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ἀστρονομία) and Astrologia (ἀστρολογία). These terms 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 fourth 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 second century B.C.), who discovered the precession of the equinoxes, and determined the length of the solar year (at 365 days, 5 hours, 55 minutes, 12 seconds), 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 Ptolemaeus in the second 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 Iulius Caesar reformed the Roman calendar he was obliged to bring an astronomer, Sosigenes, from Alexandria to help him.

Astrology, in the narrower sense of the word, and applied to predictions based upon the observation of the heavenly bodies, arose among the Chaldaeans, and in Greece did not come into vogue until after the time of Alexander the Great. In Rome the professional astrologers were called Chaldaei, or Mathematici, the latter name referring to the astronomical calculations which they made. In the Republican period they were known, but held in utter contempt. In B.C. 139 their unpopularity was so great that they were expelled from Rome and Italy. But in the turbulent times of the civil wars their reputation rose considerably, and still more under the Empire, when the most extensive demands were made upon their science. They were, indeed, repeatedly driven out of Italy and involved in trials for treason (maiestas); but this only enhanced the consideration in which they were held, the more so as they were frequently taken into counsel by the emperors and the members of the imperial family. In later times all that the Chaldaeans were forbidden to do was to consult the stars on questions referring to the emperor's life, which was made a criminal offence. The Christian emperors (but none before them) issued many prohibitions against all consultation of astrologers whatever.

In the practice of their art they used calendars written on tablets, in which were set down for every day the motion and relative distances of the stars, whether lucky or unlucky. By another set of tablets they made their calculations of every hour in detail, noting the hour of a person's birth and the relative position of the constellation dominant at the time. In accordance with this they determined the fortunes of him who was born at the hour in question. By a similar process they ascertained the times that were favourable or unfavourable to any undertaking. Among the lucky stars were Iupiter, Venus, and Luna; among the unlucky, Saturn and Mars were the chief. Mercury was lucky or unlucky, according to circumstances.

For an account of ancient astronomy and astrology, the reader is referred to Ball, Short History of Mathematics (1888); Lewis, Astronomy of the Ancients (1862); Becker, Handbuch der röm. Alterth. (1880); Whewell, History of the Inductive Sciences (3d ed. 1858); Wolf, Geschichte der Astronomie (1877); Delambre, Histoire de l'Astronomie (1827); the treatise of Iulius Firmicus in Latin, of Manetho and Ptolemy in Greek; and the Latin poem of Manilius (q.v.), entitled Astronomica.

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