Kepler's laws

1. The planets move in ellipses with the Sun at one focus.This means that the distance between a planet and the Sun changes during the year.

2. A line drawn from the planet to the Sun sweeps out equal areas in equal times
This means that planets (and other objects) move faster when they are closer to the Sun and slower when they are further away.

3. The ratio of the square of the period (T) of the planet about the Sun to the cube of the mean orbit radius (r) is a constant.

This means that if you work out the square of the period of one planet (the time it takes to go once round the Sun) and divide that number by the cube of its distance from the Sun the result that you get will be the sane for all the other planets.

The animation shows an example of laws one and two.