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Nicholas Copernicus (1473-1543) learned the works of Peurbach and Regiomontanus in the undergraduate curriculum at theuniversity of Cracow and then spent a decade studying in Italy. Upon his return to Poland, he spent the rest of his lifeas a physician, lawyer, and church administrator. During his spare time he continued his research in astronomy. The resultwas De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orbs"), which was published inNuremberg in 1543, the year of his death. The book was dedicated to Pope Paul III and initially caused litle controversy. Ananonymous preface (added by Andreas Osiander, the Protestant reformer of Nuremberg) stated that the theory put forward inthis book was only a mathematical hypothesis: the geometrical constructions used by astronomers had traditionally had onlyhypothetical status; cosmological interpretations were reserved for the philosophers. Indeed, except for the first elevenchapters of Book I, De Revolutionibus was a technical mathematical work in the tradition of the Almagest .

Diagram of the Copernican system, from De Revolutions
But in the first book, Copernicus stated that the Sun was the center of the universe and that the Earth had a triple motion
A daily rotation about its center, an annual motion around the Sun, and a conical motion of its axis of rotation. This lastmotion was made necessary because Copernicus conceptualized the Earth's annual motion as the result of the Earth beingembedded in a spherical shell centered on the Sun. Its axis of rotation therefore did not remain parallel to itself withrespect to the fixed stars. To keep the axis parallel to itself, Copernicus gave the axis a conical motion with aperiod just about equal to the year. The very small difference from the annual period accounted for the precesion of theequinoxes, an effect caused by the fact that the Earth's axis (in Newtonian terms) precesses like a top, with a period ofabout 26,000 years. (Copernicus's ideas about this precession were more cumbersome and based on faulty data.)
around this center. His theory gave a simple and elegant explanation ofthe retrograde motions of the planets (the annual motion of the Earth necessarily projected onto the motions ofthe planets in geocentric astronomy) and settled the order of the planets (which had been a convention in Ptolemy's work)definitively. He argued that his system was more elegant than the traditional geocentric system. Copernicus still retained thepriviledged status of circular motion and therefore had to construct his planetary orbits from circles upon and withincircles, just as his predecessors had done. His tables were perhaps only marginally better than existing ones.

The reception of De Revolutionibus was mixed. The heliocentric hypothesis was rejected out of hand by virtuallyall, but the book was the most sophisticated astronomical treatise since the Almagest , and for this it was widely admired. Its mathematical constructions were easilytransferred into geocentric ones, and many astronomers used them. In 1551 Erasmus Reinhold, no believer in the mobility ofthe Earth, published a new set of tables, the Prutenic Tables , based on Copernicus's parameters. These tables came to be preferred for their accuracy. Further, De revolutionibus became the central work in a network of astronomers, who dissected it in great detail. Not until ageneration after its appearance, however, can we begin point to a community of practicing astronomers who accepted heliocentriccosmology. Perhaps the most remarkable early follower of Copernicus was Thomas Digges (c. 1545-c.1595), who in A Perfit Description of the Coelestiall Orbes (1576) translated a large part of Book I of De Revolutionibus into English and illustrated it with a diagram in which the Copernican arrangement of the planets isimbedded in an infinite universe of stars.

Practice Key Terms 3

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Source:  OpenStax, Galileo project. OpenStax CNX. Jul 07, 2004 Download for free at http://cnx.org/content/col10234/1.1
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