Pan, in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn XVIII (or S18), Pan is 12.5 mi (20 km) in diameter, orbits Saturn at a mean distance of 83,000 mi (133,583 km), and has an orbital period of 0.575 earth days. The rotational period is unknown but is assumed to be the same as the orbital period. It was discovered by Mark R. Showalter at the Ames Research Center in California in 1990 while reviewing photographs taken by Voyager 1 during its flyby of Saturn in 1980. The innermost of Saturn's confirmed moons, Pan's orbit is within the Encke Division, or Encke Gap, of Saturn's A ring, where it functions as a shepherd satellite (a moon that limits the extent of a planetary ring through gravitational forces), keeping the gap open.