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's second law he again discovered
by trial and error. 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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