Nicholas Copernicus [image] Copernicus was born on February 19, 1473, in Torun, Polish Prussia, the son of a merchant. Though today he is remembered as the father of modern astronomy, in his own time he made his livelihood not as an astronomer, but as an ecclesiastic. His position as canon of the cathedral of Frombork gave him both financial security and freedom to pursue his own astronomical studies. It was at Frombork that Copernicus worked out his great book, the De Revolutionibus. The accepted model of the structure of the universe in Copernicus' time was earth-centered. The sun, the moon, the five known planets, and the stars were thought to revolve about the earth in endless, perfect circles. The model was developed by Aristotle around 350 B.C., and elaborated by Claudius Ptolemy of Alexandria around 150 A.D. Ptolemy outlined his system in a treatise which has come to be known as the Almagest, meaning "the greatest." What Ptolemy established for the first time was a working mathematical model by which the positions of the planets could be predicted accurately. In the Ptolemaic system, each planet moved in a small circle (or epicycle) whose center was carried round the earth in a larger orbit (or deferent). For fourteen centuries astronomers computed planetary positions from tables based on this analysis. Copernicus described an unfamiliar universe, with the sun, not the earth, at its center; he treated the earth as a planet among the other planets, with a yearly orbit around the sun, a daily rotation on its axis, and a conical precession. His great breakthrough is the recognition that the complex paths which we see traced by the planets could be explained by a combination of their own motion and that of the earth from which we observe them. His new model gave astronomical inquiry the direction it still follows today. But he arrived at his innovations using the traditional assumption, shared by Aristotle and Ptolemy, that the motion of heavenly bodies must be a compound of circles - an idea which was soon to be overthrown by Johannes Kepler, as he worked to build a foundation for the Copernican hypothesis. [image] Copernicus' cosmology drawing, from his manuscript of the De Revolutionibus. Copernicus' book, the De Revolutionibus, had an immediate impact on astronomical theory. "Astronomy is written for astronomers," he wrote in his preface. But in the coming century his re-examination of the structure of the universe would permanently alter the way people thought of themselves and their world. The De Revolutionibus - published only in the year of its author's death - might never have appeared in print if Georg Joachim Rheticus, a young professor of mathematics, had not persuaded Copernicus to entrust him with the manuscript for publication.