Copernicus and Kepler
Copernicus' book challenged Ptolemy's view that the earth was the center of the universe. Ptolemy was an important Greek astronomer and geographer who lived in Alexandria, Egypt, in the A.D. 100's. Ptolemy's theory required a complicated series of circular motions to account for astronomers' observations of how the planets appeared to move. Copernicus realized that if the earth and other planets traveled around the sun, a less complicated arrangement of circles could explain the observed motions of the planets. The ideas that Copernicus presented in this book differed so greatly from the traditional Ptolemaic theory that historians of science often speak of the "Copernican Revolution." But Copernicus' heliocentric (sun-centered) theory still did not accurately predict the motions of all the planets.
During the 1500's, a Danish astronomer named Tycho Brahe observed the motions of the planets far more precisely than they had ever been observed before. Brahe's work enabled Johannes Kepler, a German astronomer and mathematician, to lend new support to the heliocentric theory in 1609. Kepler used intricate calculations to show that the theory could explain the movements of the planets if the planets orbited the sun in elliptical (oval) paths rather than circular ones. Kepler's work marked the start of modern astronomy.
Excerpt adapted from the "Astronomy" article, The World Book Encyclopedia © 1999