Kepler's Laws

Kepler's Laws are three laws discovered by Johannes Kepler that describe the way the planets move round the Sun. They are:

1. A planet moves in an ellipse with the Sun at one focus.
2. The radius vector (the line joining the Sun and the planet) sweeps out equal Aareas in equal times.
3. The square of the period of revolution of a planet around the Sun is proportional to the cube of the semi-major axis of its orbit.

Sir Isaac Newton proved that these laws are true if and only if gravity follows an inverse square law. In practice, the laws are not followed exactly in the Solar System due to the mutual attraction of the planets for each other, which causes perturbations in their orbits, but the effect is small.