Using Brahe's Data Kepler theorized that the planet actually did not move in a circular motion they moved in an elliptical motion. Brahe and Kepler did not get a long well, mostly because they disagreed, so apparently Brahhe only let Kepler see some of his data. So when they worked together Brahe would give Kepler the hard to figure out Data, well, he didn't realize that it would lead to Kepler figuring out the laws of planetary motion. Kepler believed in the Copernican system, with the sun at the center of the universe. Once Kepler realized that the reason the orbits were hard to calculate mathematically was because Copernicus put all the planets in circular orbits and not eliptical everything fell together. The reason it made a difference is because ellipses are oval in shape therefore will have a shorter distance between two points and a longer distance at the oposite two points, and circles have the same distance all the way around. This makes a big differnce mathematically. This led to the first law of Planetary motion that all orbits are elipses. The second law basically states that the line joining the planet to the sun sweeps out equal areas in equal times as the planet travels around the elipses. In other words, you can calculate where the planet will be at certain time because it always travels at the same speed in the same elipses. The third law states that the ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axis. What this means is that the inner most planets take less time to travel around the sun than the outer most planets. The site linked below has pictures and formulas and is very informative. This would be a good site to learn the theories from. The information is understanable and explained will and has links to animations that even children could understand.