COPERNICUS. 115 



hundred years. Would any other hypothesis explain them? In the 

 first place, Copernicus affirms the rotation of the earth on its axis. 

 The rising and the setting of the stars is caused by this. 



The question of the rotation of the earth had been examined by 

 Ptolemy. He rejects the notion, saying: "If the earth turned in 

 twenty-four hours around its axis every point on its surface would be 

 endowed with an immense velocity, and from the rotation a force of 

 projection would arise capable of tearing the most solid buildings from 

 their foundations and of scattering their fragments in the air." The 

 force of projection depends, we know, not only on the absolute velocity 

 of points on the turning earth (and this velocity is immense), but also 

 on the angular velocity about this axis. The latter is slow. The hour 

 hand of a clock turns twice as fast as the earth. The projective force 

 at its maximum is just sufficient to diminish the weight of a ton by 

 six pounds. A feeble force of the sort is not fitted to tear trees up by 

 their roots or buildings from their foundations, as Ptolemy supposed. 



Copernicus adopted the theory of a rotating earth, although he was 

 no better able than Ptolemy to explain the difficulty. The science of 

 mechanics was not born till the time of Galileo. The reasoning of 

 Copernicus is: "The rotation of the earth being a natural movement, 

 its effects are very different from those of a violent motion; and the 

 earth, which turns in virtue of its proper nature, is not to be likened 

 to a wheel that is constrained to turn by force." He seeks to escape 

 the difficulty by a trick of scholastic philosophy. No other issue was 

 open in his day. Examples of this sort are well fitted to give us a 

 vivid idea of the state of science in those times. It was not easy for 

 our predecessors to take a forward step. More honor to them that the 

 steps were taken. 



In the preface to the 'De Revolutionibus ' Copernicus declares that 

 he was dissatisfied with the want of symmetry in the theory of eccen- 

 trics and weary of the uncertainty of the mathematical conditions. 

 Searching through the works of the ancients, he found that some of 

 them held that the earth was in motion, not stationary. Philolaus, for 

 example, taught that the earth revolved about a central fire.* Coper- 

 nicus makes no mention of the theory of Aristarchus. We must as- 

 sume that he did not know it, though his ignorance in this respect is 

 hard to explain. We have no list of his library, which was, however, 

 extensive for the time. 



"Then I too," says Copernicus, "began to meditate concerning the 

 motion of the earth; and although it appeared an absurd opinion, yet 

 since I knew that, in earlier times, others had been allowed the privi- 

 lege of imagining what circles they might choose in order to explain 



* The central fire of Philolaus was, however, not the sun ; for in his theory 

 the earth, the sun, the moon and all the planets revolved about a fire so 

 placed at the center of the system as to be forever invisible to the earth. 



