United States Naval Observatory, a federal astronomical observatory, located in Washington, D. C. It evolved from the Navy's oldest scientific institution, the Depot of Charts and Instruments, founded in 1830; the observatory was completed in 1844 and moved to its present site in 1893. It was formerly administered through the Bureau of Navigation and is now under the jurisdiction of the chief of naval operations. The principal instrument at the Washington headquarters is an Alvan Clark 26-in. refracting telescope, which has been in almost continuous operation since its installation in 1873, when it was the largest of its kind in the world. The original mounting and drive were replaced during the 1893 move. Other equipment includes a number of ordinary refracting and reflecting telescopes and special telescopes (photographic zenith tubes) used in the precise determination of time. The observatory's Flagstaff Station in Arizona has 61-in. and 40-in. reflecting telescopes; in 1978, J. Christy discovered Pluto's moon Charon with the 61-in. instrument. The main programs of the Naval Observatory involve continual observations of the positions and motions of celestial bodies for astronomical and navigational purposes and for the derivation and broadcasting of accurate time signals. Atomic clocks, cesium clocks, mercury ion clocks, and hydrogen maser frequency standards are all used for the observatory's time system, which is accurate to within a few billionths of a second per day. Since 1894 the U.S. Naval Observatory has included the Nautical Almanac Office, which publishes the American Ephemeris and Nautical Almanac. The observatory also has an extensive library.