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This interactive is used in Chapter 4. Johannes Kepler didn’t know why the planets behaved as they did, but he was a keen observer. In what became his 2nd law, he noted that planets move in a way that their orbits sweep out equal areas in equal time increments, even when those orbits are clearly elliptical. Use this Interactive to verify Kepler’s observation for yourself. Watch one of the planets or a comet move with the proper eccentricity in its orbit, or create a strange new orbit by moving the eccentricity slider. Make the time interval short or long. The graph will leave no doubt that the area swept out in each time interval always equals that of the one before.