When people first discovered that planets orbited around the sun, they thought that the orbits would be in the shape of a circle. It wasn't until 1601 that Johannes Kepler, using precise measurements taken by the astronomer Tycho Brahe, determined that the planets actually orbit in ellipses around the sun. An ellipse looks like a circle that has been "squashed" slightly, like an oval, or like an egg. Kepler also found that when a satellite orbits in an ellipse, it moves faster when it is close to the object it is orbiting and slower when it is farther away.

After discovering these two laws, Kepler went on to measure the 'eccentricity' of each planet's orbit. Eccentricity is a measure of how circular a satellite's orbit is. For a perfectly circular orbit the eccentricity is zero; elliptical orbits have eccentricities between zero and one.