A NEW ASTRONOMY 199 



i'n comparison with that of the sphere of the stars. He holds 

 all this easier of comprehension, than if the mind is confused by 

 an almost endless mass of circles, as is necessary for those who put 

 the earth in the centre of the universe. 



'So in fact the sun seated on the royal throne guides the family 

 of planets encircling it. We find thus in this arrangement a har- 

 monious connection not otherwise realized. For here one can see 

 why the forward and backward motions of Jupiter seem greater than 

 those of Saturn and smaller than those of Mars.' 



His adherence to the Greek assumption of uniform circular 

 motion leaves him still under the necessity of retaining an elabo- 

 rate system of epicycles, but he rejects Ptolemy's equant. 



... He his fabric of the heavens 

 Hath left to their disputes, perhaps to move 

 His laughter at their quaint opinions wide ; 

 Hereafter when they come to model heaven 

 And calculate the stars, how will they wield 

 The mighty frame ! how build, unbuild, contrive 

 To save appearances ! how gird the sphere 

 With centric and eccentric scribbled o'er, 

 Cycle in epicycle, orb in orb ! 



Milton, Paradise Lost, VIII. 



The epicycles of Copernicus numbered however but 34, 

 sufficing "to explain the whole construction of the world and the 

 whole dance of the planets " against the 79 to which the Ptole- 

 maic theory had gradually attained. The completeness of mathe- 

 matical detail with which the whole theory is worked out can not 

 here be adequately described. He includes so much trigonometry 

 as his astronomical work requires, also a revision of Ptolemy's star 

 catalogue. He computes a very accurate value of the equinoctial 

 precession, and interprets this correctly as due to a slow conical 

 motion of the earth's axis, like that of a top coming to rest. 



Copernicus estimates the relative sizes of moon, earth and 



