Editing Royal Observatory, Greenwich

{{Short description|Observatory in London, England}}
{{Redirect|RGO}}
{{Use British English|date=October 2011}}
{{Use dmy dates|date=September 2019}}
{{Infobox observatory
| coordinates       = {{coord|51.477|0.0|type:landmark_region:GB-ENG|display=inline,title}}
| location          = [[Greenwich]], [[London]], England
}}
The '''Royal Observatory, Greenwich''' ('''ROG''';<ref name=rmg>{{cite web |url=http://www.bshs.org.uk/travel-guide/greenwich-london |title=Royal Observatory Greenwich, London |website=BSHS Travel Guide - A Travel Guide to Scientific Sites |date= 6 September 2012|author=Rebekah Higgitt |access-date= 28 April 2017}}</ref> 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 (Greenwich)|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.<ref>{{cite web|title=List of Observatory Codes|url=http://www.minorplanetcenter.net/iau/lists/ObsCodesF.html|website=Minor Planet Center|access-date=28 April 2017}}</ref> ROG, the [[National Maritime Museum]], the [[Queen's House]] and the [[clipper ship]] ''[[Cutty Sark]]'' are collectively designated [[Royal Museums Greenwich]].<ref name=rmg/>

The observatory was commissioned in 1675 by [[Charles II of England|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.<ref>{{cite web |url=http://www.rmg.co.uk/about/history/royal-observatory |title=Greenwich and the Millennium |date=2015 |access-date=6 September 2015}}</ref> 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 {{notatypo|Astronomer Royal}}. The building was completed in the summer of 1676.<ref>Robert Chambers, Book of Days</ref> 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 S. D. Maunder#Legacy|AMAT telescope]] became operational for astronomical research in 2018.

==History==
===Chronology===
{{more citations needed section|date=November 2019}}
*1675 – 22 June, Royal Observatory founded by [[Charles II of England|King Charles II]].
*1675 – 10 August, construction began.
*1714 [[Longitude Act 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 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 chronometer]]s.
*1833 Daily time signals began, marked by dropping a [[time ball]].
*1838 –  [[Sheepshanks equatorial]], a {{convert|6.7|in|mm}} aperture [[refracting telescope]] installed.<ref>{{Cite web|url=http://www.royalobservatorygreenwich.org/articles.php?article=1171|title=The Royal Observatory Greenwich - where east meets west: Telescope: The Sheepshanks Equatorial (1838)|website=www.royalobservatorygreenwich.org|access-date=2019-10-31}}</ref> 
*1852 Time signals were distributed through telegraph lines.<ref name = "Howse1997"/>
*1884 The [[International Meridian Conference]] in [[Washington D.C.]] decides that the [[Prime meridian (Greenwich)|Greenwich Prime Meridian]] should be the [[Prime meridian]] for the whole world, which it remains for a century.
*1893  – The [[Greenwich 28 inch refractor|28-inch Great refractor]] installed.<ref>{{Cite web|url=http://www.royalobservatorygreenwich.org/articles.php?article=1174|title=The Royal Observatory Greenwich - where east meets west: Telescope: 28-inch Refractor (1893)|website=www.royalobservatorygreenwich.org|access-date=2019-10-25}}</ref> 
*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.
*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===
[[File:Illustration of Greenwich Observatory by Johann Doppelmayr.jpg|thumb|Greenwich Observatory (Latinized as {{lang|la|Observatorium Anglicanum Hoc Grenovici prope Londinum}}), as illustrated in [[Johann Gabriel Doppelmayr]]'s map of the [[celestial sphere|southern celestial hemisphere]], {{Circa|1730}}]]
There had been significant buildings on this land since the reign of William I.<ref>John Timbs' Abbeys, Castles and Ancient Halls of England and Wales</ref>{{page needed|date=August 2020}} [[Greenwich Palace]], on the site of the present-day National Maritime Museum, was the birthplace of both [[Henry VIII]] and his daughters [[Mary I of England|Mary I]] and [[Elizabeth I of England|Elizabeth I]]; [[Tudor dynasty|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.<ref>{{citation|last=Hart|first=Kelly|year=2010|title=The Mistresses of Henry VIII|publisher=The History Press|isbn=978-0-7524-5496-2|page=73}}</ref>{{page needed|date=August 2020}}

In 1676 the main building of the observatory, now known as Flamsteed House, was completed on Greenwich hill.<ref>{{Cite journal | doi=10.1119/1.2339829| title=Tricentennial Greenwich Observatory| journal=The Physics Teacher| volume=16| issue=2| pages=95| year=1978| last1=Hilton| first1=Wallace A.| bibcode=1978PhTea..16...95H}}</ref>

===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.<ref>{{cite ODNB|title=Moore, Sir Jonas (1617–1679)|id=19137|year=2004|first=Frances|last=Willmoth}}</ref> 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; {{inflation|GBP|520|1675|cursign=£|fmt=eq|r=-3}}) out of largely recycled materials on the foundations of [[Humphrey, Duke of Gloucester|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.{{citation needed|date=August 2020}}

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 {{convert|13|ft|m|abbr=off}} 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.<ref>{{Cite journal|last=Higgitt|first=Rebekah|date=2019|title='Greenwich near London': the Royal Observatory and its London networks in the seventeenth and eighteenth centuries|journal=The British Journal for the History of Science|volume=52|issue=2|language=en|pages=297–322|doi=10.1017/S0007087419000244|pmid=31084633|issn=0007-0874|doi-access=free}}</ref> By the later 18th century it incorporated additional responsibilities such as publishing the ''[[The Nautical Almanac|Nautical Almanac]]'', advising government on technical matters, disseminating time, making meteorological and magnetic observations and undertaking astrophotography and spectroscopy. The physical site<ref>{{Cite web|url=http://www.royalobservatorygreenwich.org/articles.php?article=907|title=The Royal Observatory Greenwich - where east meets west: The Buildings at Greenwich|website=www.royalobservatorygreenwich.org|access-date=2019-05-20}}</ref> and the numbers of staff<ref>{{Cite web|url=http://www.royalobservatorygreenwich.org/articles.php?article=11|title=The Royal Observatory Greenwich - where east meets west: People|website=www.royalobservatorygreenwich.org|access-date=2019-05-20}}</ref> increased over time as a result.

