In 1609 Galileo Galilei heard about a new instrument that is now known as the telescope. He set out to make his own, and when he turned it to the sky he made a series of unexpected discoveries. He found many new stars that were invisible to the eye, mountains on the Moon, and four moons in orbit around Jupiter. Later he found that the Sun's surface was spotted and seemed to rotate, and that Venus went through a series of phases like the Moon. These discoveries cast doubt on the traditional Aristotelian-Ptolemaic model of the universe, but it was Galileo’s investigation of motion that helped to finally bring down the traditional views. Galileo’s experimental work and mathematical insight led him to conclude that all bodies fall to the Earth with the same constant acceleration, and that bodies moving horizontally will continue in their motion unless impeded. This new understanding of motion made the idea of a moving Earth more plausible. Although Galileo’s work led to conflict with authorities in the Catholic Church, his work helped many future astronomers to embrace the heliocentric models of Copernicus and Kepler.