The national astronomical observatory of the U.K., founded in 1657 at Greenwich, S.E. London, England, to provide navigational information for sailors.

After World War II, it was moved to Herstmonceux Castle, Sussex; in 1990, it was transferred to Cambridge. It also operates telescopes on La Palma in the Canary Islands, including the William Herschel Telescope, commissioned in 1987.