A diagram showing the elliptical orbits of some solar system objects. Click on image for full size (15K GIF)

Ellipses are described mainly by the length of their two axes. The longest one is called the major axis, and the short one is the minor axis. The ratio of these two lengths determines the eccentricity (e) of the ellipse; it's a measure of how elliptical it is. Circles have e=0, and very stretched-out ellipses have an eccentricity nearly equal to 1.

It's important to note that planets, while they do move on ellipses, have nearly circular orbits. Comets are a good example of objects in our solar system that may have very elliptical orbits. Compare the eccentricities of the objects in the diagram.

Once Kepler figured out that planets move around the Sun on ellipses, he then discovered another interesting fact about the speeds of planets as they go around the Sun.