Editing Prime meridian (section)

===Prime meridian at Greenwich{{anchor|Greenwich}}===
[[File:Prime meridian.jpg|thumb|upright=1.2|The line of the [[Prime meridian (Greenwich)|Greenwich meridian]] at the [[Royal Observatory, Greenwich]], England]]
{{Main|Prime meridian (Greenwich)}}
In October 1884 the Greenwich Meridian was selected by delegates (forty-one delegates representing twenty-five nations) to the [[International Meridian Conference]] held in [[Washington, D.C.]], [[United States]] to be the common zero of longitude and standard of time reckoning throughout the world.<ref name=imc>{{cite book|title=International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October, 1884. Protocols of the proceedings|url=https://www.gutenberg.org/files/17759/17759-h/17759-h.htm|publisher=Project Gutenberg|access-date=30 November 2012|date=1884|archive-date=18 December 2021|archive-url=https://web.archive.org/web/20211218082731/https://www.gutenberg.org/files/17759/17759-h/17759-h.htm|url-status=live}}</ref>{{efn|Voting took place on 13 October and the resolutions were adopted on 22 October 1884.<ref>{{harvnb|Howse|1997|pp=12, 137}}</ref> The modern prime meridian, the IERS Reference Meridian, is placed very near this meridian.<ref name="who decided" />}}

The position of the historic prime meridian, based at the [[Royal Observatory, Greenwich]], was established by [[George Biddell Airy|Sir George Airy]] in 1851. It was defined by the location of the [[Meridian circle#19th century|Airy Transit Circle]] ever since the first observation he took with it.<ref name="Greenwich Observatory p.10">{{cite book  |title=Greenwich Observatory ... the story of Britain's oldest scientific institution, the Royal Observatory at Greenwich and Herstmonceux, 1675–1975 |volume=1  |first=Eric Gray |last=Forbes |page=10 |publisher=Taylor & Francis |date=1975 |isbn=9780850660937}}</ref> Prior to that, it was defined by a succession of earlier transit instruments, the first of which was acquired by the second [[Astronomer Royal]], [[Edmond Halley]] in 1721. It was set up in the extreme north-west corner of the Observatory between Flamsteed House and the Western Summer House. This spot, now subsumed into Flamsteed House, is roughly 43 metres (47 yards) to the west of the Airy Transit Circle, a distance equivalent to roughly 2 seconds of longitude.{{sfn|Dolan|2013a}} It was Airy's transit circle that was adopted in principle (with French delegates, who pressed for adoption of the [[Paris meridian]] abstaining) as the Prime Meridian of the world at the 1884 International Meridian Conference.<ref name="Time">{{cite book|
title=TIME from Earth Rotation to Atomic Physics
|last1=McCarthy
|first1=Dennis
|author-link1=Dennis McCarthy (scientist)
|last2=Seidelmann
|first2=P. Kenneth
|pages=244–5
|location=Weinheim
|publisher=Wiley-VCH
|date=2009}}</ref><ref name="RMG">{{cite web|url= http://www.rmg.co.uk/explore/astronomy-and-time/astronomy-facts/history/the-prime-meridian-at-greenwich|title= The Prime Meridian at Greenwich|author= ROG Learning Team|date= 23 August 2002|work= Royal Museums Greenwich|access-date= 14 June 2012|archive-date= 7 November 2015|archive-url= https://web.archive.org/web/20151107023957/http://www.rmg.co.uk/explore/astronomy-and-time/astronomy-facts/history/the-prime-meridian-at-greenwich|url-status= live}}</ref>

All of these Greenwich meridians were located via an astronomic observation from the surface of the Earth, oriented via a [[plumb line]] along the direction of gravity at the surface. This astronomic Greenwich meridian was disseminated around the world, first via the [[lunar distance method]], then by chronometers carried on ships, then via telegraph lines carried by [[submarine communications cable]]s, then via radio time signals. One remote longitude ultimately based on the Greenwich meridian using these methods was that of the [[North American Datum#North American Datum of 1927|North American Datum 1927]] or NAD27, an ellipsoid whose surface best matches [[mean sea level]] under the [[United States]].