SATELLITE

SATELLITE

SATELLITE, in astronomy, secondary object that revolves in a closed orbit about a planet or star, referred to as the primary of the satellite. The best-known satellite is the earth's moon--just as the earth itself is a satellite of the sun--although the moon and earth are close enough in size to be considered sometimes as a double-planet system. The motion of most of the solar system's known satellites about their planets is direct--that is, from west to east--and in the same direction as the rotation of their planets. Only a few satellites of the large outer planets revolve in the retrograde direction--that is, from east to west--and opposite the direction of rotation of their planets; they probably were captured by the planet's gravitational fields some time after the formation of the solar system. Many astronomers believe that Pluto, which moves in an independent orbit about the sun, is an escaped satellite of Neptune; Pluto itself was recently discovered to have a satellite. Information about the individual satellites is given in the articles on the planets that they orbit.