Galileo discovers 3 moons

On this date in 1610, Galileo wrote a letter describing his discovery of three of Jupiter's moons. He had recently made some improvements to his telescope, and he discovered them in December. As he continued to observe them over the next few months, a fourth celestial body appeared, and he realized that they were actually orbiting the giant planet. Since most people at that time still believed in the Ptolemaic theory - that the Earth was the center of the universe and everything revolved around us - it was an important discovery. It went a long way toward confirming Copernicus's controversial theory that the Earth went around the Sun.