut, 

 and Halley. But the agreement between theory and obser- 

 vation is by no means perfect, and the discrepancies are in 

 many cases greater than can be explained as being due to 

 the necessary imperfections in our observations. 



The two most striking cases are perhaps those of Mercury 

 and the moon. Leverrier's explanation of the irregularities 

 of the former ( 288) has never been fully justified or 

 generally accepted ; and the position of the moon as given 

 in the Nautical Almanac and in similar publications is 

 calculated by means of certain corrections to Hansen's 

 tables which were deduced by Pfofessor Newcomb from 

 observation and have no justification in the theory of 

 gravitation. 



291. The calculation of the paths of comets has be- 

 come of some importance during this century owing to 

 the discovery of a number of comets revolving round the 

 sun in comparatively short periods. Halley's comet 

 (chapter XL, 231) reappeared duly in 1835, passing through 

 its perihelion within a few days of the times predicted by 

 three independent calculators ; and it may be confidently 

 expected again about 1910. Four other comets are now 

 known which, like Halley's, revolve in elongated elliptic 

 orbits, completing a revolution in between 70 and 80 years ; 



