204 THE GREEK ASTRONOMY. 



studied with so much advantage as in the history of 

 astronomy. 



In truth, however, Hipparchus did not leave to 

 his successors the task of pursuing into detail those 

 views of the heavens to which his discoveries led 

 him. He examined with scrupulous care almost 

 every part of the subject. We must briefly mention 

 some of the principal points which were thus set- 

 tled by him. 



The verification of the laws of the changes 

 which he assigned to the skies, implied that the 

 condition of the heavens was constant, except so far 

 as it was affected by those changes. Thus, the doc- 

 trine that the changes of position of the stars were 

 rightly represented by the precession of the equi- 

 noxes, supposed that the stars were fixed with 

 regard to each other; and the doctrine that the 

 unequal number of days, in certain subdivisions of 

 months and years, was adequately explained by the 

 theory of epicycles, assumed that years and days 

 were always of constant lengths. But Hipparchus 

 was not content with assuming these bases of his 

 theory, he endeavoured to prove them. 



1. Fixity of the Stars. The question neces- 

 sarily arose after the discovery of the precession, 

 even if such a question had never suggested itself 

 before, whether the stars which were called fixed, 

 and to which the motions of the other luminaries 

 are referred, do really retain constantly the same 

 relative position. In order to determine this funda- 



