Nicolaus Copernicus (1473–1543) was a Polish astronomer whose book De revolutionibus orbium coelestium (in English, On the Revolutions of the Celestial Spheres) refuted the geocentric, or earth-centered, theory of the structure of the universe. The Greek philosopher Aristotle believed that the sun, moon, planets, and stars all revolved in concentric spheres around the earth. The Greco-Roman astronomer and mathematician Ptolemy expanded this concept, devising mathematical models to calculate and predict the motion and position of the heavenly bodies while still accepting the earth-centered paradigm. Copernicus was troubled that Ptolemy’s geocentric model of the universe failed to consider Aristotle’s requirement for the uniform circular motion of all celestial bodies. On the Revolutions of the Celestial Spheres refuted the geocentric theory and argued instead for an astronomical model that placed the sun at the center of the solar system—a view referred to as the heliocentric model. The Copernican model conceived a universe in which the distances of the planets from the sun had a direct relationship to the size of their orbits.