In the early 17th century, Kepler discovered the following three laws of planetary motion:
Kepler discovered these laws empirically. While he searched for a reason behind the laws, he was never able to discover why the laws were true. He only new that they worked. It wasn't until Newton formulated his three laws of motion and law of gravity that the "why" behind Kepler's laws was found. Newton was the first person to be able to derive Kepler's laws from fundamental principles.
Follow the links for each derivation.
Note: The derivation for the 1st Law involves doing Newton's Laws in polar coordinates, which we don't talk about in class. Here is a brief summary. It also involves doing conic sections in polar coordinates, so here is a brief summary of that. If the Word-generated html is too messy, here are PDFs of the originals: Conic Sections and Polar Coordinates.