As a young man, the Polish cleric Nicolaus Copernicus (1473–1543) was drawn to the vitality of the Italian Renaissance. After studies at the University of Kraków, he departed for Italy, where he studied astronomy, medicine, and church law. Copernicus noted that astronomers still depended on the work of Ptolemy for their most accurate calculations, but he felt that Ptolemy’s cumbersome and occasionally inaccurate rules detracted from the majesty of a perfect creator. He preferred an alternative ancient Greek idea: that the sun, rather than the earth, was at the center of the universe.

Finishing his university studies and returning to a position in church administration in East Prussia, Copernicus worked on his hypothesis from 1506 to 1530. Without questioning the Aristotelian belief in crystal spheres or the idea that circular motion was divine, Copernicus theorized that the stars and planets, including the earth, revolved around a fixed sun.

The Copernican hypothesis had enormous scientific and religious implications, many of which the conservative Copernicus did not anticipate. First, it put the stars at rest, their apparent nightly movement simply a result of the earth’s rotation. Thus, his hypothesis destroyed the main reason for believing in crystal spheres capable of moving the stars around the earth. Second, Copernicus’s theory suggested a universe of staggering size. If, in the course of a year, the earth moved around the sun and yet the stars appeared to remain in the same place, then the universe was unthinkably large. Third, by using mathematics instead of philosophy to justify his theories, he challenged the traditional hierarchy of the disciplines. And by characterizing the earth as just another planet, Copernicus destroyed the basic idea of Aristotelian physics — that the earthly sphere was quite different from the heavenly one.

Other events were almost as influential as the Copernican hypothesis in creating doubts about traditional astronomy. In 1572, a new star appeared and shone very brightly for almost two years. The new star, which was actually a distant exploding star, made an enormous impression on people. It seemed to contradict the idea that the heavenly spheres were unchanging and therefore perfect. In 1577, a new comet suddenly moved through the sky, cutting a straight path across the supposedly impenetrable crystal spheres. It was time, as a sixteenth-century scientific writer put it, for “the radical renovation of astronomy.”1