$*9*i9*] The Use of Epieydes by Coppernicus 123 



which he played in the overthrow of the Ptolemaic system 

 is so conspicuous, that we are sometimes liable to forget 

 that, so far from rejecting the epicycles and eccentrics of 

 the Greeks, he used no other geometrical devices, and was 

 even a more orthodox " epicyclist " than Ptolemy himself, 

 as he rejected the equants of the latter.* Milton's famous 

 description (Par. Lost, VIII. 82-5) of 



"The Sphere 



With Centric and Eccentric scribbled o'er, 

 Cycle and Epicycle, Orb in Orb," 



applies therefore just as well to the astronomy of Copper- 

 nicus as to that of his predecessors; and it was Kepler 

 (chapter vn.), writing more than half a century later, not 

 Coppernicus, to whom the rejection of the epicycle and 

 eccentric is due. 



92. One point which was of importance in later 

 controversies deserves special mention here. The basis 

 of the Coppernican system was that a motion of the 

 earth carrying the observer with it produced an apparent 

 motion of other bodies. The apparent motions of the 

 sun and planets were thus shewn to be in great part 

 explicable as the result of the motion of the earth round 

 the sun. Similar reasoning ought apparently to lead 

 to the conclusion that the fixed stars would also appear 

 to have an annual motion. There would, in fact, be a 

 displacement of the apparent position of a star due to 

 the alteration of the earth's position in its orbit, closely 

 resembling the alteration in the apparent position of the 

 moon due to the alteration of the observer's position 

 on the earth which had long been studied under the name 

 of parallax (chapter n., 43). As such a displacement 

 had never been observed, Coppernicus explained the 

 apparent contradiction by supposing the 'fixed stars so 



* Recent biographers have called attention to a cancelled passage 

 in the manuscript of the De Revolutionibus in which Coppernicus 

 shews that an ellipse can be generated by a combination of circular 

 motions. The proposition is, however, only a piece of pure mathe- 

 matics, and has no relation to the motions of the planets round the 

 sun. It cannot, therefore, fairly be regarded as in any way an 

 anticipation of the ideas of Kepler (chapter vn.). 



