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Kepler's first law states "all planets move in elliptical orbits with the Sun at one focus and the other focus empty". Applied to Earth satellites, the center of the Earth becomes one focus, with the other focus empty. For circular orbits, the two foci coincide.

Artist concept of the planets orbiting around the Sun. The equation "r = a(1-e^2)/(1+e cos theta)" flashes up. Next an artist concept of a satellite orbiting the Earth is shown. The Earth is at one focus, with the other one empty. Last a circular orbit, with the two foci coinciding, is shown.


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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