Johannes Kepler was a German astronomer and mathematician of the late sixteenth and early seventeenth centuries. His work was largely based on the work of his mentor, Tycho Brahe. Kepler was able to use Bahe's precise measurements (made before telescopes) to determine, mostly by trial and error, three laws that described the motion of the five planets then known.
The picture to the right depicts this law.
Expressed mathematically, then, this says that:
T is proportional to L3/2
or
T2 is proportional to L3
Though they fit with observation and certainly seemed to work, he had no theoretical basis for explaining why they were true. The theoretical framework came in the form of Newton's Law of Gravity, nearly a century later. Newton's universal gravitation can be used to prove that Kepler's Laws do indeed describe the motion of planetary bodies in orbit.
