3i6 POPULAR SCIENCE MONTHLY. 



THE PEEDECESSOES OF COPEENICUS. 



By EDWARD S. HOLDEN, Sc.D., LL.D., 



LIBRARIAN OF THE U. S. MILITARY ACADEMY. 



THE records of the earliest Greek astronomy are very meager. 

 Pythagoras, in the sixth century B, C, held that the heavenly 

 bodies, the earth included, were spheres. Pythagoras is supposed to 

 have known that lunar phases were caused by illumination from the 

 sun; and the curved line separating the bright and dark parts of the 

 moon throughout the month would naturally suggest that it was not 

 a flat disc but a globe. He imagined all the stars to be fixed to a 

 crystal sphere which daily turned round the earth and produced their 

 rising and setting. Each of the seven planets (sun, moon. Mercury, 

 Venus, Mars, Jupiter, Saturn) was attached to a sphere of its own, 

 and their turning made harmonious sound — the music of the spheres. 

 The distances of the several spheres were assigned in accordance with 

 certain laws of music that Pythagoras had himself discovered. The 

 idea of a spherical earth is thus some twenty-five hundred years old. 



Philolaus, a Phythagorean of the fifth century B. C, maintained 

 that the earth and all the planets (including the sun) revolved about 

 a central fire. The idea of a moving earth was, therefore, not un- 

 familiar after his time and Copernicus quotes the Phythagoreans as 

 authorities in the first chapters of his book De Revolutionibus Orbium 

 Ccelestium (1543). But the sun was not the central fire in their 

 system, as it is in nature. "This world Pythagoras and his followers 

 asserted to be one of the stars, and they also said that there was 

 another opposite to it, similar to it; and they called that one Antic- 

 thona; and he said that both were in one sphere which revolved from 

 east to west, and by this revolution the sun was circled round us ; now 

 he was seen, and now he was not seen. And he said that the fire was 

 in the center of these, considering the fire to be a more noble body 

 than the water and than the earth, and giving the noblest center" 

 (Dante, Convito, iii., chap. v.). The Pythagoreans took the sun to 

 be about three times the distance of the moon from the earth. 



We know too little of the reasons that led Aristarchus of Samos, 

 in the third century B. C, to hold that the sun was motionless at the 

 center of the celestial sphere and that the earth revolved about him, 

 rotating on her axis as she went. He taught also that the fixed stars 

 are at rest, and measured the sun's apparent diameter, fixing it at 

 half a degree. The little that remains of his writings gives the very 

 highest idea of his originality and practical genius. 



