De revolutionibus orbium coelestium (On the revolutions of the heavenly spheres), written by Polish astronomer Nicolaus Copernicus (1473–1543) and published just before his death, placed the sun at the center of the universe and argued that the Earth moved across the heavens as one of the planets. Copernicus anticipated his ideas would be controversial and waited more than 30 years to publish his book. De Revolutionibus opens with a brief argument for the heliocentric universe and follows with an extensive set of mathematical proofs and astronomical tables. Copernicus was not trying to disparage the accepted wisdom of astronomers and religious thinkers; instead he sought to uncover a more elegant order in the universe. His ideas were revolutionary, but they built on an existing line of thinking. The movement of Mercury and Venus had long perplexed philosophers and astronomers. Plato and Eudoxus noted that these planets never strayed far from the sun; it was almost as if they were tethered to the sun, as they could only move a bit ahead of or lag a bit behind it. In the fifth century, Martianus Capella had argued that Mercury and Venus orbited the sun, which in turn rotated around the Earth. Aristarchus of Samos had proposed a heliocentric system and the Pythagoreans before him had argued that the sun was the "central fire." Although not part of the mainstream, these were all ideas that Copernicus built upon. While Copernicus made revolutionary contributions to astronomy, his conception of the solar system was fundamentally different from that of present-day science. His model still assumed perfect circular motion in the heavens. This meant that, like Ptolemy, he needed to use circles on circles, or epicycles, to account for the movement of the planets. Copernicus's circles were much smaller than those used in the Ptolemaic system, but they still were required to make his model work. Later astronomers, including Johannes Kepler (1571–1630), Galileo (1564–1642), and Isaac Newton (1642–1727), all built upon the work of Copernicus to advance humanity’s understanding of the solar system.