# Kepler’s 3^{rd} Law: P^{2} = a^{3}

**Kepler’s 3rd Law: P2 = a3** Kepler’s 3^{rd} **law **is a mathematical formula. It means that if you know the period of a planet’s orbit (P = how long it takes the planet to go around the Sun), then you can determine that planet’s distance from the Sun (a = the semimajor axis of the planet’s orbit).

It also tells us that planets that are far away from the Sun have longer periods than those close to the Sun. They move more slowly around the Sun.

Check out the table of orbital data for the planets. You will notice that the **orbital period (P)** of the planets gets longer as you go from Mercury to Pluto.

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