5. Competing with Ptolemy

Abstract

In 1515, a full Latin translation of Ptolemy’s account of his geocentric cosmology was printed. Copernicus soon realized that his own radical cosmology would need a comparable book—with a full set of observed planetary positions—to be taken seriously. ‘Competing with Ptolemy’ explains how he spent more than two decades collecting the specific observations for his magnum opus, De revolutionibus. But how could he measure the position of a slowly moving planet? How would he deal with the precession of the equinoxes? This was the starting problem for Copernicus; he had to get precession under control before he could get into serious details of the planetary motions.