ASTRONOMY 109 



omy for the law of gravitation itself, and now astronomers 

 tell us that this law as we usually state it may not be strictly 

 correct. In connection with celestial mechanics should be 

 mentioned one incident which forms one of the more sensa- 

 tional happenings in the history of science. Previous to 

 1845 the planet Neptune was unknown. However, another 

 planet, Uranus, showed certain irregularities in its motion 

 which seemed then unaccountable. About this time two men, 

 Adams and Leverrier, unknown to each other, showed by very 

 careful reckoning that these irregularities could be explained 

 by the attraction of another planet, and even calculated its 

 position with such accuracy that it was readily found with a 

 telescope. This has always been regarded as one of the great- 

 est triumphs of applied mathematics. 



Other branches of physics besides mechanics are made 

 use of in astronomy. The velocity of light was first deter- 

 mined by astronomical observations, and though we can now 

 measure it more accurately by other methods, the value so 

 found is used to find the distance from the earth to the sun, 

 which forms the baseline for all other measurements of length 

 in astronomy. 



In the lecture on physics you were .told that an English 

 physicist, Clerk Maxwell, had found from theoretical consid- 

 erations that a beam of light must exert a slight pressure upon 

 any object upon which it falls, and that such an effect had 

 afterwards been shown to exist, by direct experiment. This 

 fact of light-pressure gives us an explanation of the sun's 

 corona, the tails of comets, and several other phenomena. 



The study of the radiation from heated bodies allows us 

 to guess at the temperature of our sun and stars, and the 

 recent study of electrons, of which the last two lecturers told 

 you so much, suggests explanations of the aurora borealis, the 

 effect of the sun upon the compass-needle, and many of the 

 phenomena of sun-spots and nebulae. 



While celestial mechanics in one form or another is one 



