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 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.