An orbit is the path in space that one heavenly body makes in its movement around another heavenly body. The Moon, for example, makes an orbit around Earth, while Earth and the other planets make orbits around the Sun. The technical name for the orbiting body is satellite. The orbited body is called a primary. Because primaries are also in motion, the orbits described by satellites are elliptical rather than circular.
Satellites form stable orbits by counterbalancing two forces—their movement away from the primary and the force of gravity drawing them back toward the primary. In other words, in the absence of gravity a satellite would move in a straight line, which would soon take it away from its primary; in the absence of satellite motion, gravity would draw a satellite and its primary together until they collided.
Robinson, J. Hedley, and James Muirden. Astronomy Data Book. 2d ed. New York: John Wiley & Sons, 1979.
Smoluchowski, Roman. The Solar System: The Sun, Planets, and Life. New York: Scientific American Books, 1983.
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