INDUCTION OF COPERNICUS. 391 



geocentric hypothesis ascribed to the planets those 

 motions which were apparent only, and which really 

 arose from the motion of the earth round the sun 

 in the new hypothesis, the latter scheme must much 

 simplify the planetary theory. Kepler 2 enumerates 

 eleven motions of the Ptolemaic system, which are 

 at once exterminated and rendered unnecessary by 

 the new system. Still, as the real motions, both of 

 the earth and the planets, are unequable, it was 

 requisite to have some mode of representing their 

 inequalities; and, accordingly, the ancient theory 

 of eccentrics and epicycles was retained, so far as 

 was requisite for this purpose. The planets revolved 

 round the sun by means of a Deferent, and a great 

 and small Epicycle ; or else by means of an Eccen- 

 tric and Epicycle, modified from Ptolemy's, for rea- 

 sons which we shall shortly mention. This mode 

 of representing the motions of the planets con- 

 tinued in use, till it was expelled by the discoveries 

 of Kepler. 



Besides the daily rotation of the earth on its 

 axis, and its annual circuit about the sun, Coper- 

 nicus attributed to the axis a "motion of declina- 

 tion," by which, during the whole annual revo- 

 lution, the pole was constantly directed towards the 

 same part of the heavens. This constancy in the 

 absolute direction of the axis, or its moving parallel 

 to itself, may be more correctly viewed as not indi- 

 cating any separate motion. The axis continues in 



2 Myst. Cosm. cap. 1. 



