* 8385] Precession 1 1 1 



Of much more interest than the detailed discussion of tre- 

 pidation and of geometrical schemes for representing it is 

 the interpretation of precession as the result of a motion of 

 the earth's axis. Precession was originally recognised by 

 Hipparchus as a motion of the celestial equator, in which 

 its inclination to the ecliptic was sensibly unchanged. 

 Now the ideas of Coppernicus make the celestial equator 

 dependent on the equator of the earth, and hence on its 

 axis ; it is in fact a great circle of the celestial sphere 

 which is always perpendicular to the axis about which the 

 earth rotates daily. Hence precession, en the theory of 

 Coppernicus, arises from a slow motion of the axis of the 

 earth, which moves so as always to remain inclined at the 

 same angle to the ecliptic, and to return to its original 

 position after a period of about 26,000 years (since a 

 motion of 5o"*2 annually is equivalent to 360 or a complete 

 circuit in that period); in other words, the earth's axis 

 has a slow conical motion, the central line (or axis) of the 

 cone being at right angles to the plane of the ecliptic. 



85. Precession being dealt with, the greater part of the 

 remainder of the third book is devoted to a discussion in 

 detail of the apparent annual motion of the sun round the 

 earth, corresponding to the real annual motion of the earth 

 round the sun. The geometrical theory of the Almagest 

 was capable of- being immediately applied to the new system, 

 and Coppernicus, like Ptolemy, uses an eccentric. He 

 makes the calculations afresh, arrives at a smaller and more 

 accurate value of the eccentricity (about ^ T instead of ^), 

 fixes the position of the apogee and perigee (chapter n., 39), 

 or rather of the equivalent aphelion and peiihelion (i.e. the 

 points in the earth's orbit where it is respectively farthest 

 from and nearest to the sun), and thus verifies Albategnius's 

 discovery (chapter in., 59) of the motion of the line of 

 apses. The theory of the earth's motion is worked out in 

 some detail, and tables. are given whereby the apparent place 

 of the sun at any time can be easily computed. 



The fourth book deals with the theory of the moon. As 

 has been already noticed, the moon was the only celestial 

 body the position of which in the universe was substantially 

 unchanged by Coppernicus, and it might hence have been 

 expected that little alteration would have been required in 



