The idea that Earth revolved around the Sun was one of the most significant scientific theories in history. It was important for three main reasons. First, it led to an understanding of planetary motion. Second, it led to the discovery of the other planets. Third, it led to other scientific discoveries. For example, over time it led to modern space programs and the many inventions those programs inspired. The idea that Earth revolved around the Sun led to the theory that Earth was shaped like a ball. In the year 1577, an astronomer named Tycho Brahe watched a comet, which over 74 days passed near Earth. Tycho's observations helped Johannes Kepler describe how objects moved in space.