During the renaissance a priest named Nicolaus Copernicus thought, from looking at the way the planets moved, that the Earth was not the center of everything. Based on previous works, he said that the Earth was a planet and all the planets moved around the sun. This brought back the old idea of heliocentrism. A physicist called Galileo Galilei built his own telescopes, and used them to look more closely at the stars and planets for the first time. He agreed with Copernicus. The Catholic Church decided that Galileo was wrong. He had to spend the rest of his life under house arrest. Heliocentric ideas were soon improved by Johannes Kepler and Isaac Newton who invented the theory of gravity.