Royal Observatory, Greenwich
The Royal Observatory, Greenwich (formerly the Royal Greenwich Observatory or RGO), in London played a major role in the history of astronomy and navigation, and is best known as the location of the prime meridian. It is situated on a hill in Greenwich Park, overlooking the River Thames.
The observatory was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August. 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." He appointed John Flamsteed as the first AR. The building was completed in the summer of 1676. The building was often given the title "Flamsteed House", in reference to its first occupant.
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.
There had been significant buildings on this land since the reign of William I. Greenwich Palace, near the site of the present-day Maritime Museum, was the birthplace of Henry VIII; the Tudors used Greenwich Castle, which was built on the land that the Observatory now stands on. Greenwich Castle was apparently a favourite place for Henry VIII to house his mistresses, so that he could easily travel from the Palace to see them.
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 the creation of the observatory, with John Flamsteed being installed as its director. 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 Sir Christopher Wren, probably with the assistance of 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.
The original observatory at first housed the scientific instruments to be used by Flamsteed in his work on stellar tables, and over time also incorporated a number of additional responsibilities such marking the official time of day, and housing Her Majesty's Nautical Almanac Office.
Moore donated two clocks, built by Thomas Tompion, which were installed in the 20 foot high Octagon Room, the principal room of the building. They were of unusual design, each with a pendulum 13 feet (3.96 metres) in length mounted above the clock face, giving a period of four seconds and an accuracy, then unparalleled, of seven seconds per day.
British astronomers have long used the Royal Observatory as a basis for measurement: four separate meridians have been drawn through the building. The basis of longitude, the Prime Meridian, established in 1851 and adopted at an international conference in 1884, passes through the Airy transit circle of the observatory. It was long marked by a brass strip in the courtyard, now upgraded to stainless steel, and, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky.
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 were confused to discover that in the Ordnance Survey system the longitude of the international Greenwich meridian was not 0° but 0°00'00.417" (about 8m) East. Besides the change of the reference line, imperfections of the surveying system added another discrepancy to the definition of the origin, so that the Bradley line itself is now 0°00'00.12" East of the Ordnance Survey Zero Meridian (about 2.3m).
This old astronomical prime meridian has been replaced by a more modern prime meridian. When Greenwich was an active observatory, geographical coordinates were referred to a local oblate spheroid called a datum, whose surface closely matched local mean sea level, called the geoid. Several datums were in use around the world, all using different spheroids, because mean sea level undulates by as much as 100 metres worldwide. Modern geodetic reference systems, such as the World Geodetic System and the International Terrestrial Reference Frame, use a single oblate spheroid, fixed to the Earth's gravitational centre. The shift from several spheroids to one worldwide spheroid caused all geographical coordinates to shift by many metres, sometimes as much as several hundred metres. The Prime Meridian of these modern reference systems is 102.5 metres east of the Greenwich astronomical meridian represented by the stainless steel strip. Thus the strip is now 5.31 arcseconds West. The modern location of the Airy Transit is 
International time since the end of the 19th century (before 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 coincides in 2013 with a meridian halfway between Airy's circle and the IERS origin : .
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 for polar 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 ball installed 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).
Bomb attack of 1894
The Observatory underwent an attempted bombing on 15 February 1894. This was possibly the first "international terrorist" incident in Britain. The bomb was accidentally detonated while being held by 26-year-old French anarchist Martial Bourdin in Greenwich Park, near the Observatory building. Bourdin died about 30 minutes later. It is not known why he chose the observatory, or whether the detonation was intended to occur elsewhere. Novelist Joseph Conrad used the incident in his novel The Secret Agent.