=== Positional astronomy and star charts ===
[[File:Airys Transit Circle.jpg|thumb |upright|The [[George Biddell Airy|Airy]] [[Transit Circle]] at the Royal Observatory, Greenwich, used for over a century (1851–1953) as the reference point when charting the heavens and determining times, thus earning for it the epithet "the centre of time and space"]]
When the observatory was founded in 1675, one of the best star catalogues was [[Tycho Brahe]]'s 1000-star catalogue from 1598.<ref name="Jones FRS">{{Cite web|url=http://www.geomag.bgs.ac.uk/data_service/data/yearbooks/ARpdf/AR_00825.pdf |archive-url=https://web.archive.org/web/20120601155223/http://geomag.bgs.ac.uk/data_service/data/yearbooks/ARpdf/AR_00825.pdf |archive-date=2012-06-01 |url-status=live|title=The Royal Greenwich Observatory|first=Harold |last=Spencer Jones |author-link=Harold Spencer Jones}}</ref>  However, this catalogue was not accurate enough to determine longitudes.<ref name="Jones FRS" /> One of Flamsteed's first orders of business was creating more accurate charts suitable for this purpose.<ref name="Jones FRS" />

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.<ref name="Jones FRS" /> Bradley was the third [[Astronomer Royal]], and his tenure started in 1742.<ref>{{Cite journal|title=1963QJRAS...4...47W Page 47|journal = Quarterly Journal of the Royal Astronomical Society|volume = 4|pages = 47|bibcode = 1963QJRAS...4...47W|last1 = Woolley|first1 = Richard|year = 1963}}</ref>

In the early 19th century, the main positional devices were the Troughton Transit instrument and a [[mural instrument|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.<ref name="2001JAHH....4..115S Page 115">{{Cite journal|title=2001JAHH....4..115S Page 115|journal = Journal of Astronomical History and Heritage|volume = 4|issue = 2|pages = 115|bibcode = 2001JAHH....4..115S|last1 = Satterthwaite|first1 = Gilbert E.|year = 2001| doi=10.3724/SP.J.1440-2807.2001.02.02 | s2cid=116777770 }}</ref>

[[Positional astronomy]] was one of the primary functions of Greenwich for the Admiralty.<ref name="Ratcliff">{{Cite book|url=https://books.google.com/books?id=LWhECgAAQBAJ&q=positional+astronomy+greenwich&pg=PA30|title=The Transit of Venus Enterprise in Victorian Britain|last=Ratcliff|first=Jessica|date=2015-07-28|publisher=Routledge|isbn=978-1-317-31639-8|language=en}}</ref> 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.<ref name="Ratcliff"/> One of the difficulties with positional astronomy, is accounting for the [[Atmospheric refraction|refraction of light through Earth's atmosphere]].<ref>{{Cite book|url=https://books.google.com/books?id=BDysAAAAIAAJ&q=positional+astronomy+greenwich|title=Greenwich Observatory ... the Story of Britain's Oldest Scientific Institution, the Royal Observatory at Greenwich and Herstmonceux, 1675-1975|last1=Forbes|first1=Eric Gray|last2=Meadows|first2=Arthur Jack|last3=Howse|first3=Derek|date=1975|publisher=Taylor & Francis Group|isbn=978-0-85066-095-1|language=en}}</ref> Sources of error include the precision of the instrumentation, and then there has to be accounting for [[precession]], [[nutation]], and [[aberration (astronomy)|aberration]].<ref>{{Cite book|url=https://books.google.com/books?id=qB0oAAAAYAAJ&q=bradley+star+charts+refraction&pg=PA295|title=New International Encyclopedia|date=1914|publisher=Dodd, Mead|language=en}}</ref> Sources of error in the instrument have to be tracked down and accounted for to produce more accurate results.<ref name="2001JAHH....4..115S Page 115"/>

The transit circle makes two measurements; along with a clock, the time a star passed a certain point in the sky as the [[Earth's rotation|Earth rotates]], and the vertical angle of the location of the star.<ref name="UNESCO 2015" /> 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.<ref name="UNESCO 2015" />

===1832 transit of Mercury===
The [[Shuckburgh telescope]] of the Royal Observatory in Greenwich was used for the 1832 [[transit of Mercury]].<ref name="Priestley and Weale-1831" /> 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.<ref name="Priestley and Weale-1831">{{Cite book|url=https://books.google.com/books?id=utAUAQAAMAAJ&q=Shuckburgh+telescope+eastern+equatorial&pg=RA1-PA103|title=Monthly Notices of the Royal Astronomical Society|date=1831|publisher=Priestley and Weale|language=en}}</ref> By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken.<ref name="Priestley and Weale-1831" /> They also reported the peculiar effects that they compared to pressing a coin into the Sun.<ref name="Priestley and Weale-1831" /> The observer remarked:

{{quote2|I afterwards observed, that immediately around the planet there was a dusky tinge, making it appear as if, in a small degree sunk below the sun's surface;"|''Royal Astronomical Society, Vol II, No. 13''<ref name="Priestley and Weale-1831" /> }}

===Greenwich Meridian===
{{Main|Prime meridian (Greenwich)}}
[[File:Prime meridian 001.jpg|thumb|upright|The building housing the origin of the [[Prime meridian (Greenwich)|Greenwich Prime Meridian]], marked by the brass strip in the foreground. The apex of the roof opens up to the left above the white vertical strips (joints in the eaves are visible), enabling telescopic star sightings to be made.]]

