In addition to determining that the orbits of the planets around the Sun were ellipses, Kepler also noticed that their speeds varied throughout their journies Kepler noted that the planets seemed to move fastest when they were at their closest point to the Sun (called perihelion) and slowest when they were at their farthest point from the Sun (called aphelion). Using some rather brilliant insights of geometry, Kepler discovered that:

This is now known as Kepler's Second Law or The Equal Areas Law.

The motion this law describes also tells us that the average distance from a planet to the Sun is equal to the length of the semimajor axis. That's why astronomers love the term so much!

You can watch this law in effect with the Orbit-Area-O-Tron 2000. Simply enter in an eccentricity and watch as three areas are swept out from the orbit. Where is the planet moving fastest, when it is close to the Sun or far from the Sun? When is it moving slowest?