A site or facility for observing astronomical or meteorological phenomena.

The earliest recorded observatory was in Alexandria, north Africa, built by Ptolemy Soter in about 300 B.C.

The modern observatory dates from the invention of the telescope. Observatories may by ground based, carried on aircraft, or sent into orbit as satellites, in space stations, and on the space shuttle.

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.