British astronomers have long used the Royal Observatory as a basis for measurement. Four separate [[Meridian (geography)|meridians]] have passed through the buildings, defined by successive instruments.<ref>{{cite web|last1=Dolan|first1=Graham|title=The Greenwich Meridian before the Airy Transit Circle|url=http://www.thegreenwichmeridian.org/tgm/articles.php?article=8|website=The Greenwich Meridian|access-date=2 May 2015}}</ref> The basis of [[longitude]], the meridian that passes through the [[George Biddell Airy|Airy]] [[Meridian circle|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).<ref name = Howse1997>{{cite book|first=Derek|last=Howse|title=Greenwich time and the longitude|year=1997|pages=[https://archive.org/details/greenwichtimelon0000hows/page/12 12, 98, 137]|isbn=978-0-85667-468-6|publisher=Phillip Wilson|place=London|url=https://archive.org/details/greenwichtimelon0000hows/page/12}}</ref> 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 [[Principal Triangulation of Great Britain|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 [[Retriangulation of Great Britain|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&nbsp;m) east.<ref>{{cite book|first=Derek|last=Howse|url=https://archive.org/stream/GreenwichTime/Howse-GreenwichTime#page/n3/mode/1up|title=Greenwich time and the discovery of the longitude|year=1980 |page=171}}</ref> 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 [[United Kingdom Ordnance Survey Zero Meridian|Ordnance Survey Zero Meridian]] (about 2.3&nbsp;m).<ref>{{cite web|first=Brian|last=Adams|url=https://s3-eu-west-1.amazonaws.com/charlesclose/Sh40.pdf |archive-url=https://web.archive.org/web/20140407092126/https://s3-eu-west-1.amazonaws.com/charlesclose/Sh40.pdf |archive-date=2014-04-07 |url-status=live|title=Charles Close Society|year=1994|pages=14–15}}</ref>

This old astronomical prime meridian has been replaced by a more precise prime meridian. When Greenwich was an active observatory, geographical coordinates were referred to a local [[oblate spheroid]] called a [[datum (geodesy)|datum known as a geoid]], whose surface closely matched local mean sea level. 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 local 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 the [[IERS Reference Meridian]], in full the International Earth Rotation and Reference Systems Service Reference Meridian (in short called the IRM), which is 102.5 metres east of the Airy Greenwich astronomical meridian represented by the stainless steel strip, which is now 5.31&nbsp;[[arcsecond]]s west. The modern location of the Airy Transit is {{coord|51|28|40.1|N|0|0|5.3|W|type:landmark_globe:earth_region:GB_scale:1000|name=Airy Transit}} as the IRM is at 0 degree in longitude nowadays.<ref>{{cite journal|title=Why the Greenwich meridian moved |first1=Stephen |last1=Malys |first2=John H. |last2=Seago |first3=Nikolaos K. |last3=Palvis |first4=P. Kenneth |last4=Seidelmann |first5=George H. |last5=Kaplan |journal=Journal of Geodesy |volume=89 |issue=12 |pages=1263–1272 |date=1 August 2015 |doi=10.1007/s00190-015-0844-y|doi-access=free |bibcode=2015JGeod..89.1263M }}</ref>

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 Reference Meridian|IERS origin]]: {{coord|51|28|40.1247|N|0|0|2.61|W|type:landmark_globe:earth_region:GB_scale:1000}}.<ref>{{cite web|first1=John H.|last1=Seago|first2=P. Kenneth|last2=Seidelmann|url=http://www.agi.com/downloads/resources/white-papers/AAS%2013-486.pdf|title=The mean-solar-time origin of Universal Time and UTC|work=Paper presented at the AAS/AIAA Spaceflight Mechanics Meeting, Kauai, HI, USA, March 2013. Reprinted from Advances in the Astronomical Sciences v. 148. pp. 1789, 1801, 1805|url-status=dead|archive-url=https://web.archive.org/web/20131227020944/http://www.agi.com/downloads/resources/white-papers/AAS%2013-486.pdf|archive-date=27 December 2013}}</ref>

===Greenwich Mean Time===
{{Main|Greenwich Mean Time}}
[[File:Greenwich clock 1-manipulated.jpg|thumb|The [[Shepherd Gate Clock]] at the gates of the Royal Greenwich Observatory. This clock shows [[Greenwich Mean Time]] all year round, ie. it is not set to [[British Summer Time]] in the summer.]]

A key instrument for determining time was the Airy Transit Circle (ATC), which was used primarily from 1851 to 1938.<ref name="UNESCO 2015">{{Cite web|url=https://www.rmg.co.uk/discover/explore/airys-transit-circle-and-dawn-universal-day|title=Airy's Transit Circle and the dawn of the Universal Day|date=2015-08-18|website=Royal Museums Greenwich {{!}} UNESCO World Heritage Site In London|language=en|access-date=2019-11-08}}</ref>  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".  (Note, however, that this Prime Meridian is obsolete; the [[Prime meridian#IERS Reference Meridian|ITRF Zero Meridian]], which is more than 100 meters east, is the modern standard defining longitude.)  The time was determined by marking the time a star of known location would pass through the aimpoint of the telescope.<ref name="UNESCO 2015" /> In a reverse case, this type of instrument was also used for making star charts.<ref name="UNESCO 2015" />  The stars whose position was known precisely enough for being used for time determination, were called "clock stars".<ref name="UNESCO 2015" />

By 1925, confusion about whether GMT was reckoned from noon or from midnight led (in 1928) to the IAU retiring GMT for astronomical and chronological purposes, replacing it with [[Universal Time]] (UT).<ref>{{cite web | title=3rd General Assembly of the IAU | website=IAU | access-date=13 May 2023| url=https://iau.org/static/resolutions/IAU1928_French.pdf}}</ref>  In 1929, UT was redefined as a statistical combination of multiple observatories.<ref>{{cite journal | last1=Malys | first1=Stephen | last2=Seago | first2=John H. | last3=Pavlis | first3=Nikolaos K. | last4=Seidelmann | first4=P. Kenneth | last5=Kaplan | first5=George H. | title=Why the Greenwich meridian moved | journal=Journal of Geodesy | publisher=Springer Science and Business Media LLC | volume=89 | issue=12 | date=2015-08-01 | issn=0949-7714 | doi=10.1007/s00190-015-0844-y | pages=1263–1272| bibcode=2015JGeod..89.1263M | doi-access=free }}</ref>  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.

