Johannes Kepler (1571-1630) continued the study of planetary motion. He used Tycho Brahe's observations and his own mathematical mind to develop three basic laws of planetary motion. The first of Kepler's laws is that the orbit of each planet is elliptical. In other words, the planets do not move in perfect circles. The orbit will vary slightly each time making it elliptical. The second law Kepler devised deals with equal areas. Every planet revolves around the sun. At each position, an imaginary line may be drawn connecting the planet to the sun. The imaginary line will cover an equal amount of area in an equal amount of time. Therefore, Kepler concluded that a planet will move faster, the closer it is to the Sun. Kepler's third law states that the orbital period of a planet is proportional to its distance to the sun. This can be expressed by the equation of a "planets orbital period squared equals its mean solar distance cubed."* Kepler's laws really helped support the heliocentric theory, although Kepler did not determine the forces that produced the planetary motion. However, Galileo Galilei and Sir Isaac Newton did determine the forces.
* Lutgens and Tarbuck, p. 497