258 THE POPULAR SCIENCE MONTHLY. 



toward the end of his life, and regretted that he had not placed 

 the sun at the center of the world, the only place that became it. 

 Three centuries before Christ Aristarchus of Samos, according to 

 Archimedes, composed a work, now lost, defending the doctrine 

 of the movement of the earth against the opinions of philosophers 

 to the contrary, in which he said that " the sun continues immov- 

 able and the earth moves around the sun, describing a circular 

 course of which that star occupies the center." Passing to the 

 Romans, this system of Aristarchus was modified into one like 

 that of Tycho Brahe. 



In his review of the ancient systems, Copernicus was most 

 drawn, according to M. Biot, " to that of the Egyptians, which 

 made Mercury and Venus revolve round the sun, and put Mars, 

 Jupiter, Saturn, and the sun in motion round the earth ; and to 

 that of Apollonius of Perga, which made the sun the common 

 center of all the planetary motions, while the sun itself revolved 

 around the earth an arrangement that became the system of 

 Tycho Brahe. Copernicus was impressed with these systems be- 

 cause he found that they represented well the limited excursions 

 of Mars and Venus around the sun, explaining their movements, 

 direct, stationary, and retrograde, an advantage which the sys- 

 tem of Apollonius extended to the superior planets. The astro- 

 nomical planets were thus no longer simple sports of the imagina- 

 tion to him. He had studied them experimentally, and had found 

 the conditions which they must satisfy. The hardest part of his 

 discovery was made. On the other hand, he perceived that the 

 Pythagoreans had taken away the earth from the center of the 

 world and put the sun there. It seemed to him that Apollonius's 

 system would be simpler and more symmetrical if it was modified 

 in this sense, so as to suppose the sun fixed in the center, and the 

 earth revolving round it. He had seen also that Nicetas, Herac- 

 lides, and other philosophers, while they placed the earth in the 

 center of the world, had ventured to give it a movement of rota- 

 tion upon itself, producing the phenomena of the rising and set- 

 ting of the stars and the alternations of day and night. He still 

 more approved the theory of Philolaus, who, taking the earth 

 away from the center of the world, had given it a rotation on its 

 axis and another motion of annual revolution around the sun. 

 And, although it might seem difficult and even absurd to take the 

 earth from the center and make a simple planet of it, yet, as other 

 astronomers before him had taken the liberty of imagining circles 

 in the sky to explain phenomena, he thought he might be per- 

 mitted to look for some other arrangement, with a moving earth, 

 which would establish a more simple order in the motions of the 

 stars. Thus, taking what is true from each system and rejecting 

 all in them that was false and complicated, he composed that ad- 