The term "GMT" continues to be promoted by the Observatory and the UK in general, despite no longer being measured in any way.  [[Coordinated Universal Time]] (UTC) forms the basis of modern civil time, and is based on the best attributes of UT1 (the modern form of UT, now measured from extra-galactic radio sources) and [[International Atomic Time]] (TAI, time kept by accurate clocks).

===Greenwich Time Ball===
[[File:London, Greenwich, Royal Greenwich Observatory -- 2016 -- 4695.jpg|thumb|The [[time ball]] is the red ball on a post – when it drops a certain time is signalled. This allowed clocks to be set from afar with great accuracy, particularly the [[Marine chronometer|chronometers]] of ships on the [[River Thames]] below, prior to sailing. The observatory would first determine the time by stellar observations.]]
The red [[time ball]] of Greenwich was established in 1833, and is noted as a public time signal.<ref name="rmg.co.uk">{{cite web | url=https://www.rmg.co.uk/see-do/we-recommend/attractions/see-time-ball-drop | title=The Greenwich Time Ball }}</ref> The time ball in modern times is normally in a lowered position, then starting at 12:55{{nbsp}}pm, the ball begins to rise, then at 12:58 it reaches the top; at 1{{nbsp}}pm the ball drops.<ref name="rmg.co.uk"/>

To help mariners at the port and others in line of sight of the observatory to synchronise their clocks to GMT, Astronomer Royal [[John Pond]] installed a very visible time ball that drops precisely at 1{{nbsp}}pm (13:00) every day atop the observatory in 1833. Initially it was dropped by an operator; from 1852 it was released automatically via an electric impulse from the [[Shepherd Gate Clock|Shepherd Master Clock]].<ref>{{cite web |url=http://londonist.com/2009/12/from_the_royal_observatory_the_gree |title=From The Royal Observatory: The Greenwich Time Ball |newspaper=Londonist.com |date=  9 December 2009}}</ref> The ball is still dropped daily at 13:00 (GMT in winter, [[British Summer Time|BST]] in summer).<ref>{{cite web|url=http://wwp.greenwichmeantime.com/info/timeball.htm|title=Greenwich Time Ball}}</ref>

The original time ball system was built by Messrs Maudslay and Field, and cost £180.<ref name="Laurie-1958">{{Cite journal|title=1958Obs....78..113L Page 113|journal = The Observatory|volume = 78|pages = 113|bibcode = 1958Obs....78..113L|last1 = Laurie|first1 = P. S.|year = 1958}}</ref> The five-foot diameter ball was made of wood and leather.<ref name="Laurie-1958"/> In the original ball system, it was hoisted by a rope up from the Octagon room, and there was a catch at the top to hold it.<ref name="Laurie-1958"/> This could then be triggered by hand, while observing the time on an astronomical month clock, that was regulated to the [[mean solar time]].<ref name="Laurie-1958"/>

By dropping the ball, the public, mariners, and clock makers could then get a time signal by viewing it from afar.<ref name="Laurie-1958"/> The ball drop would be repeated at 2{{nbsp}}pm also if possible.<ref name="Laurie-1958"/>

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.<ref>{{Cite book|url=https://books.google.com/books?id=ygJCDwAAQBAJ&q=greenwich+time+ball&pg=PA61|title=Marine Chronometers at Greenwich|last=Betts|first=Jonathan|date=2018-01-04|publisher=Oxford University Press|isbn=9780199641383|language=en}}</ref>

In rare occasions where the ball could get stuck due to icing or snow, and if the wind was too high it would not be dropped.<ref name="Laurie-1958"/><ref>{{Cite book|url=https://books.google.com/books?id=adA0AQAAMAAJ&q=greenwich+time+ball&pg=RA2-PA15|title=Observations Made at the Royal Observatory, Greenwich in the Year ... in Astronomy, Magnetism and Meteorology|date=1921|publisher=H.M. Stationery Office|language=en}}</ref> In 1852, it was established to distribute a time signal by the telegraph wires also.<ref name="Laurie-1958"/>

The time ball was extremely popular with the public, chronometers, railways, mariners, and there was a petition to have another time ball established in [[Southampton]] also.<ref name="Laurie-1958"/>

=== 1890s ===
[[File:Royal Observatory Greenwich - site plan.tif|thumb|Royal Observatory Greenwich, site plan]]
The 1890s marked the addition of a new larger refractor, the 28-inch Grubb in the Great Equatorial Dome. Because the new telescope was longer than the old Great refractor, the new dome had to be bigger; thus the famous "onion dome" that expands beyond the diameter of the turret was established. For the tricentennial, it was revitalized with a [[fiberglass|fibre-glass]] dome; the old one made of [[papier-mâché]] and iron had been taken down.

The telescope was installed by 1893, with 28-inch diameter glass [[Doublet (lens)|doublet lens]] made by Grubb from Chance of Birmingham glass.<ref name="Observatory-1897">{{Cite book|url=https://books.google.com/books?id=A1U3AQAAMAAJ&q=28+inch+grubb&pg=PR18|title=Observations Made in Astronomy, Magnetism and Meteorolgy|last=Observatory|first=Royal Greenwich|date=1897|publisher=H.M. Stationery Office|language=en}}</ref>{{page needed|date=August 2020}} The new dome was made by [[T. Cooke & Sons|T. Cooke and Sons]].<ref name="Observatory-1897" />{{page needed|date=August 2020}} This replaced a smaller drum-shaped dome.<ref name="Observatory-1897" />

