The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north. It played a major role in the history of astronomy and navigation, and because the Prime Meridian passed through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC). The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and the clipper ship Cutty Sark are collectively designated Royal Museums Greenwich.

The observatory was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August. The old hilltop site of Greenwich Castle was chosen by Sir Christopher Wren, a former Savilian Professor of Astronomy; as Greenwich Park was a royal estate, no new land needed to be bought. 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 Astronomer Royal. The building was completed in the summer of 1676.Robert Chambers, Book of Days The building was often called "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 almost exclusively as a museum, although the Annie Maunder Astrographic Telescope (AMAT) telescope became operational for astronomical research in 2018.

History Chronology

1675 – 22 June, The Royal Observatory founded by Charles II. 1675 – 10 August, construction began. 1714 - The 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 - The fifth 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. 1838 - Sheepshanks equatorial, a aperture refracting telescope installed. 1852 - Time signals were distributed through telegraph lines. 1884 - The International Meridian Conference in Washington D.C. decides that the Greenwich Prime Meridian should be the Prime meridian for the whole world, which it remains for a century. 1893 – The 28-inch Great refractor installed. 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, originally generated from a Dent astronomical regulator (Dent no. 2016). 1931 - Yapp telescope ordered. 1948 - Office of the Astronomer Royal was moved to Herstmonceux in East Sussex. 1957 - Royal Observatory completed its move to Herstmonceux, becoming the Royal Greenwich Observatory (RGO). The Greenwich site was renamed the Old Royal Observatory. 1984 - The IERS Reference Meridian replaces the Greenwich Meridian as the Prime Meridian for the world. Its location is closely related to its predecessor, but runs approximately 102 metres east of it. 1990 - RGO moved to Cambridge. 1998 - RGO closed. Greenwich site was returned to its original name, the Royal Observatory, Greenwich, and was made part of the National Maritime Museum. 2011 - The Greenwich museums, including the ROG, became collectively the Royal Museums Greenwich.

Site

There had been significant buildings on this land since the reign of William I.John Timbs' Abbeys, Castles and Ancient Halls of England and Wales 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.

In 1676 the main building of the observatory, now known as Flamsteed House, was completed on Greenwich hill.

Establishment The establishment of a Royal Observatory was proposed in 1674 by Sir Jonas Moore who, in his role as Surveyor-General of the Ordnance, persuaded King Charles II to create the observatory, with John Flamsteed 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 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, the forerunner of Greenwich Castle, which resulted in the alignment being 13 degrees away from true North, somewhat to Flamsteed's chagrin.

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 in length mounted above the clock face, giving a period of four seconds and an accuracy, then unparalleled, of seven seconds per day.

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. Over time the institution became a more established institution, thanks to its links to long-lasting government boards (the Board of Ordnance and Board of Longitude) and oversight by a Board of Visitors, founded in 1710 and made up of the President and Members of the council of the Royal Society. 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. The physical site and the numbers of staff increased over time as a result.

Positional astronomy and star charts

When the observatory was founded in 1675, one of the best star catalogues was Tycho Brahe's 1000-star catalogue from 1598. However, this catalogue was not accurate enough to determine longitudes. One of Flamsteed's first orders of business was creating more accurate charts suitable for this purpose.

One of the noted charts made at Greenwich was by the Astronomer Royal James Bradley, who between 1750 and 1762 charted sixty thousand stars, so accurately his catalogues were used even in the 1940s. Bradley was the third Astronomer Royal, and his tenure started in 1742.

In the early 19th century, the main positional devices were the Troughton Transit instrument and a mural circle, but after George Biddell Airy took over as Astronomer Royal in 1835, he embarked on a plan to have better instruments at Greenwich observatory.

Positional astronomy was one of the primary functions of Greenwich for the Admiralty. 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. One of the difficulties with positional astronomy, is accounting for the refraction of light through Earth's atmosphere. Sources of error include the precision of the instrumentation, and then there has to be accounting for precession, nutation, and aberration. Sources of error in the instrument have to be tracked down and accounted for to produce more accurate results.

The transit circle makes two measurements; along with a clock, the time a star passed a certain point in the sky as the Earth rotates, and the vertical angle of the location of the star. The instrument can be used to plot the locations of stars, or alternately, with an accurate star chart, the time at the location of the instrument.

1832 transit of Mercury The Shuckburgh telescope of the Royal Observatory in Greenwich was used for the 1832 transit of Mercury. 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. By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken. They also reported the peculiar effects that they compared to pressing a coin into the Sun. The observer remarked:

Greenwich Meridian

British astronomers have long used the Royal Observatory as a basis for measurement. Four separate meridians have passed through the buildings, defined by successive instruments. The basis of longitude, the meridian that passes through the Airy transit circle, first used in 1851, was adopted as the world's Prime Meridian at the International Meridian Conference at Washington, DC, on 22 October 1884 (voting took place on 13 October). Subsequently, nations across the world used it as their standard for mapping and timekeeping. The Prime Meridian was marked by a brass (later replaced by stainless steel) strip in the Observatory's courtyard once the buildings became a museum in 1960, 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 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. 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 2.3 m).