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HE 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 unfamiliar after his time and Copernicus quotes the Phythagoreans as authorities in the first chapters of his book De Revolutionibus Orbium Cælestium (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 Anticthona; 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.