The [[William Lassell|Lassell]] two-foot reflector was a famous metal-mirror telescope that had been used to discover the Moons [[Triton (moon)|Triton]] (orbiting Neptune) and [[Hyperion (moon)|Hyperion]] (orbiting Saturn).<ref name="East meets west">{{Cite web|url=http://www.royalobservatorygreenwich.org/articles.php?article=1046|title=The Royal Observatory Greenwich - where east meets west: Telescope: The Lassell 2-foot Reflector (1847)|website=www.royalobservatorygreenwich.org|access-date=2019-11-28}}</ref> It was donated to the observatory in the 1880s, but was taken down in the 1890s.<ref name="East meets west" />

The 1890s also saw the construction of the Altazimuth Pavilion, completed in 1896 and designed by William Crisp.<ref>{{Cite web|url=http://www.royalobservatorygreenwich.org/articles.php?article=918|title=The Royal Observatory Greenwich - where east meets west: The Altazimuth Pavilion|website=www.royalobservatorygreenwich.org|access-date=2019-11-08}}</ref>
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.<ref>{{cite web | url=http://www.royalobservatorygreenwich.org/articles.php?article=974 | title=The Christie Enclosure | publisher=The Royal Observatory Greenwich | accessdate=6 April 2021 }}</ref>

The Observatory underwent an attempted bombing on 15 February 1894. This was possibly the first "international terrorist" incident in Britain.<ref>{{Cite book|
title=Symbolism in terrorism : motivation, communication, and behavior| last=Matusitz | first = Jonathan Andre|isbn=9781442235793| publisher = Rowman & Littlefield | location=Lanham| oclc=891148726| date=16 September 2014 }}</ref> 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. The novelist [[Joseph Conrad]] used the incident in his 1907 novel ''[[The Secret Agent]]''.<ref>{{cite web|url=http://www.nmm.ac.uk/server/show/conWebDoc.413|title=Propaganda by Deed – the Greenwich Observatory Bomb of 1894|url-status=dead|archive-url=https://web.archive.org/web/20070930035329/https://www.nmm.ac.uk/server/show/conWebDoc.413|archive-date=30 September 2007|date=20 August 2015}}</ref>

===Early 20th century===
For major parts of the twentieth century, the Royal Greenwich Observatory was not at Greenwich, because it moved to [[Herstmonceux]] in East Sussex in 1957. The last time that all departments were in Greenwich was 1924: in that year electrification of the railways affected the readings of the [[magnetism|Magnetic]] and [[meteorology|Meteorological]] Departments, and the Magnetic Observatory moved to [[Abinger]] in Surrey. 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.<ref>{{cite web |url=http://www.royalobservatorygreenwich.org/articles.php?article=911 |title=Abinger Magnetic Observatory (1923–1957) |website=The Royal Observatory Greenwich|access-date= 3 May 2017}}</ref>

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]],<ref>{{cite web |url=http://www.royalobservatorygreenwich.org/articles.php?article=912 |title=Bristol & Bradford on Avon (1939–1948) |website=The Royal Observatory Greenwich|access-date= 3 May 2017}}</ref> and [[Bath, Somerset|Bath]],<ref>{{cite web |url=http://www.royalobservatorygreenwich.org/articles.php?article=913 |title=Bath (1939–1949) |website=The Royal Observatory Greenwich|access-date= 3 May 2017}}</ref> and activities in Greenwich were reduced to the bare minimum.

On 15 October 1940, during [[the Blitz]], the Courtyard gates were destroyed by a direct bomb hit. The wall above the Gate Clock collapsed, and the clock's dial was damaged. The damage was repaired after the war.<ref>{{cite web |url=http://www.royalobservatorygreenwich.org/articles.php?article=1085 |title=The Royal Observatory Greenwich – The Shepherd Gate Clock |website=Royal Observatory Greenwich |access-date= 3 May 2017}} A very detailed history of the Shepherd Gate Clock.</ref>

===The Royal Observatory at Herstmonceux===
[[File:Royal Greenwich Observatory, Herstmonceux - April 2012.jpg |thumb |upright 2.0|Former Royal Greenwich Observatory, [[Herstmonceux]], East Sussex]]
After the Second World War, in 1947, the decision was made to move the Royal Observatory to [[Herstmonceux Castle]]<ref>{{cite web |url=http://www.royalobservatorygreenwich.org/articles.php?article=5 |title=The Herstmonceux years... 1948–1990 |website=The Royal Observatory Greenwich|access-date= 3 May 2017}}</ref> and 320 adjacent acres (1.3&nbsp;km<sup>2</sup>), 70&nbsp;km south-southeast of Greenwich near [[Hailsham]] in East Sussex, due to [[light pollution]] in London. The Observatory was officially known as the ''Royal Greenwich Observatory, Herstmonceux''. 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, such as the [[HM Nautical Almanac Office|Nautical Almanac Office]], Chronometer Department, the library, and observing equipment.<ref name="McKechnie-2015" />

The largest telescope at Greenwich at that time, the [[Yapp telescope]] 36-inch reflector, was moved out to Herstmonceux in 1958.<ref name="the-observatory.org">{{Cite web|url=https://www.the-observatory.org/dome-b|title = The 36-inch Yapp Reflector}}</ref> There it was reconstructed in Dome B of the facility.<ref name="the-observatory.org"/> There it was used for astronomy in the 1960s, 1970s, and 1980s. It was left behind at Herstmonceux in 1990 in its dome when the organization moved once again.<ref name="McKechnie-2015">{{Cite book|url=https://books.google.com/books?id=p_JUCgAAQBAJ&q=yapp+moved+to+herstmonceux&pg=PA214|title=General Theory of Light Propagation and Imaging Through the Atmosphere|last=McKechnie|first=T. Stewart|date=2015-08-05|publisher=Springer|isbn=9783319182094|language=en}}</ref>

The tricentennial of [[Sir Isaac Newton]] had passed during the Second World War, delaying festivities. One of the ground-swells was to build a 'big better' telescope in honour of the celebrated inventor of the Newtonian reflecting telescope. Some two decades of development led to the commissioning of the Isaac Newton Telescope at Herstmonceux. It proved so successful that the cloudy weather was felt to be a bottleneck to its productivity, and plans were made to get it to a higher spot with better weather.

