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Kepler's 3rd Law: The Square of the Period = the Cube of the Distance - Windows to the Universe

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A graph showing Kepler's 3rd law.
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# Kepler's 3rd Law: P2 = a3

Kepler's 3rd 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), because P2 = a3.

It also tells us that planets which are far away from the Sun have longer periods than those close to the Sun. They move more slowly around the Sun. Look at the formula: if you make the distance, a, large, then P must also be large.

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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