410 A SHORT HISTORY OF SCIENCE 



this opinion, whose words I have seen fit to quote here, that they may 

 be accessible to all : 



Some maintain that the Earth is stationary, but Philolaus the Pythag- 

 orean says that it revolves in a circle about the fire of the ecliptic, like the 

 sun and moon. Heraklides of Pontus and Ekphantus the Pythagorean make 

 the Earth move, not changing its position, however, confined in its falling and 

 rising around its own centre in the manner of a wheel. 



Taking this as a starting-point, I began to consider the mobility of 

 the Earth ; and although the idea seemed absurd, yet because I knew 

 that the liberty had been granted to others before me to postulate all 

 sorts of little circles for explaining the phenomena of the stars, I 

 thought I also might easily be permitted to try whether by postulating 

 some motion of the Earth, more reliable conclusions could be reached 

 regarding the revolution of the heavenly bodies, than those of my 

 predecessors. 



And so, after postulating movements, which, farther on in the book, 

 I ascribe to the Earth, I have found by many and long observations 

 that if the movements of the other planets are assumed for the circular 

 motion of the Earth and are substituted for the revolution of each 

 star, not only do their phenomena follow logically therefrom, but the 

 relative positions and magnitudes both of the stars and all their orbits, 

 and of the heavens themselves, become so closely related that in none 

 of its parts can anything be changed without causing confusion in 

 the other parts and in the whole universe. Therefore, in the course 

 of the work I have followed this plan : I describe in the first book all 

 the positions of the orbits together with the movements which I 

 ascribe to the Earth, in order that this book might contain, as it 

 were, the general scheme of the universe. Thereafter in the remaining 

 books, I set forth the motions of the other stars and of all their orbits 

 together with the movement of the Earth, in order that one may see 

 from this to what extent the movements and appearances of the 

 other stars and their orbits can be saved, if they are transferred to the 

 movement of the Earth. Nor do I doubt that ingenious and learned 

 mathematicians will sustain me, if they are willing to recognize and 

 weigh, not superficially, but with that thoroughness which Philosophy 

 demands above all things, those matters which have been adduced 

 by me in this work to demonstrate these theories. In order, how- 

 ever, that both the learned and the unlearned equally may see that 



