Royal Greenwich Observatory, a former astronomical observatory in Britain, originally located in Greenwich, a borough of London. Founded in 1675, the observatory was Britain's oldest scientific institution when it closed in 1998. It was known for its work in timekeeping, navigation, and astronomy. The meridian passing through Greenwich is the prime meridian (0 longitude), the starting point for measuring longitude and calculating standard time.

The Royal Greenwich Observatory was founded by King Charles II. In 1948 it was moved to Herstmonceux Castle in Sussex, and in 1990, to Cambridge. The original Greenwich site became a museum of astronomy and time. With the closure of the Royal Greenwich Observatory in 1998, its programs were transferred to other institutions, including the Astronomy Technology Centre in Edinburgh, Scotland.