Editing Royal Observatory, Greenwich (section)

=== 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" />