Born Galileo Galilei, his main contributions were, in astronomy, the use of the telescope in observation and the discovery of sunspots, lunar mountains and valleys, the four largest satellites of Jupiter, and the phases of Venus. In physics, he discovered the laws of falling bodies. Galileo was born near Pisa, on February 15, 1564. Galileo was taught by monks at Vallombroso and then entered the University of Pisa in 1581 to study medicine. In 1589 he became professor of mathematics at Pisa, where he is reported to have shown his students the error of Aristotle's belief that speed of fall is proportional to weight, by dropping two objects of different weight simultaneously from the Leaning Tower. At Padua, Galileo invented a calculating compass for the practical solution of mathematical problems. He showed little interest in astronomy, although beginning in 1595 he preferred the Copernican theory that the earth revolves around the sun to the Aristotelian and Ptolemaic assumption that planets circle a fixed earth. In 1609 he heard that a spyglass had been invented in Holland. In August of that year he presented a telescope, about as powerful as a modern field glass, to the doge of Venice. By December 1609, Galileo had built a telescope of 20 times magnification, with which he discovered mountains and craters on the moon. He also saw that the Milky Way was composed of stars, and he discovered the four largest satellites of Jupiter. By December 1610 he had observed the phases of Venus, which contradicted Ptolemaic astronomy and confirmed his preference for the Copernican system. Galileo's most valuable scientific contribution was his founding of physics on precise measurements.