The Influence of Copernicus
That new cosmic view arrived in 1543 with a book titled On the Revolutions of the Heavenly Spheres, by the Polish astronomer Nicolaus Copernicus. Like Aristarchus, whom he may have read about in the writings of the ancient Greek author Plutarch, Copernicus put the sun at the center of the solar system. Copernicus was motivated largely by a philosophical conviction that the structure of the universe must be simple and elegant. The Ptolemaic universe was anything but. Today, we date the modern era of astronomy and cosmology to the “Copernican Revolution” fostered by Copernicus' groundbreaking book.
But Copernicus' theory failed to gain immediate acceptance because many people were not ready to give up the idea that Earth was the center of the universe. Religious authorities, in particular, were committed to an Earth centered cosmos. Realizing that he risked being denounced for contradicting the official position of the church, Copernicus had delayed publishing his book until he was on his deathbed.
Copernicus' theory, moreover, could not account precisely for the movements of the planets. Like the Greeks, Copernicus had assumed planetary movements to be circular—cross sections of great invisible spheres. The circle and sphere were perfect geometrical forms, and the heavens simply had to be the embodiment of perfection. But nature pays no attention to human ideas about how it should behave. The planets do not move in circular orbits, as the German mathematician and astronomer Johannes Kepler discovered in the 1600's.