Herstmonceux Castle, East Sussex, was home to the Royal Greenwich Observatory from 1957 until 1990, when it was again moved to Cambridge until its closure in 1998.

The castle was built in the 1440s by Sir Roger Fiennes, Treasurer of the Household of Henry VI of England, on the site of his manor house, with construction completed by his son, Lord Dacre. It is considered one of the finest early brick buildings in England, built with local labour and the help of craftsmen from the Low Countries. The castle remained in the hands of the Fiennes family until 1708 when it was bought by George Naylor, a lawyer of Lincoln's Inn. Following his death in 1730, it passed through the hands of various successors. Naylor's half-brother, Robert Hare, demolished the interior, which had fallen into disrepair, in 1776, effectively turning the castle into a picturesque ruin. It lay in an abandoned state until 1913 when a major reconstruction was begun by Lieutenant-Colonel Claude Lowther, based on plans drawn up by architect, Walter Godfrey.

Most of the outer walls, along with the north and south gatehouses and walled gardens had remained intact, so Lowther rebuilt the impressive south range, and retained the exterior style of the original building, with the major change of incorporating the four original internal courtyards into one. The existing interiors largely date to this period, including much of the interior panelling and woodwork. The restoration work was completed, by the next owner, Sir Paul Latham, in 1933. Latham installed the magnificent carved library staircase and doors, which were acquired from Wheatley Hall, Doncaster, and also re-flooded the moat for the first time since 1570.

In 1946, the Admiralty bought the site and grounds of Herstmonceux Castle as the new location for the Royal Observatory. The Observatory had been housed at Greenwich since 1675, but by the mid-20th century, light pollution in London had necessitated the move. Following the move, which was completed c.1957, the newly located Observatory was henceforth known as the Royal Greenwich Observatory (RGO), to distinguish itself from its earlier incarnation and from the site itself in Greenwich Park, now a museum and properly referred to as the Royal Observatory, Greenwich.

During the time of the RGO, various buildings were added to the castle grounds to facilitate the work of the Observatory, including the Solar Dome, West Building, and the Equatorial Group of Telescopes. In 1988-1990, the Observatory moved again to Cambridge, following a decision of the Particle Physics and Astronomy Research Council (PPARC, later SERC). The site at Herstmonceux was sold in May 1989 and closed in 1990. The Castle and estate was later bought in 1993 by Queen's University, Ontario. The Equatorial Group of Telescopes is currently under the care of the Observatory Science Centre, and can be visited by the public, along with the castle grounds and gardens.