Kepler put forward three laws of planetary motion: 1. Planets move on ellipses with the sun at one focus of the ellipse. 2. The orbit of a planet sweeps out equal areas in equal times. Thus, planets move more quickly when they are closer to the sun, more slowly when they are farther away. 3. Larger orbits have a longer orbital period. In particular, the square of the period is proportional to the cube of the long axis of the ellipse. This relates the orbits of different planets to one another.
