[3] At that time the king also created the position of Astronomer Royal, to serve as the director of the observatory and to "apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation."

The scientific work of the observatory was relocated elsewhere in stages in the first half of the 20th century, and the Greenwich site is now maintained almost exclusively as a museum, although the AMAT telescope became operational for astronomical research in 2018.

[8][page needed] Greenwich Palace, on the site of the present-day National Maritime Museum, was the birthplace of both Henry VIII and his daughters Mary I and Elizabeth I; the Tudors used Greenwich Castle, which stood on the hilltop that the Observatory presently occupies, as a hunting lodge.

Greenwich Castle was reportedly a favourite place for Henry VIII to house his mistresses, so that he could easily travel from the Palace to see them.

[9][page needed] In 1676 the main building of the observatory, now known as Flamsteed House, was completed on Greenwich hill.

Flamsteed House, the original part of the Observatory, was designed by Sir Christopher Wren, probably assisted by Robert Hooke, and was the first purpose-built scientific research facility in Britain.

It was built for a cost of £520 (£20 over budget; equivalent to £99,000 in 2023) out of largely recycled materials on the foundations of Duke Humphrey's Tower, the forerunner of Greenwich Castle, which resulted in the alignment being 13 degrees away from true North, somewhat to Flamsteed's chagrin.

The original observatory housed the astronomer royal, his assistant and his family as well as the scientific instruments to be used by Flamsteed in his work on stellar tables.

[12] By the later 18th century it incorporated additional responsibilities such as publishing the Nautical Almanac, advising government on technical matters, disseminating time, making meteorological and magnetic observations and undertaking astrophotography and spectroscopy.

[18] The Astronomer Royal Airy was an advocate of this and the transit circle instrument he had installed in 1851 was used for a century for positional astronomy.

[22] It was equipped with a filar micrometer by Peter Dollond and was used to provide a report of the events as seen through the small refractor.

Since the first triangulation of Great Britain in the period 1783–1853, Ordnance Survey maps have been based on an earlier version of the Greenwich meridian, defined by the transit instrument of James Bradley.

When the Airy circle (5.79 m to the east) became the reference for the meridian, the difference resulting from the change was considered small enough to be neglected.

When a new triangulation was done between 1936 and 1962, scientists determined that in the Ordnance Survey system the longitude of the international Greenwich meridian was not 0° but 0°00'00.417" (about 8 m) east.

[26] International time from the end of the 19th century until UT1 was based on Simon Newcomb's equations, giving a mean sun about 0.18 seconds behind UT1 (the equivalent of 2.7 arcseconds) as of 2013; it coincided in 2013 with a meridian halfway between Airy's circle and the IERS origin: 51°28′40.1247″N 0°0′2.61″W / 51.477812417°N 0.0007250°W / 51.477812417; -0.0007250.

[21] It was agreed that the (Prime) "meridian line marked by the cross-hairs in the Airy Transit Circle eyepiece would indicate 0° longitude and the start of the Universal Day".

[29] In 1948, the Office of the Astronomer Royal was moved to Herstmonceux in East Sussex and in 1957, the observatory closed, ceasing time measurement operations.

[33] The reason why 12 noon was not chosen was because astronomers at the observatory would record when the Sun crossed the meridian at that time on that day.

[38] In 1898 the Christie Enclosure was established to house sensitive magnetic instruments that had been disrupted by the use of iron at the main facility.

[40] The bomb was accidentally detonated while being held by 26-year-old French anarchist Martial Bourdin in Greenwich Park, near the Observatory building.

Prior to this, the observatory had had to insist that the electric trams in the vicinity could not use an earth return for the traction current.

[42] After the onset of World War II in 1939, many departments were temporarily evacuated out of range of German bombers, to Abinger, Bradford on Avon, Bristol,[43] and Bath,[44] and activities in Greenwich were reduced to the bare minimum.

[45] After the Second World War, in 1947, the decision was made to move the Royal Observatory to Herstmonceux Castle[46] and 320 adjacent acres (1.3 km2), 70 km south-southeast of Greenwich near Hailsham in East Sussex, due to light pollution in London.

On 1 December 1967, the Isaac Newton Telescope of the Royal Greenwich Observatory at Herstmonceux was inaugurated by Queen Elizabeth II.

[53] The centre has established itself as a noted tourist and education attraction in its own right, featuring many old observatory items as exhibits.

[55] When the RGO was closed as an institution, the HM Nautical Almanac Office transferred to the Rutherford Appleton Laboratory (Harwell Science and Innovation Campus, Chilton, Oxfordshire), while other work went to the UK Astronomy Technology Centre in Edinburgh.

[57][58] AMAT is a cluster of four separate instruments, to be used for astronomical research; it had achieved first light by June 2018, and contains:[59] The telescopes and the works at the site required to operate them cost about £150,000, from grants, museum members and patrons, and public donations.

[64] Notable exhibits include John Harrison's pioneering chronometer, known as H4, for which he received a large reward from the Board of Longitude, and his three earlier marine timekeepers; all four are the property of the Ministry of Defence.

Many additional horological artefacts are displayed, documenting the history of precision timekeeping for navigational and astronomical purposes, including the mid-20th-century Russian-made F.M.

It also houses the astronomical instruments used to make meridian observations and the 28-inch equatorial Grubb refracting telescope of 1893, the largest of its kind in the UK.