On 1 December 1967, the [[Isaac Newton Telescope]] of the Royal Greenwich Observatory at Herstmonceux was inaugurated by Queen Elizabeth II.<ref>{{cite journal
  | last1 = Smith
  | first1 = F. Graham
  | author-link = Francis Graham Smith
  | last2 = Dudley
  | first2 = J.
  | title = The Isaac Newton Telescope
  | journal = [[Journal for the History of Astronomy]]
  | volume = 12
  | number = 1
  | pages = 1–18
  | date = 1982
  | bibcode = 1982JHA....13....1S
  | url = http://articles.adsabs.harvard.edu/full/1982JHA....13....1S
  | access-date = 31 January 2020
  | doi = 10.1177/002182868201300101
  | s2cid = 116989341
 }}</ref> The telescope was the biggest telescope by aperture in the British Isles.<ref>{{Cite book|url=https://books.google.com/books?id=F8EuToKH45QC&q=the+98+inch+isaac+newton+mirror+exhibit&pg=PA359|title=New Scientist|date=1983-08-04|publisher=Reed Business Information|language=en}}</ref> It was moved to [[Roque de los Muchachos Observatory]] in Spain's [[Canary Islands]] in 1979. In 1990 the RGO moved to [[Cambridge]].<ref>{{cite web|url=http://www.lib.cam.ac.uk/deptserv/manuscripts/RGO_history/|title=A Personal History Of The Royal Greenwich Observatory at Herstmonceux Castle, 1948–1990 by G.A. Wilkins}}</ref> At Herstmonceux, the castle grounds became the home of the International Study Centre of [[Queen's University, Kingston|Queen's University, Kingston, Canada]], and The Observatory Science Centre,<ref>[http://www.the-observatory.org/ the-observatory.org]</ref> which is operated by an educational charity Science Project.

The Observatory Science Centre opened in April 1995.<ref name=":bn">{{Cite web|url=https://ehive.com/collections/3501/the-observatory-science-centre|title=The Observatory Science Centre on eHive|website=eHive|language=en|access-date=2019-11-09}}</ref> Some of the remaining telescopes, which were left behind in the move, have public observation events as part of operations of the centre.<ref name=":bn" /> The centre has established itself as a noted tourist and education attraction in its own right, featuring many old observatory items as exhibits.<ref>{{Cite web|url=https://kingfishervisitorguides.com/brighton-hove/observatory-science-centre/|title=Observatory Science Centre – Brighton & Hove days out|date=2015-12-03|website=Brighton and Hove|language=en|access-date=2019-11-09}}</ref> It was getting 60,000 visitors per year in the early 21st century.<ref name=":bn" />

===The Royal Observatory at Cambridge===
[[File:Greenwich House - geograph.org.uk - 373583.jpg|thumb|Greenwich House at Cambridge]]
In 1990 the Royal Observatory moved from Herstmonceux to a new site at [[Cambridge]], adjacent to the [[University of Cambridge|University]]'s [[Institute of Astronomy, Cambridge|Institute of Astronomy]], where it occupied Greenwich House just to the north of the [[Cambridge Observatory]]. By now, the RGO's focus had moved from carrying out observations from the British Isles to providing technical support, acting as a conduit between scientists in British universities and the powerful British-owned telescopes (such as the [[Isaac Newton Telescope]], the Anglo-Dutch [[Jacobus Kapteyn Telescope]], and the [[William Herschel Telescope]]) on the [[Canary Islands]] and [[Hawaii]].<ref name="independent.co.uk">[https://www.independent.co.uk/news/science/a-closed-subject-1255152.html Charles Arthur, ''A closed subject?''], The Independent, 10 June 1997 (accessed 12 November 2019)</ref>

After abandoning a plan to privatise the RGO and the [[Royal Observatory Edinburgh]], the [[Particle Physics and Astronomy Research Council]] (PPARC) as the RGO's funding body made the decision to close the institution and the Cambridge site by 1998.<ref name="independent.co.uk"/> 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|Chilton]], [[Oxfordshire]]), while other work went to the [[UK Astronomy Technology Centre]] in Edinburgh. The old observatory site at Greenwich returned to its original name  – the Royal Observatory, Greenwich – and was made part of the [[National Maritime Museum]].

In 2002 the UK joined the [[European Southern Observatory]], building the [[VISTA (telescope)|VISTA infrared telescope]] at the [[Paranal Observatory]] as an in-kind contribution.

The Astronomer Royal [[Martin Rees]] called PPARC "irresponsible" for how it handled the RGO.<ref>{{Cite web|url=https://www.newscientist.com/article/mg15520903-900-times-up-for-the-greenwich-observatory/|title=Time's up for the Greenwich observatory|website=www.newscientist.com|access-date=2019-11-15 | date = 12 July 1997 | last = Motluk | first = Alison}}</ref>

===Greenwich site returns to active use===
In 2018 the [[Annie S. D. Maunder|Annie Maunder]] Astrographic Telescope (AMAT) was installed at the ROG in Greenwich.<ref>{{cite web |url=https://www.theguardian.com/science/2018/jun/25/star-attraction-royal-observatory-seeks-volunteers-to-use-new-telescope |title=Star attraction: Royal Observatory seeks volunteers to use new telescope |newspaper=The Guardian |author= Ian Sample |date= 25 June 2018}}</ref><ref>{{cite web |title=Altazimuth Pavilion |url=https://www.rmg.co.uk/see-do/we-recommend/attractions/altazimuth-pavilion |publisher=Royal Observatory, Greenwich |access-date=25 June 2018|date=5 August 2015 }}</ref> AMAT is a cluster of four separate instruments, to be used for astronomical research; it had achieved [[first light (astronomy)|first light]] by June 2018, and contains:<ref>{{cite web |url=https://www.rmg.co.uk/discover/behind-the-scenes/blog/first-light |title=First Light: a new era for the Royal Observatory |publisher=Royal Observatory, Greenwich|date=25 June 2018  |author=Kate Wilkinson |access-date= 27 June 2018}}</ref>

* A 14-inch reflector that can take high-resolution images of the sun, moon and planets.
* An instrument dedicated to observing the sun.
* An instrument with interchangeable filters to view distant [[nebula]]e at different optical wavelengths.
* A general-purpose telescope.

