54 A Short History of Astronomy [CH. n. 



The annual motion r r ' was, as has been stated, estimated 

 by Hipparchus as being at least 36" (equivalent to one 

 degree in a century), and probably more. Its true value is 

 considerably more, namely about 50". 



An important consequence of the motion of the equator 

 thus discovered is that the sun in its annual journey round 

 the ecliptic, after starting from the equinoctial point, returns 

 to the new position of the equinoctial point a little before 

 returning to its original position with respect to the stars, 

 and the successive equinoxes occur slightly earlier than they 



7 

 FIG. 24. The precession of the equinoxes. 



otherwise would. From this fact is derived the name pre- 

 cession of the equinoxes, or more shortly, precession, which 

 is applied to the motion that we have been considering. 

 Hence it becomes necessary to ^recognise, as Hipparchus 

 did, two different kinds of year, the tropical year or period 

 required by the sun to return to the same position with 

 respect to the equinoctial points, and the sidereal year or 

 period of return to the same position with respect to the 

 stars. If r T' denote the motion of the equinoctial point 

 during a tropical year, then the sun after starting from the 



