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 immovable 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 because 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 system of Apollonius extended to the superior planets. The astronomical planets were thus no longer simple sports of the imagination 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, Heraclides, and other philosophers, while they placed the earth in the center of the world, had ventured to give it a movement of rotation upon itself, producing the phenomena of the rising and setting 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 permitted 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-