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.

The telescope was installed in the Altazimuth Pavilion,<ref name="skyatnightmagazine">{{Cite web|url=https://www.skyatnightmagazine.com/news/new-telescope-for-royal-observatory-greenwich/|title=New telescope for Royal Observatory Greenwich|website=skyatnightmagazine|language=en|access-date=2019-11-15}}</ref> from which the multi-purpose telescope is controlled by a computer system.<ref name="skyatnightmagazine" />

== Magnetic observations ==
[[File:"The Magnetic Pavilion" - Royal Observatory Greenwich ca 1900 (7890151714).jpg|thumb|The Magnetic Pavilion, 1900]]
The first magnetic observation was taken in 1680 by the first Astronomer Royal, John Flamsteed, using a magnetic needle from the Royal Society.<ref name="Malin-1996">{{Cite journal|title=1996QJRAS..37...65M Page 65|journal = Quarterly Journal of the Royal Astronomical Society|volume = 37|pages = 65|bibcode = 1996QJRAS..37...65M|last1 = Malin|first1 = S. R. C.|year = 1996}}</ref> The second and third Astronomers Royal, [[Edmond Halley]] and then James Bradley, also took some magnetic measurements during their tenure.<ref name="Malin-1996" />

In the 19th century George Airy established the Magnetical and Meteorological Department.<ref name="Malin-1996" />

The first Magnetic House was built next to the observatory but by 1900 a second, about 300–400 metres from the main observatory, was built to reduce magnetic interference.<ref name="HMSO-1921">{{Cite book|url=https://books.google.com/books?id=adA0AQAAMAAJ&q=magnetic+pavilion+built+greenwich&pg=RA2-PR2|title=Observations Made at the Royal Observatory, Greenwich in the Year ... in Astronomy, Magnetism and Meteorology|date=1921|publisher=H.M. Stationery Office|language=en}}</ref> Both houses were made of non-magnetic materials.<ref name="HMSO-1921" /> The older building was called the Magnet House, but iron added to buildings in the 1890s at the observatory was throwing off measurements, so the instruments were moved to the Magnetic Pavilion.<ref name="HMSO-1921" /> A new Magnetograph House was also completed by 1914.<ref name="HMSO-1921" />

One of the special events that occurred in the study of magnetism was when [[François Arago]] and [[Alexander von Humboldt]] took magnetic observations at Greenwich in 1822.<ref name="1986JBAA...97...26H Page 26">{{Cite journal|title=1986JBAA...97...26H Page 26|journal = Journal of the British Astronomical Association|volume = 97|pages = 26|bibcode = 1986JBAA...97...26H|last1 = Howard-Duff|first1 = I.|year = 1986}}</ref> In 1825 Arago won the [[Copley Medal|Copley Gold Medal]] for this research<ref name="1986JBAA...97...26H Page 26"/> (see also [[Arago's rotations]]).

==Observatory museum==
[[File:Royal observatory Greenwich.JPG|thumb|upright=1.5|Tourists flock to the Observatory museum, 2009]]

The observatory buildings at Greenwich became a museum of astronomical and navigational tools, which is part of the Royal Museums Greenwich.<ref>{{cite web|url=http://www.rmg.co.uk/|title=Royal Museums Greenwich : Sea, Ships, Time and the Stars : RMG|date=25 August 2015}}</ref> Notable exhibits include [[John Harrison]]'s pioneering [[marine chronometer|chronometer]], known as [[John Harrison#H4|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 (United Kingdom)|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. Fedchenko clock (the most accurate pendulum clock ever built in multiple copies). It also houses the astronomical instruments used to make meridian observations and the 28-inch equatorial [[Howard Grubb|Grubb]] [[refracting telescope]] of 1893, the largest of its kind in the UK. The [[Shepherd Gate Clock|Shepherd Clock]] outside the observatory gate is an early example of an [[Clock#Synchronized or slave clocks|electric slave clock]].

In 1997 the observatory site was getting 400,000 visitors per year.<ref>{{Cite journal|last=Masood|first=Ehsan|date=1997-08-01|title=Royal observatory could return to Greenwich site|journal=Nature|language=en|volume=388|issue=6644|pages=705|doi=10.1038/41849|bibcode=1997Natur.388R.705.|issn=1476-4687|doi-access=free}}</ref>

In February 2005 a £16&nbsp;million redevelopment comprising a new [[planetarium]] and additional display galleries and educational facilities was started; the ROG reopened on 25 May 2007 with the new 120-seat [[Peter Harrison Planetarium]].<ref>{{cite web |url=http://www.rmg.co.uk/work-services/news-press/press-release/reopening-new-royal-observatory-greenwich |title=Press Release: Reopening of the new Royal Observatory, Greenwich |website=Royal Museums Greenwich |date=27 June 2007}}</ref>

For a year between 2016 and 2017 the Museum reported 2.41 million visitors.<ref>{{Cite web|url=https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/628588/HC_108_NMM_16-17_Web.pdf |archive-url=https://web.archive.org/web/20190725195606/https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/628588/HC_108_NMM_16-17_Web.pdf |archive-date=2019-07-25 |url-status=live|title=National Maritime Museum Annual Report and Accounts 2016-2017}}</ref>

