INDUCTION OF COPERNICUS. 263 



grounds on which Copernicus adopted his theory ; as the craving foi 

 these qualities was the feeling which led him to seek for a new theory 

 It is manifest that in this, as in other cases of discovery, a clear and 

 steady possession of abstract Ideas, and an aptitude in comprehending 

 real Facts under these general conceptions, must have been leading 

 characters in the discoverer's mind. He must have had a good geo- 

 metrical head, and great astronomical knowledge. He must have seen, 

 with peculiar distinctness, the consequences which flowed from his 

 suppositions as to the relations of space and time, the apparent 

 motions which resulted from- the assumed real ones ; and he must alsc 

 have known well all the irregularities of the apparent motions for 

 which he had to account. We find indications of these qualities in 

 his expressions. A steady and calm contemplation of the theory is 

 what he asks for, as the main requisite to its reception. If you sup- 

 pose the earth to revolve and the heaven to be at rest, you will find, 

 he says, "si serio animadvcrtas" if you think steadily, that the appar- 

 ent diurnal motion will follow. And after alleging his reasons for his 

 system, he says, 1 " We are, therefore, not ashamed to confess, that the 

 whole of the space within the orbit of the moon, along with the centre 

 of the earth, moves round the sun in a year among the other planets ; 

 the magnitude of the world being so great, that the distance of the 

 earth from the sun has no apparent magnitude when compared with 

 the sphere of the fixed stars." "All which things, though they be 

 difficult and almost inconceivable, and against the opinion of the 

 majority, yet, iu the sequel, by God's favor, we will make clearer than 

 the sun, at least to those who are not ignorant of mathematics." 



It will easily be understood, that since the ancient geocentric hypoth- 

 esis ascribed to the planets those motions which were apparent only, 

 and which really arose from the motion of theearth round the sun in 

 the new hypothesis, the latter scheme must much simplify the plan- 

 etary 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 represent- 

 ing their inequalities ; and, accordingly, the ancient theory of eccen- 

 trics 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 



1 Nicolai Copcrnici Torinensis de Rewlutionilus Orblum Ccukstiiun Lllri VI 

 Norimbergse, M.D.XLIII. p. 9. 



2 Mi/st. Cosm. cap. 1. 



