The United States Naval
Observatory, established in 1842 at Washington, D.C., as an outgrowth of the Navy Department's Depot
of Charts and Instruments, has responsibilities in the broad areas of navigation, time, and fundamental celestial reference systems. Because of these duties, it is one of the few institutions in the world to undertake astrometryÑthe determination, through continual observations, of the positions and motions of the Sun, the Moon, the planets, and principal stars. The data collected form the basis of the Nautical Almanac and ephemeris time.The
observatory's Washington site formerly housed photographic zenith tubes to
determine mean solar time, as well as two transit circles. (Transit circles are telescopes that make observations only as an object crosses the meridian. They are used to measure accurate positions of celestial bodies.) Still operating in Washington is a 26-in (66-cm) refracting telescope, the instrument Asaph Hall used to discover the two moons of Mars in 1877. Many atomic clocks are employed to determine atomic time, and the master clock establishes standard time for the United States. At the observatory's site in Flagstaff, Ariz., a 61-in (155-cm) astrometric reflector measures distances of faint nearby stars, and a 40-in (102-cm) Ritchey Chretien reflector observes comets and minor planets.