Copernicus Moves the Sun to the Center

For more than a thousand years, most European scholars accepted the model of Ptolemy, a second-century astronomer who placed a motionless Earth at the center of the universe. In 1543 the Polish astronomer Nicolaus Copernicus published On the Revolutions of the Heavenly Spheres, arguing that the Earth and other planets orbit the Sun. His model explained puzzles such as the apparent backward motion of planets more simply, though it still used circular orbits and was not yet more accurate than Ptolemy's.

An unsigned preface added by the theologian Andreas Osiander presented the theory as a mere calculating device, which softened its impact. Its true significance came later, when Kepler, Galileo and Newton built on it. The Copernican idea mattered for the Enlightenment because it showed that a long-accepted authority could be overturned by reasoning and observation.