Circular Orbits:
  Limiting case of an ellipse.
•Centripetal acceleration (v2/r) caused by Gravity
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•Period found by
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•Kepler’s 3rd Law just comes from this
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•Given P and a (and G) we can find the mass of a planet or star, and in building an alien solar system, Kepler’s laws must be valid!
From Horizons, by Seeds