200 THE GREEK ASTRONOMY. 



senting them, appear also in other discoveries of 

 his, which we must not pass unnoticed. The Pre- 

 cession of the Equinoxes, in particular, is one of the 

 most important of these discoveries. 



The circumstance here brought into notice was 

 a Change of Longitude of the Fixed Stars. The 

 longitudes of the heavenly bodies, being measured 

 from the point where the sun's annual path cuts the 

 equator, will change if that path changes. Whether 

 this happens, however, is not very easy to decide ; 

 for the sun's path among the stars is made out, not 

 by merely looking at the heavens, but by a series 

 of inferences from other observable facts. Hippar- 

 chus used for this purpose eclipses of the moon ; for 

 these, being exactly opposite to the sun, afford data 

 in marking out his path. By comparing the eclipses 

 of his own time with those observed at an earlier 

 period by Timocharis, he found that the bright star, 

 Spica Virginis, was six degrees behind the equi- 

 noctial point in his own time, and had been eight 

 degrees behind the same point at an earlier epoch. 

 The suspicion was thus suggested, that the longi- 

 tudes of all the stars increase perpetually ; but Hip- 

 parchus had too truly philosophical a spirit to take 

 this for granted. He examined the places of Re- 

 gulus, and those of other stars, as he had done those 

 of Spica; and he found, in all these instances, a 

 change of place which could be explained by a cer- 

 tain alteration of position in the circles to which 

 the stars are referred, which alteration is described 

 as the Precession of the Equinoxes. 



