The Tercentenary of the Royal Greenwich Observatory was celebrated in 1975. The Royal Observatory was founded in Greenwich in 1675, but problems with light, magnetic, and atmospheric pollution so close to central London led to the relocation of various departments to other locations. By 1957 the institution had reconsolidated at Herstmonceux Castle in East Sussex. Once it left Greenwich it was known as the Royal Greenwich Observatory, while its original site in Greenwich was termed the Old Royal Observatory. The ORO buildings were restored and opened to the public by the National Maritime Museum, who were key partners with the RGO in the tercentenary celebrations. Responsibility for the RGO had passed in 1965 from the Admiralty to the Science Research Council, who had a supervisory role in the celebrations.

At the time of the tercentenary celebrations, Alan Hunter was director of the RGO, and Humphry Smith, the head of the Time Department, was chairman of the Tercentenary Committee. Another key contributor was Celia Hewerdine, secretary to the Tercentenary Committee after Patricia Hanning.

A major event of the tercentenary was a garden party on 18 July at which Princess Anne unveiled a bust of John Flamsteed, the first Astronomer Royal, and inaugurated a commemorative sundial. In addition, open days were held for members of various scientific societies and establishments and for the general public. An international historical conference on the Royal Observatory, Greenwich, was held at the NMM the week before the garden party and an international scientific symposium, 'The galaxy and the local group', was held at the RGO the week after the garden party. A tercentenary exhibition was installed by the NMM at the Queen's House, Greenwich, and was visited by the queen on 20 May. A commemorative evensong was held at Westminster Abbey on 20 July.

Physical souvenirs of the tercentenary were also organised: Professor W. H. McCrea was commissioned to write a short history of the RGO and The Times published a special supplement. The Post Office issued a commemorative stamp showing Flamsteed House. Bookmarks and a commemorative Wedgwood plate were commissioned, as well as postcards, transparencies, and leaflets.

RGO staff at all levels were closely involved in the festivities, and the RGO Club held its own celebration, with historical music and dancing.

The collection includes correspondence, memoranda, notes, minutes, drafts, photographs, and promotional material. Some files are restricted for data protection reasons.

(Historical source: G. A. Wilkins, A Personal History of the Royal Greenwich Observatory at Herstmonceux Castle, 1948-1990, published online, 2009, https://www.lib.cam.ac.uk/collections/departments/manuscripts-university-archives/significant-archival-collections/royal-0 )