Today the buildings include a museum of astronomical and navigational tools, which is part of the National Maritime Museum, notably including John Harrison's prize-winning longitude marine chronometer, the H4, and its three predecessors, although all four are the property of the Ministry of Defence. Several 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. Fedchenko clock (the most accurate pendulum clock ever built in multiple copies). It also houses the 28-inch Grubb refracting telescope of 1893, the largest of its kind in the UK. The Shepherd Clock outside the observatory gate is an early example of an electric slave clock. In February 2005 construction began on a £15 million redevelopment project to provide a new planetarium and additional display galleries and educational facilities. The 120-seat Peter Harrison Planetarium opened on 25 May 2007.
Royal Observatory, Greenwich vs. Royal Greenwich Observatory
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During much of the twentieth century, the Royal Greenwich Observatory was not at Greenwich. The last time that all departments were there was 1924: in that year electrification of the railways affected the readings of the Magnetic and Meteorological Department and forced its move to Abinger. Indeed prior to this, the observatory had to insist that all the electric trams in the vicinity could not use an earth return for the traction current. After the onset of World War II in 1939, many departments were evacuated, along with the rest of London, to the countryside (Abinger, Bradford, and Bath) and activities in Greenwich were reduced to the bare minimum.
After the War, in 1947, the decision was made to move to Herstmonceux Castle and 320 adjacent acres (1.3 km²), 70 km south-southeast of Greenwich near Hailsham in East Sussex, due to light pollution in London. Although the Astronomer Royal Harold Spencer Jones moved to the castle in 1948, the scientific staff did not move until the observatory buildings were completed, in 1957. Shortly thereafter, other previously dispersed departments were reintegrated at Herstmonceux.
The Isaac Newton Telescope was built at Herstmonceux in 1967, but was moved to Roque de los Muchachos Observatory in Spain's Canary Islands in 1979. In 1990 the RGO moved again, to Cambridge. Following a decision of the Particle Physics and Astronomy Research Council, it closed in 1998. Her Majesty's Nautical Almanac Office was transferred to the Rutherford Appleton Laboratory after the closure. Other work went to the UK Astronomy Technology Centre in Edinburgh. The castle grounds are now the home of the International Study Centre of Queen's University, Kingston, Canada and The Observatory Science Centre, which is operated by an educational charity Science Project.
- 1675 Royal Observatory, Greenwich founded.
- 1714 Longitude Act establishes the Board of Longitude and the Longitude prize.
- 1924 Hourly time signals (Greenwich Time Signal) from the Royal Observatory, Greenwich were first broadcast on 5 February.
- 1948 Astronomer Royal moves to Herstmonceux.
- 1957 Royal Observatory completes its move to Herstmonceux, becoming the Royal Greenwich Observatory (RGO). The Greenwich site becomes the Old Royal Observatory.
- 1990 RGO moves to Cambridge.
- 1998 RGO closes. Greenwich site becomes the Royal Observatory, Greenwich again, part of the National Maritime Museum.
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- Greenwich Observatory: ... the Royal Observatory at Greenwich and Herstmonceux, 1675–1975. London: Taylor & Francis, 1975 3v. (Vol. 1. Origins and early history (1675–1835), by Eric G. Forbes. ISBN 0-85066-093-9; Vol. 2. Recent history (1836–1975), by A.J. Meadows. ISBN 0-85066-094-7; Vol. 3. The buildings and instruments by Derek Howse. ISBN 0-85066-095-5)
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|Wikimedia Commons has media related to Royal Observatory, Greenwich.|
- Online catalogue of the Royal Greenwich Observatory Archives (held at Cambridge University Library)
- "Where the Earth's surface begins—and ends", Popular Mechanics, December 1930
- HM Nautical Almanac Office
- The Royal Observatory, Greenwich, now part of the National Maritime Museum
- Aerial View of The Royal Observatory, Greenwich at Google Maps
- The National Maritime Museum
- Castle in the sky – The story of the Royal Greenwich Observatory at Herstmonceux
- A Personal History of the Royal Greenwich Observatory at Herstmonceux Castle, 1948–1990 by George Wilkins, a former member of staff
- The Observatory Science Centre
- Isaac Newton Group of Telescopes
- A pictorial catalogue of meridian markers