M 79, s.\i The Motion of the Earth 105 



the earth turns in the second case, but that the two 

 rotations are in opposite directions. A similar explanation 

 evidently applies to more complicated cases. 



Hence the apparent daily rotation of the celestial sphere 

 about an axis through the poles would be produced equally 

 well, either by an actual rotation of this character, or by 

 a rotation of the earth about an axis also passing through 

 the poles, and at the same rate, but in the opposite 

 direction, i.e. from west to east. This is the first motion 

 which Coppernicus assigns to the earth. 



79. The apparent annual motion of the sun, in accordance 

 with which it appears to revolve round the earth in a path 

 which is nearly a circle, can be equally well explained by 

 supposing the sun to be at rest, and the earth to describe 

 an exactly equal path round the sun, the direction of the 

 revolution being the same. This is virtually the second 

 motion which Coppernicus gives to the earth, though, on 

 account of a peculiarity in his geometrical method, he 

 resolves this motion into two others, and combines with 

 one of these a further small motion which is required for 

 precession.* 



80. Coppernicus's conception then is that the earth 

 revolves round the sun in the plane of the ecliptic, while 

 rotating daily on an axis which continually points to the 

 poles of the celestial sphere, and therefore retains (save for 

 precession) a fixed direction in space. 



It should be noticed that the two motions thus assigned 

 to the earth are perfectly distinct ; each requires its own 

 proof, and explains a different set of appearances. It was 

 quite possible, with perfect consistency, to believe in one 

 motion without believing in the other, as in fact a very 

 few of the 16th-century astronomers did (chapter v., 105). 



In giving his reasons for believing in the motion of the 



* To Coppernicus, as to many of his contemporaries, as well as to 

 the Greeks, the simplest form of a revolution of one body round 

 another was a motion in which the revolving body moved as if 

 rigidly attached to the central body. Thus in the case of the earth 

 the second motion was such that the axis of the earth remained 

 inclined at a constant angle to the line joining earth and sun, and 

 therefore changed its direction in space. In order then to make the 

 axis retain a (nearly) fixed direction in space, it was necessary to add 

 a third motion. 





