Kepler's Third Law: The Harmonic Law

After determining his first two Laws of Planetary Motion, Kepler continued to study the orbits of the planets. Ten years later, he discovered a relation between the time of a planet's orbit nad its distance from the Sun:

The squares of the orbital periods of the planets around the Sun are proportional to the cubes of the orbital semimajor axes.

What does this mean? This means that if you know either how much time a planet's orbit around the Sun takes you can easily know it's average distance from the Sun, or vice-versa! Now you will often see Kepler's Third Law written like this:

P2=a3

Where P is the orbital period in Earth years and a is the length of the semimajor axis (average distance from the Sun) in Astronomical Units.

Now, you can try out Kepler's Third Law using the Kepler-Calc-O-Tron 2000. Just type in either the orbital period or the average distance from the Sun, press the appropriate button, and *presto* you'll have your answer! Try looking up the values for some of the planets in your textbook and try them out!

 Planet Orbital Period: (in Earth years) Planet Distance From Sun: (in Astronomical Units)