Page:The Harvard Classics Vol. 51; Lectures.djvu/116

106 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.

THE COPERNICAN THEORY

Copernicus was born at Thorn in Poland (1473) of a German mother. Educated first in medicine, he studied astronomy in Vienna, and he was later in Italy (1495–1505) at the height of the Renaissance. When he returned home, his uncle, the bishop of Ermeland, presented him with a clerical position at Frauenburg. Here for forty years he labored to bring astronomical calculations and observations into harmony, and finally, long after he had become convinced of the soundness of the heliocentric view, published the work which marks the first great step in modern science, a work which he saw for the first time on his deathbed in 1543.