Charon, also called (134340) Pluto I, is the largest of the five known moons of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS). It is a very large moon in comparison to its parent body, having half the diameter and one eighth the mass of Pluto. Its gravitational influence is such that the barycenter of the Pluto–Charon system lies outside Pluto.