The great Polish astronomer Nicolaus Copenicus (Polish name; Mikolaj Kopernik), was born in 1473, in the city of Tourun, on the Vistuala River, in Poland. He came from a well-to-do family. As a young man, Copenicus studied at the University of Cracow, where he became interested in astronomy. In his mid-twenties he went to Italy, where he studied law and medicine at the Universities of Bologna and Padua, and later received a doctorate in canon law from the University of Ferrara. Copenicus spent most of his adult life on the staff of the cathedral at Frauenburg (Polish:Frombork), where he was a canon. Copenicus was never a professional astronomer, and the great work which has made him famous was accomplished in his spare time. During his stay in Italy, Copenicus had become aquainted with the idea of Greek philosopher, Aristarchus of Samos (third century B.C.), that the Earth and other planets revolved about the sun. Copenicus became convinced of the correctness of his heliocentric hypothesis, and when he was about forty he began to circulate among his friends a short, handwritten manuscript setting forth in preliminary form his own ideas on the subject. Copenicus spent many years taking the observations and making the calculations that were necessary for the composition of his great book, De revolutionibus orbium coelestium (On the Revolution of the Celestial Spheres), in which he describes his theory in detail, and sets forth the evidence for it. In 1533, when he was sixty years old, Copenicus delivered a series of lectures in Rome, in which he presented the principal points of his theory, without incurring papal disaproval. However , it was not until he was in his late sixties that Copenicus finally decided to have his book published; and it was not until the day he died, May 24th, 1543, that he received the first copy of his book from the printer. In his book, Copenicus correctly stated that the earth rotates on it's axis; that the moon revolves around the earth; and that the earth and other planets all revolve around the sun. However, like his predeccesors, he badly underestimated the scale of the solar system. Also, he was wrong in believing that the orbits consist of circles or of epicycles. Thus, his theory was not only complicated mathematically, but inexact as well. Nevertherless, his book promptly aroused great interest. It also motivated other astronomers, most notably the great Danish astronomer, Tycho Brahe, to make more accurate obsevations of planetary motions. It was from the observational data accumulated by Tycho that Johaness Kepler was finally able to deduce the correct laws of planetary motion. Though Aristarchus of Samos had propounded the heliocentric hypothesis more than seventeen centuries before Copenicus, it is appropriate that Copenicus has received the bulk of the credit. Aristarchus had made an inspired guess, but had never presented his theory in sufficent detail to make it scientifically useful. When Copenicus worked out the mathematics of the hypothesis in detail, he transformed it into a useful scientific theory-one that could be used for prediction, that could be checked against astronomical observations, and that could be meaningfully compared with the older theory that the earth was the center of the universe. It is clear that the Copenican theory has revolutionized our conception of the universe, and has led to major changes in our philosophical outlook. But in the evaluating the importance of Copenicus , it should be remembered that astronomy does not have the great range of practical applications that physics,chemistry, and biology do. In principle, one could construct such devices as a television set, an automobile, or a modern chemical factory without the slightest knowledge or application of Copenicus's theories. (One could not do so without applying the ideas of Faraday, Maxwell, Lavoisier, and Newton.) But to consider only the direct infleunce on Copenicus on technology would be to completely miss his true significance. Copenicus's book was the indispensable prologue to the work of both Galileo and Kepler. They in turn were the major predecessors of Newton, and it was their discoveries which enabled Newton to formulate his laws of motion and gravitation. Historically, the publication of De revolutionibus orbium coelestium was the starting point of modern astronomy-and, more importantly, the starting point of modern science.