enough by it to want more detail. His model was later used to create the calendar of Pope Gregory XIII. However, the idea that the earth moved around the sun was doubted by most of Copernicus' contemporaries. It contradicted not only empirical observation, due to the absence of an observable stellar parallax, but more significantly at the time, the authority of Aristotle. The discoveries of Kepler and Galileo gave the theory credibility. Kepler was an astronomer who, using the accurate observations of Tycho Brahe, proposed that the planets move around the Sun not in circular orbits but in elliptical ones. Together with his other laws of planetary motion, this allowed him to create a model of the Solar System that was an improvement over Copernicus' original system. Galileo's main contributions to the acceptance of the heliocentric system were his mechanics, the observations he made with his telescope, as well as his detailed presentation of the case for the system. Using an early theory of inertia, Galileo could explain why rocks dropped from a tower fall straight down even if the earth rotates. His observations of the moons of Jupiter, the phases of Venus, the spots on the Sun, and mountains on the Moon all helped to discredit the Aristotelian philosophy and the Ptolemaic theory of the Solar System. Through their combined discoveries, the heliocentric system gained support, and at the end of the 17th century it was generally accepted by astronomers. This work culminated in the work of Newton, and his Principia formulated the laws of motion and universal gravitation which dominated scientists' view of the physical universe for the next three centuries. By deriving Kepler's laws of planetary motion from his mathematical description of gravity, and then using the same principles to account for the trajectories of comets, the tides, the precession of the equinoxes, and other phenomena, Newton removed the last doubts about the validity of the heliocentric model of the cosmos. This work also demonstrated that the motion of objects on Earth and of celestial bodies could be described by the same principles. His prediction that the Earth should be shaped as an oblate spheroid was later vindicated by other scientists. His laws of motion were to be the solid foundation of mechanics; his law of universal gravitation combined terrestrial and celestial mechanics into one great system that seemed to be able to describe the whole world