(1473-1543) Polish astronomer who developed the theory that the earth is a moving planet

Nicolaus Copernicus (Mikolaj Kopernik in Polish) was born in Thorn (now Torun, Poland). Following his father's death he was brought up by a wealthy and aristocratic uncle. He attended Jagiellonian University of Kraków. Through the influence of his uncle Copernicus was appointed a canon of the cathedral chapter of Frauenburg (now Frombork, Poland). The income from this position supported him for the rest of his life. The chapter gave him permission to continue his education in Italy, and Copernicus received a doctor's degree in law from the University of Ferrara. He also studied medicine at the University of Padua. When he returned to Poland, he acted as medical adviser to his uncle and served as canon.

In Copernicus' time, astronomers generally accepted a theory formulated by Ptolemy 1,400 years earlier. Ptolemy had said that the Earth was the center of the universe, and had no motion. In his efforts to justify that the Earth was the center of the universe, Ptolemy had been forced to propose some vastly elaborate and complex orbits. One of the apparent contradictions that Ptolemy had to explain was the fact that at certain times of the year some planets seemed to be motionless for a few days, then actually to move backward. This behavior was shared by all the planets except Mercury and Venus. Even these two planets seemed to behave strangely, appearing to have orbits strictly limited to a region close to the sun. Instead of working from known observable facts to devise a theory to explain these behaviors, Ptolemy made the facts fit his preconceived idea of the Earth's position in the universe.

Copernicus, however, found Ptolemy's theory to be seriously flawed. Turning to the most up-to-date tables of planetary positions available, Copernicus tried to find a simple hypothesis that would explain them. He discovered almost immediately that he had to ignore Ptolemy's theory altogether. Instead, Copernicus assumed that the Sun was at the center of the Solar System with the planets orbiting around it. On this basis he was able to formulate an accurate and far simpler theory.

Working out his theory in full mathematical detail, Copernicus found that he could predict planetary positions to a far greater accuracy than had been possible under Ptolemy's system. In addition, the behavior of Venus and Mercury were easily explained -- the two planets as viewed from Earth never moved outside a region close to the Sun because their orbits always lay closer to the Sun that to the Earth's orbit. Conversely, since the other planets moved in orbits that were always farther from the Sun, the Earth periodically "overtook" them and caused them to appear to move backward across the sky.

Copernicus explained his theory in a book he titled Concerning the Revolutions of the Celestial Spheres, but decided against publishing it for fear of religious reprisals. However, word of Copernicus' theory spread to many of Europe's scholars and aroused great interest and speculation. In 1543, Copernicus, by then an old and ailing man, was persuaded by his friends to publish his book. Before sending it to the publisher, however, Copernicus took the wise precaution of adding a flattering dedication to Pope Paul III. The publisher was still worried about the reception it would have, however, and added a preface without Copernicus' knowledge. In it he stated that the Copernican theory was not advanced as a description of actual facts but as a novel and convenient device for the more accurate calculation of planetary positions. This preface undermined the impact of the book and damaged the astronomer's reputation, and Copernicus was by then too ill to defend himself. He died only a few hours after seeing the first copy of his book.

Despite the offending preface, Copernicus' work laid the foundations for the telescopic discoveries of Galileo, the planetary laws of Johannes Kepler, and the gravitation principle of Sir Isaac Newton.

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Mercury
Venus
Galileo
Johannes Kepler
Sir Isaac Newton

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