A diagram showing the path of a planet around the Sun.
Click on image for full size
Kepler's 2nd Law: The Speeds of Planets
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.
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