Epimetheus, in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn XI (or S11), Epimetheus is an irregularly shaped (nonspherical) body measuring about 89 mi (144 km) by 67 mi (108 km) by 61 mi (98 km); it orbits Saturn at a mean distance of 94,090 mi (151,422 km) and has equal orbital and rotational periods of 0.6942 earth days. It was discovered by R. Walker, Stephen M. Larson, and John W. Fountain in 1978 and confirmed in 1980 by Dale P. Cruikshank at the Univ. of Hawaii at Manoa from Voyager 1 photographs. Its surface is cratered, with several craters more than 18 mi (30 km) in diameter, and marked with both large and small ridges, valleys, and grooves as well. Epimetheus and Janus are co-orbital; that is, they share the same average orbit. About every fourth year—at closest approach—the lower, faster satellite overtakes the other, they exchange angular momentum, and the lower one is boosted into the higher orbit while the higher one drops to the lower orbit. The two moons may have formed from the disruption of a single satellite early in the formation of Saturn's satellite system.