Copernicus

   For centuries astronomers had believed in the theory stated by Ptolemy in the 100's A.D. This was the belief that the earth was the center of the universe and that the other planets and the sun moved around it. This theory is called the geocentric ("earth-centered") theory, from the Greek words ge, meaning "Earth," and kentron, meaning "center."
   In the 1500's a Pole named Nicholas Copernicus came across ancient writings arguing that the sun was the center of the universe. This was the heliocentric theory, from the Greek word helios, mean­ing "Sun." The ancient theory interested and excited Copernicus, and he began a long period of study and observation. He became convinced that all the known facts of astronomy of his time were best explained by the heliocentric theory. His conclusions were published in 1543 in a book that was entitled On the Revolutions of the Heavenly Spheres.
   The book of Copernicus caused little excitement at the time. Few people believed in the heliocentric theory. It seemed to contradict the evidence of the senses. Anyone could "see" that the Sun and planets moved around the Earth. Anyone could "feel" that the Earth was solid and not moving.
   Copernicus could not test and prove the helio­centric theory with the instruments or the mathe­matics available to him. Proof of the heliocentric theory had to wait for the work of two later scien­tists, a German named Kepler and an Italian named Galileo.