$ 76, 77] The Motion f jf' the Ea*-tv 101 



ciples or postulates, but on account of their novelty takes 

 a little more trouble than his predecessor (cf. chapter n., 

 47) to make them at once appear probable. With 

 these postulates as a basis he proceeds to develop, by 

 means of elaborate and rather tedious mathematical reason- 

 ing, aided here and there by references to observations, 

 detailed schemes of the various celestial motions ; and it 

 is by the agreement of these calculations with observations, 

 far more than by the general reasoning given at the 

 beginning, that the various postulates are in effect justified. 

 His first postulate, that the universe is spherical, is 

 supported by vague and inconclusive reasons similar to 

 those given by Ptolemy and others ; for the spherical form 

 of the earth he gives several of the usual valid arguments, 

 one of his proofs for its curvature from east to west being 

 the fact that eclipses visible at one place are not visible 

 at another. A third postulate, that the motions of the 

 celestial bodies are uniform circular motions or are com- 

 pounded of such motions, is, as might be expected, sup- 

 ported only by reasons of the most unsatisfactory character. 

 He argues, for example, that any want of uniformity in 

 motion 



"must arise either from irregularity in the moving power, 

 whether this be within the body or foreign to it, or from some 

 inequality of the body in revolution. . . . Both of which things 

 the intellect shrinks from with horror, it being unworthy to hold 

 such a view about bodies which are constituted in the most 

 perfect order." 



77. The discussion of the possibility that the earth may 

 move, and may even have more than one motion, then 

 follows, and is more satisfactory though by no means con- 

 clusive. Coppernicus has a firm grasp of the principle, 

 which Aristotle had also enunciated, sometimes known as 

 that of relative motion, which he states somewhat as 

 follows : 



" For all change in position which is seen is due to a motion 

 either of the observer or of the thing looked at, or to changes 

 in the position of both, provided that these are different. For 

 when things are moved equally relatively to the same things, 



