Naval Observatory, United States, a division of the Office of the Chief of Naval Operations, Navy Department. The Naval Observatory is the countrys oldest national observatory. It calculates the correct time with atomic clocks and electronic devices, and checks the calculations with astronomical observations. It also detects and records fluctuations in the rotation speed of Earth. It comprises a main station in Washington, D.C., and substations in Flagstaff, Arizona; Richmond, Florida, near Miami; and Black Birch, New Zealand. It also runs the radio telescopes in Green Bank, West Virginia.

The Naval Observatory is the only institution in the United States making continual observations of the positions and motions of the moon, planets, sun, and stars, and monitoring the position of Earth. It also maintains the U.S. reference for accurate time. The observatory distributes its data through almanacs and related publications, electronic data links, and satellites. U.S. and international civilian and scientific agencies use this information primarily for navigation and scientific research. The Astronomical Almanac, the Nautical Almanac, and the Air Almanac, which are used in navigation, are compiled from these observations.

The Naval Observatory grew out of the Depot of Charts and Instruments, established in 1830 to inspect and store various navigational instruments for the navy. In 1893, the observatory was moved to its present location at 34th Street and Massachusetts Avenue, Washington, D.C. Among large instruments owned by the observatory are a 26-inch (66-cm) refracting telescope; and, at Flagstaff, Arizona, 40-inch (102-cm) and 61-inch (155-cm) reflecting telescopes used to measure distances to the stars. In 1978, the telescope at Flagstaff was used to discover Pluto's only known satellite, since named Charon. The refracting telescope in Washington, D.C., was used to discover the two moons of Mars in 1877. The observatory also operates telescopes in Florida and New Zealand.