Royal Greenwich Observatory

The Royal Observatory, Greenwich, (known as the Royal Greenwich Observatory or RGO when the working institution moved from Greenwich toHerstmonceux after World War II) played a major role in the history of astronomyand navigation, and is best known as the location of the prime meridian. The observatory is situated on a hill in Greenwich Park, overlooking the River Thames.

Greenwich Prime meridian

A prime meridian, based at the Royal Observatory, Greenwich, in London,[1] was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships andtonnage used it as the reference meridian on their charts and maps. In October of that year, at the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the official prime meridian due to its popularity. However, France abstained from the vote and French maps continued to use the Paris meridian for several decades. In the 18th century, London lexicographer, Malachy Postlethwayt published his African maps showing the ‘Meridian of London’ intersecting the Equator a few degrees west of the later meridian and Accra, Ghana.[2]


  • 1675 Royal Observatory founded.
  • 1714 Longitude Act established the Board of Longitude and Longitude rewards. The Astronomer Royal was, until the Board was dissolved in 1828, always an ex officio Commissioner of Longitude.
  • 1767 Astronomer Royal Nevil Maskelyne began publication of the Nautical Almanac, based on observations made at the Observatory.
  • 1818 Oversight of the Royal Observatory was transferred from the Board of Ordnance to the Board of Admiralty; at that time the observatory was charged with maintaining the Royal Navy’s Marine chronometers.
  • 1833 Daily time signals began, marked by dropping a Time ball.
  • 1899 The New Physical Observatory (now known as the South Building) was completed.
  • 1924 Hourly time signals (Greenwich Time Signal) from the Royal Observatory were first broadcast on 5 February.
  • 1948 Office of the Astronomer Royal was moved to Herstmonceux.
  • 1957 Royal Observatory completed its move to Herstmonceux, becoming the Royal Greenwich Observatory (RGO). The Greenwich site is renamed the Old Royal Observatory.
  • 1990 RGO moved to Cambridge.
  • 1998 RGO closed. Greenwich site is returned to its original name, the Royal Observatory, Greenwich, is made part of theNational Maritime Museum.


The establishment of a Royal Observatory was proposed in 1674 by Sir Jonas Moore who, in his role as Surveyor General at the Ordnance Office, persuaded King Charles II to create the observatory, with John Flamsteed installed as its director.[5] The Ordnance Office was given responsibility for building the Observatory, with Moore providing the key instruments and equipment for the observatory at his own personal cost. Flamsteed House, the original part of the Observatory, was designed by SirChristopher 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) out of largely recycled materials on the foundations of Duke Humphrey’s Tower, which resulted in the alignment being 13 degrees away from true North, somewhat to Flamsteed’s chagrin.


Greenwich Mean Time

Greenwich Mean Time (GMT) was until 1954 based on celestial observations made at Greenwich. Thereafter, GMT was calculated from observations made at other observatories. GMT is more properly called Universal Time at present, and is calculated from observations of extra-galactic radio sources, and then converted into several forms, including UT0 (UT at the remote observatory), UT1 (UT corrected forpolar motion), and UTC (UT in discrete SI seconds within 0.9 s of UT1).

To help others synchronise their clocks to GMT, AR John Pond had a time ballinstalled atop the observatory in 1833. It still drops daily to mark the exact moment of 1 pm (13:00) year-round (GMT during winter and BST during summer).

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 as a museum.