In July 2024, Royal Museums Greenwich announced plans for a redevelopment of the Royal Observatory site to improve the visitor experience.<ref>{{cite web |title=Royal Museums Greenwich Royal Observatory Greenwich Project |url=https://docs.planning.org.uk/20240816/53/_GRNW_DCAPR_120992/txz08nlz8v4xxfds.pdf |website=Planning.org |access-date=8 April 2025}}</ref><ref>{{cite web |title=Planning Application Summary: Full Regeneration of the Royal Observatory Site |url=https://planning.royalgreenwich.gov.uk/online-applications/applicationDetails.do?keyVal=_GRNW_DCAPR_120991&activeTab=summary |website=Royal Borough of Greenwich |access-date=8 April 2025}}</ref><ref>{{cite news |title=Major refurbishment of the Royal Observatory Greenwich proposed |url=https://www.ianvisits.co.uk/articles/major-refurbishment-of-the-royal-observatory-greenwich-proposed-74958/ |access-date=8 April 2025 |work=IanVisits |date=23 August 2024}}</ref>
{{clear}}

==Gallery==
{{gallery <!-- Landscape format pictures -->
|height=200
|width=250
|File:Royal Observatory Greenwich London.jpg
   |The centuries-old Flamsteed House overlooking [[Greenwich Park]] in east London, seen looking south. The statue at left is of Major General [[James Wolfe]], who died capturing [[Quebec City|Quebec]] in 1759, and was buried in [[St Alfege Church, Greenwich]].|File:RoyalObservatoryShepherd1824 edited.jpg
   |Flamsteed House in 1824
|File:Royal Greenwich Observatory Postcard c1902.jpg
   |Royal Observatory, Greenwich, {{Circa|1902}}, as depicted on a postcard
|File:Herstmonceux Observatory aerial view.jpg
   |Aerial view of the Royal Greenwich Observatory, [[Herstmonceux]] site in [[East Sussex]]; the dome that formerly housed the [[Isaac Newton Telescope]] is the single dome to the right. The telescope was moved to [[La Palma]] in the [[Canary Isles]] in 1979.
|File:Greenwich observatory laser.jpg
   |Laser projected from the observatory marking the  [[Prime meridian (Greenwich)|Greenwich Prime Meridian]] line (1999)
|File:Royal Observatory.jpg
   |Laser at night (1999)
|File:EH1078998 Royal Observatory Former Great Equatorial Building 01.JPG
   |Entrance to the Great Equatorial building (2013)
|File:Royal Greenwich Observatory and Greenwich Park.jpg
  |The [[Queen's House]] (centre left) at Greenwich, with the Royal Observatory on the skyline behind (2017)
|File:Imperial Standards of Length, Greenwich.jpg
   |[[Imperial units|Imperial standard lengths]] on the wall of the Royal Observatory, Greenwich, London – 1 yard (3 feet), 2 feet, 1 foot, 6 inches (1/2-foot), and 3 inches. The separation of the inside faces of the marks is exact at an ambient temperature of {{convert|60|°F|°C}} and a rod of the correct measure, resting on the pins, will fit snugly between them.<ref>{{citation|last=Bennett|first=Keith|editor=Bucher, Jay L.|title=The Metrology Handbook|publisher=American Society for Quality Measurement|location=Milwaukee, WI|year=2004|page=8|isbn=978-0-87389-620-7}}.</ref><ref>{{citation|last=Walford|first=Edward|title=Old and New London|year=1878|volume=VI|url=http://www.british-history.ac.uk/report.aspx?compid=45276}}</ref>
}}

{{gallery   <!-- Portrait format pictures -->
|height=250
|width=200
|File:28-inch telescope, Royal Observatory and tree, Greenwich, London, UK, 2015.jpg
   |Dome of the [[Greenwich 28 inch refractor]] telescope and tree (2015)
|File:The Greenwich Observatory - geograph.org.uk - 1350541.jpg
   |Dome of the Great Equatorial Building overlooking [[Greenwich Park]]
|File:The Altazimuth Pavilion in Greenwich Park.jpg 
   |Modern view of the Altazimuth Pavilion
|File:Royalobs.jpg
   |Royal Observatory, Greenwich (2006)
}}

==See also==
* [[List of astronomical observatories]]

==References==
{{Reflist}}

==Further reading==
*''Greenwich Observatory:&nbsp;... 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}})

==External links==
{{Commons category}}
{{NSRW Poster|Greenwich Observatory}}
*{{official website|http://www.rmg.co.uk/}} of the Royal Museums Greenwich (RMG) – including section on Royal Observatory Greenwich (ROG)
**[http://www.rmg.co.uk/royal-observatory ROG on RMG website]
*[http://janus.lib.cam.ac.uk/db/node.xsp?id=EAD%2FGBR%2F0180 Online catalogue of the Royal Greenwich Observatory Archives (held at Cambridge University Library)]
*[https://books.google.com/books?id=qOIDAAAAMBAJ&pg=PA927 "Where the Earth's surface begins—and ends"], ''Popular Mechanics'', December 1930.
*[https://web.archive.org/web/20110709074037/http://www.ukho.gov.uk/HMNAO/Pages/Home.aspx HM Nautical Almanac Office]
*[http://maps.google.co.uk/maps?f=q&hl=en&q=Greenwich&ie=UTF8&t=k&om=1&z=19&ll=51.477815,-0.001556&spn=0.00073,0.002438&iwloc=A Aerial View of The Royal Observatory, Greenwich] at Google Maps
*[http://www.ing.iac.es/PR/int_info/history.htm Castle in the sky – The story of the Royal Greenwich Observatory at Herstmonceux]
* [http://www.royalobservatorygreenwich.org/devblog/wp-content/uploads/2015/05/RGO-Herstmonceux-site-plan-c.1967.jpg Map of the Royal Greenwich Observatory at Herstmonceux]
*[http://www.lib.cam.ac.uk/deptserv/manuscripts/RGO_history/  A Personal History of the Royal Greenwich Observatory at Herstmonceux Castle, 1948–1990 by George Wilkins, a former staff member]
*[http://www.the-observatory.org/ The Observatory Science Centre]
*[http://www.ing.iac.es Isaac Newton Group of Telescopes]
*[http://www.thegreenwichmeridian.org/ The Greenwich Meridian]

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