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were at first synonymous expressions among the ancients, both signifying "the science of the stars." But afterwards Astrology came to mean that part of the science which deals with the supposed influence of the stars on the destinies of men. Among the Greeks, Astronomy, the origin of which they themselves ascribed to the Assyrians, Babylonians and Egyptians, was for centuries the subject of' philosophical speculation without a sufficient groundwork in observation, because mathematics and mechanics had not reached the requisite degree of perfection. The list of observing astronomers opens with Eudoxus of Cnidus in the first half of the 4th century, B.C., who assumed that the earth was spherical, and tried to explain the phenomena of the heavens by a complicated theory of concentric spheres. Aristotle too maintained and proved the spherical form of the earth, which, he took to be the immovable centre of the universe. Astronomy was first raised into a real science after B.C. 300 at Rhodes and Alexandria, in the Museum of which town the first observatory was built, and Aristyllus and Timochares determined the places of the fixed stars with comparative accuracy, though as yet with very rude apparatus. A great step in advance was taken by Aristarchus of Samos, who observed the summer solstice at Alexandria in B.C. 279, maintained the earth's rotation on her axis and revolution round the sun, and made an attempt, by no means contemptible, to ascertain the size and distance of the sun and moon. His successor Eratosthenes also rendered essential service to the progress of the science; thus he came very near to determining the exact obliquity of the ecliptic. The true founder of scientific Astronomy, and the greatest independent observer of antiquity, was Hipparchus of Nicaea(in the 2nd century B.C.), who discovered the precession of the equinoxes, and determined the length of the solar year (at 365 days 5 hours 55' 12") as well as the time of the moon's revolution, and the magnitude and distances of the heavenly bodies. The last important astronomer of antiquity, and the greatest after Hipparchus, is Claudius Ptolemoeus (in the 2nd century A.D.). In his chief work, commonly known by its Arabic name of Almagest, he digested the discoveries of his predecessors, especially Hipparchus, and his own, into a formal system, which passed current all through the Middle Ages. According to it the earth is a sphere resting motionless in the middle of the equally spherical universe, while the sun, moon, planets and fixed stars roll at various distances around her. The Romans regarded Astronomy as an idle speculation, and gave little attention to it. When Caesar reformed the Roman Calendar, he had to bring an astronomer from Alexandria, Sosigenes, to help him.
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