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the Sun (perihelion), as in C, it moves much faster than when farther away (aphelion), as in A. In other words, the three shaded areas A, B, and C are equal. Any planet traveling along the elliptical path, takes the same amount of time to cover the distance the three red arrows show. This is why planets move faster when closer to the Sun.10
Kepler’s clever study of the relative speeds and positions of the planets in their elliptical orbits about the Sun led to his first two laws. These laws also explained planetary brightness variations and some peculiar non-uniform motions that could not be clarified by circular motion and epicycle theory. When Kepler modified Copernicus’ theory, he really simplified our solar system model enabling us to predict motions more accurately. We could apply his laws to any orbiting object.
10 Chaisson, McMillan. Astronomy Today. 45.
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