Dione (dĪōˈnē) [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn IV (or S4), Dione is 695 mi (1,120 km) in diameter, orbits Saturn at a mean distance of 234,500 mi (377,400 km), and has an orbital period of 2.737 earth days—the rotational period is unknown but is assumed to be the same as the orbital period. It was discovered in 1684 by the Italian-French astronomer Gian Domenico Cassini. Aside from Titan, Dione is the densest of Saturn's satellites; it is believed to be composed primarily of water ice with a considerable fraction of denser material, such as silicate rock. The trailing hemisphere is more heavily cratered than the leading hemisphere, which is the reverse of the cratering on most of the other Saturnian satellites. Another moon, Helene, is co-orbital with Dione; that is, it orbits Saturn at the same distance as Dione, and precedes Dione by about 60°. Dione also forms a satellite pair with Enceladus; that is, the two moons interact gravitationally.