The principal results of ancient astronomy go by the name of Ptolemy (the Ptolemaic system), but are mainly due to the labors of Hipparchus.

Hipparchus knew the latitude and longitude of 150 fixed stars within a fraction of a degree, when, in the year 134 B. C., a new star of the first magnitude suddenly appeared. Encouraged by this extraordinary event, he applied himself diligently to astronomical measurements, establishing the position of more than 1,000 fixed stars. It was no doubt this sound basis of accurate quantitative data, and the familiarity with his subject which such work provided, that led to his great achievements. He discovered the precession of the equinoxes, and measured it with considerable accuracy; he measured the length of the day with an error of but six minutes; but his great achievement was a mathematical device whereby the position of the sun and, with less accuracy, the positions of the moon and planets could be calculated.

The essential features of this device consisted in imagining the sun to move in a circle of which the earth was not quite the center; this is the excentric of ancient astronomy. Another more difficult idea was that of epicycles. These two mathematical ideas did very good service in the work of Hipparchus, for the practical purposes of the calendar. But later, in the hands of Ptolemy, and in the succeeding centuries, they ceased to be arbitrary assumptions, or even mere theories, and in the Middle Ages became dogmas which were held most tenaciously and blindly. As astronomical knowledge slowly increased, it became necessary to make the theory more and more complex in order to fit the facts, and, long before the work of Copernicus, astronomical theories had reached a degree of absurdity that could not have endured in any other age. Yet more than one of the astronomers of antiquity had believed that the earth moves, either rotating on its axis, or revolving round the sun, or both.

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