Kepler's 3rd Law: P2 = a3



A graph showing Kepler's 3rd law.
Click on image for full size (7K GIF)
Kepler's 3rd law is a mathematical formula. It means that if you know how long it takes a planet to go around the Sun (P), then you can determine that planet's distance from the Sun (a).

This formula also tells us that planets far away from the Sun take longer to go around the Sun than those that are close to the Sun.

Check out the table of orbital data for the planets. You will notice that Venus' orbital period (P) is longer than Mercury's, and the Earth's period is longer than Venus', and Mars' period is longer than Earth's...

Kepler's 3rd Law: P2 = a3



A graph showing Kepler's 3rd law.
Click on image for full size (7K GIF)
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).

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.

Kepler's 3rd Law: P2 = a3



A graph showing Kepler's 3rd law.
Click on image for full size (7K GIF)
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.


Last modified prior to September, 2000 by the Windows Team

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