394 HISTORY OF FORMAL ASTRONOMY. 



Copernicus is not exempt from giving us, in his 

 work, an example of this character of the inventive 

 spirit. The axiom that the celestial motions must 

 be circular and uniform, appeared to him to have 

 strong claims to acceptation ; and his theory of the 

 inequalities of the planetary motions is fashioned 

 upon it. His great desire was to apply it more 

 rigidly than Ptolemy had done. The time did not 

 come for rejecting the axiom, till the observations 

 of Tycho Brahe and the calculations of Kepler had 

 been made. 



I shall not attempt to explain, in detail, Coper- 

 nicus's system of the planetary inequalities. He 

 retained epicycles and eccentrics, altering their 

 centers of motion ; that is, he retained what was 

 true in the old system, translating it into his own. 

 The peculiarities of his method consisted in making- 

 such a combination of epicycles as to supply the 

 place of the equant 1 , and to make all the motions 

 equable about the centers of motion. This device 

 was admired for a time, till Kepler's elliptic theory 

 expelled it, with all other forms of the theory of 

 epicycles: but we must observe that Copernicus 

 was aware of some of the discrepancies which be- 

 longed to that theory as it had, up to that time, 

 been propounded. In the case of Mercury's orbit, 

 which is more eccentric than that of the other 

 planets, he makes suppositions which are complex 

 indeed, but which show his perception of the im- 

 4 See p. 235. 



