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:
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:
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!