Nicolaus Copernicus was a Polish astronomer. He became famous for explaining that the Earth is a spinning planet that travels around the sun. These motions make the sun, the other planets and the stars appear to move in the sky. The ancient Greek astronomer Ptolemy had said that the Earth did not move. He said the sun, planets and stars moved around the Earth. In 1543, Copernicus explained his ideas in a book, on the Revolutions of the Heavenly Spheres. Later, scientists proved that Copernicus's explanation was correct. How to cite this article: To cite this article, World Book recommends the following format: "Nicolaus Copernicus." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.