Kepler's second law states that as a planet orbits the Sun, a line joining the planet and the Sun sweeps out equal areas in equal times. As a result, a planet must move faster in its orbit when it is near the Sun than when it is far from the Sun. If the planet moves in a circular orbit so that its distance from the Sun doesn't change, the planet's speed is constant. The more elongated the orbit, the greater than variation in orbital speed.

The simple animation below shows this change in orbital speed. You can alter the orbit shape by clicking on the buttons at the top of the blue region. The buttons are labeled with the orbital "eccentricity," a measure of the ellipticity of the orbit. Eccentricity = 0.0 corresponds to a circle. The larger the eccentricity, the more elongated the orbit. An eccentricity =1.0 corresponds to an open orbit.

To stop the animation, click on the blue area. It takes a moment or two to stop running.

Choose an eccentricity (E) for the orbit by clicking a button below