52 A Short History of Astronomy [CH. n. 



at least 36" annually, and possibly more. The agreement 

 between the motions of different stars was enough to 

 justify him in concluding that the change could be 

 accounted for, not as a motion of individual stars, but 

 rather as a change in the position of the equinoctial 

 points, from which longitudes were measured. Now these 

 points are the intersection of the equator and the ecliptic : 

 consequently one or another of these two circles must have 

 changed. But the fact that the latitudes of the stars had 

 undergone no change shewed that the ecliptic must have 

 retained its ppsition and that the change had been caused 



N 

 FIG. 21.- The increase of the longitude of a star. 



by a motion of the equator. Again, Hipparchus measured 

 the obliquity of the ecliptic as several of his predecessors 

 had done, and the results indicated no appreciable change. 

 Hipparchus accordingly inferred that the equator was, as 

 it were, slowly sliding backwards (i.e. from east to west), 

 keeping a constant inclination to the ecliptic. 



The argument may be made clearer by figures. In 

 fig. 21 let TM denote the ecliptic, TN the equator, s a 

 star as seen by Timocharis, s M a great circle drawn per- 

 pendicular to the ecliptic. Then s M is the latitude, TM 

 the longitude. Let s' denote the star as seen by Hipparchus ; 



