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Kepler's second law, the law of areas, states "the line joining the planet to the Sun sweeps over equal areas in equal time intervals". When a satellite orbits. the line joining it to the Earth sweeps over equal areas in equal periods of time. If areas 1, 2, and 3 are equal. times 1, 2, and 3 are also equal. Therefore, the speed of the satellite changes, depending on its distance from the center of the Earth.

Video shows artist concept of planets orbiting the Sun. Then switches to a satellite orbiting the Earth. Three different area A1, A2, and A3 are shaded around the orbit. Each of these areas is a different shape, but the same size. Times T1, T2, and T3 are show as the time to cover the parts of the orbit that hit each of the areas. Areas that are closer to the Earth have a longer path for the satellite to cover, so the satellite has to move faster.


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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