NEWTON'S THEORY OF PLANETARY MOTIONS 67 



This simple expedient the ancients seem never to have 

 thought of, for those were the halcyon days of deductive 

 philosophy, when man sought to solve nature by pre- 

 sumptuously trying to read the mind of the Infinite by 

 divination, instead of indirectly, through His works. 

 They reasoned, metaphysically, that the Creator would 

 not choose any but the ' ' perfect curve ' ' for the paths of 

 the celestial bodies ; and they would have accounted it 

 blasphemy in anyone who might have suggested an alter- 

 native idea. Accordingly, when systematic observation 

 showed certain vagaries of movement on the part of the 

 planets, instead of adopting the obvious course and ex- 

 perimenting with other possible curves, they invented the 

 device known as epicycles ; that is to say, they imagined 

 the planet, in addition to revolving in one great circle, to 

 revolve also in a second, smaller circle, whose center, 

 rather than the planet's center, progressed along the cir- 

 cumference of the main curve. One such epicycle proving 

 insufficient, nowise daunted, they postulated a second 

 epicycle grafted upon the first, and so on indefinitely, 

 until, by the time of Copernicus, they had as many as 

 seventy such arrangements piled one upon the other ! 



Such, then, was the state of theoretical astronomy at 

 the close of the regime of the Ptolemaic system, whose 

 rule, though not actually ended, was at least foredoomed 

 by the publication of Nicholas Copernicus' (1473-1543) 

 great work, De Orbium Coelestium Revolutionibus, in the 

 very year of the author's death. In this treatise Coper- 

 nicus taught that the sun, and not the earth, is the center 

 of our system, that the moon revolves around the earth, 

 and that the earth and all the rest of the then known 

 planets revolve around the sun in circular orbits. The 

 only material mistake he made lay in perpetuating this 

 last doctrine, inherited, as it was, from the older system ; 

 but in extenuation it should be mentioned that the gen- 

 eral reform brought about by him was so basic as to ren- 

 der for a time unavailable the tangled skein of mathe- 

 matical material accumulated by his predecessors. 



As things turned out, it would have been a lasting re- 

 flection on the perspicacity of philosophers had the dis- 



