Founding of the Royal Greenwich observatory

22nd June 1675

Royal Observatory
Royal Observatory
© NMM London

The Royal Observatory , home of Greenwich Mean Time and the Prime Meridian line, is one of the most important historic scientific sites in the world. It was founded by Charles II in 1675 and is, by international decree, the official starting point for each new day, year and millennium (at the stroke of midnight GMT as measured from the Prime Meridian).

The launch of A Universal Appeal in May 2004, means that the Royal Observatory is entering one of the most exciting periods in its history. A Universal Appeal is an initiative to raise £1m towards the Time and Space project, an ambitious redevelopment of the site which includes a new Planetarium, brand new gallary spaces and an education centre.

The Observatory, part of the National Maritime Museum, is one of the most famous features of Maritime Greenwich – since 1997 a UNESCO World Heritage Site. Visitors to the Observatory can stand in both the eastern and western hemispheres simultaneously by placing their feet either side of the Prime Meridian - the centre of world time and space. The Observatory galleries unravel the extraordinary phenomena of time, space and astronomy, the Planetarium lets visitors explore the wonders of the heavens and Flamsteed House, Sir Christopher Wren's original building, also has London's only public camera obscura.

John Flamsteed
© NMM London
Charles II appointed John Flamsteed as his first Astronomer Royal in March 1675. The Observatory was built to improve navigation at sea and 'find the so-much desired longitude of places' - one's exact position east and west - while at sea and out of sight of land, by astronomical means. This was inseparable from the accurate measurement of time, for which the Observatory became generally famous in the 19th century. A disaster at sea in 1707 killed over 2000 men and prompted greater calls for more reliable means of navigation. In 1714, Parliament established a panel of experts, the Board of Longitude, and offered a massive £20,000 reward (equivalent of about £2 million today) to anyone who could solve the problem of finding longitude at sea. It took nearly 60 years for the prize to be claimed. In the end it went not to a famous astronomer, scientist or mathematician, but to a little-known Yorkshire carpenter turned clockmaker, John Harrison.
Harrison's marine timekeeper H4
© NMM London, MoD Art Collection

Harrison's H4 was to change navigation forever. All four of his ground-breaking timekeepers are kept in full working order on display in the Harrison gallery - the highlight of a visit to the Observatory. Find out more about John Harrison and his 50-year quest to solve the longitude problem in our online gallery .

Harrison's marine timekeeper H4 Repro ID D0789-1
© NMM London, MoD Art Collection

The Royal Observatory is also the source of the Prime Meridian of the world, Longitude 0° 0' 0''. Every place on the Earth is measured in terms of its distance east or west from this line. The line itself divides the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres.

The Prime Meridian is defined by the position of the large 'Transit Circle' telescope in the Observatory's Meridian Building. This was built by Sir George Biddell Airy, the 7th Astronomer Royal, in 1850. The cross-hairs in the eyepiece of the Transit Circle precisely define Longitude 0º for the world. Read more about Airy and the Transit Circle in our online feature .

The Meridian Line
The Meridian Line at the Royal Observatory at dusk
Since the late-19th century, the Prime Meridian at Greenwich has served as the co-ordinate base for the calculation of Greenwich Mean Time. Before this, almost every town in the world kept its own local time. There were no national or international conventions to set how time should be measured, or when the day would begin and end, or what the length of an hour might be. However, with the vast expansion of the railway and communications networks during the 1850s and 1860s, the worldwide need for an international time standard became imperative.

The Greenwich Meridian was chosen to be the Prime Meridian of the World in 1884. Forty-one delegates from 25 nations met in Washington DC for the International Meridian Conference. By the end of the conference, Greenwich had won the prize of Longitude 0º by a vote of 22 in favour to 1 against (San Domingo), with two abstentions (France and Brazil). There were two main reasons for the victory:

  1. the USA had already chosen Greenwich as the basis for its own national time-zone system.
  2. at the time, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian.

The decision, essentially, was based on the argument that by naming Greenwich as Longitude 0º, it would inconvenience the least number of people. Therefore, the Prime Meridian at Greenwich became the centre of world time, and the starting point of each new day, year and millennium.

In 1960, shortly after the transfer of the Royal Greenwich Observatory (RGO) to Herstmonceux (and later Cambridge), Flamsteed House was transferred to the National Maritime Museum's care and over the next seven years the remaining buildings on the site were also transferred and restored for Museum use. Here the collections of scientific, especially astronomical, instruments has continued to grow. Following the closure of the RGO at Cambridge in October 1998, the site is now again known as the Royal Observatory, Greenwich.