There are three main laws of planetary motion which were formulated by Kepler. They are used to describe the motions of planets, moons and other satellites in orbit around larger bodies.
Kepler’s first law says that the orbits of the planets are ellipses, with the Sun at one focus of the ellipse. For a long time it was believed that the planets orbited around the Sun in perfect circles. In fact, the path of an orbit is not circular but a different shape called an ellipse - which is basically a circle that has been stretched on two ends. These ends are called “centers”, or focuses (foci). The Sun lies near one of these focuses of the elliptical path.
Kepler’s second law tells us that in an orbit, the imaginary line between a planet and the Sun sweeps out equal areas in equal times. You see, when a planet is in the section of its orbit that is closest to the Sun, it moves more quickly. When it is further away from the Sun, it moves more slowly. However, no matter what sections of the orbit you measure, the line between a planet and the Sun will always cover the same amount of area in a given time.
To see a simulation of Kepler's second law click here (requires a Java enabled browser).
Finally, Kepler’s third law says that the ratio of the cube of the distance of a planet from the Sun and the square of the orbital period is the same for all of the planets. Basically this law describes a relationship between the distance of a planet from the Sun, and the time it takes for that planet to orbit the Sun. If you cube the distance, the number you come up with will be pretty much exactly the same as the square of the time it takes the planet to complete an orbit around the Sun.
For more on Kepler see History Page 5.
