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