Or, to be perfectly accurate, the center of mass of the system of objects (the orbiter and the orbitee's combined center of mass - for all intents and purposes the center of mass of the earth for any man made satellite) must lie at one of the foci of an ellipse describing the orbital path of the object. Which of course for a circular orbit of any man made object means exactly what jens said.

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Schroedinger's Cat

Dead And Alive