Kepler's Second Law: The Speeds of Planets

A diagram showing the path of a planet around the Sun. Click on image for full size (9K GIF) Kepler's second law he again discovered by trial and error. After some experimentation, Kepler realized that the line connecting the planet and the Sun sweeps out equal area in equal time. Look at the diagram to the left. What Kepler found is that it takes the same amount of time for the blue planet to go from A to B as it does to go from C to D. But the distance from C to D is much larger than that from A to B. It has to be so that the green regions have the same area. So the planet must be moving faster between C and D than it is between A and B. This means that when planets are near the Sun in their orbit, they move faster than when they are further away. Kepler's work led him to one more important discovery about the distances of planets.

A table of orbital data for the planets

Last modified prior to September, 2000 by the Windows Team

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