ri6 PRECESSION OF THE EQUINOXES. 



Only, this movement of the starry sphere in longitude, or 

 parallel to the plane of the Ecliptic, is extremely slow, com- 

 pared with the movement of the same sphere in right ascen- 

 sion, or parallel to the Equator of the world. While the former 

 traverses in twenty-four hours the 360 of the circle, the latter 

 occupies (in round numbers) 25,000 years. 



Who was the first discoverer of the slow movement of the 

 heavens ? Hipparchus. This great astronomer, on compar- 

 ing his own observations with the more ancient ones of 

 Aristillus and Timocharis, succeeded in ascertaining that the 

 constellation which, 150 years before him, corresponded to 

 the spring equinox, did not, in his time, any longer coincide 

 exactly with the same equinoctial point, but had outstripped 

 or preceded it about 2. This is what we mean by the pre- 

 cession of the equinoxes. 



Hipparchus was at first of opinion that this movement 

 affected only the constellations of the Zodiac; but he soon 

 became assured of its universality. He perceived that if it 

 does not alter the parallels of latitude ; because it has occurred 

 parallel with the Ecliptic, it makes the position of the equinox 

 retrograde from east to west, and the sun pass slowly through 

 the same constellations in the reverse of the order in which 

 he annually traverses them. 



We know this movement now to nearly the fraction of a 

 second. By an inappreciable daily quantity, it rises, at the 

 end of the year, to $o".3, in a century, to about i|, 

 in twenty centuries, to 30, or the twelfth part of the Zodiac. 

 It is for this reason the Ram, which, in the days of Hip- 



