Editing Greenwich Mean Time (section)

== History ==

{{Main|History of longitude}}

[[Image:Greenwich clock.jpg|thumb|The [[Shepherd Gate Clock]] at the gates of the [[Royal Observatory, Greenwich]] is permanently kept on Greenwich Mean Time.]]

As the [[United Kingdom]] developed into an advanced [[maritime nation]], British mariners kept at least one [[marine chronometer|chronometer]] on GMT to calculate their [[longitude]] from the Greenwich meridian,{{efn|For explanation, see [[History of longitude#Chronometers]] but simplistically, there is a 15° difference of longitude for each hour that the time of local noon differs from Greenwich noon.}}  which was considered to have longitude zero degrees, by a convention adopted in the [[International Meridian Conference]] of 1884. Synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. But this practice, combined with mariners from other nations drawing from [[Nevil Maskelyne]]'s method of [[Lunar distance (navigation)|lunar distance]]s based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most [[time zone]]s were based upon GMT, as an offset of a number of hours (and occasionally half or quarter hours) "ahead of GMT" or "behind GMT".

Greenwich Mean Time was adopted across the island of [[Great Britain]] by the [[Railway Clearing House]] in 1847 and by almost all railway companies by the following year, from which the term ''[[railway time]]'' is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "[[local mean time]]" to be the official time.{{sfn|Howse|1997|page=114}} On 14 May 1880, a letter signed by "Clerk to Justices" appeared in ''The Times'', stating that "Greenwich time is now kept almost throughout England, but it appears that Greenwich time is not legal time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 p.m."<ref>CLERK TO JUSTICES. [http://find.galegroup.com/ttda/infomark.do?&source=gale&prodId=TTDA&userGroupName=qubelfast&tabID=T003&docPage=article&searchType=&docId=CS168214190&type=multipage&contentSet=LTO&version=1.0 "Time, Actual And Legal"]. Times, London, England, 14 May 1880: 10. The Times Digital Archive. Web. 18 August 2015.</ref><ref>{{cite book |last=Bartky |first=Ian R. |date=2007 |title=One Time Fits All: The Campaigns for Global Uniformity |url=https://books.google.com/books?id=rC6sAAAAIAAJ&q=%22Central+european+time%22+German+1893&pg=PA126 |publisher=Stanford University Press |page=134 |isbn=978-0804756426 |access-date=18 August 2015}}</ref> This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted in the [[Isle of Man]] in 1883, in [[Jersey]] in 1898 and in [[Bailiwick of Guernsey|Guernsey]] in 1913. [[Ireland]] adopted GMT in 1916, supplanting [[Dublin Mean Time]].{{sfn|Myers|2007}} Hourly [[time signal]]s from Greenwich Observatory were first broadcast by [[shortwave radio]] on 5 February 1924 at 17:30:00 UTC,<ref>{{cite news |title=Greenwich Time by Wireless— New Scheme Today |work=Liverpool Daily Post  |date=5 February 1924 |page=6 |quote=the last four seconds of the preceding minute will be heard as 'clicks' when the signal is about to be given, representing the 55th, 56th, 57th, 58th and 59th second, and the final click, which will be a little louder than the others}}</ref> providing a rival accurate time-source to the [[time ball]] at the Greenwich Observatory.<ref>{{cite dictionary |entry=Greenwich Time Signal |title=Historical Dictionary of British Radio |first= Sean |last=Street |publisher=Scarecrow Press |date=2015 |page=156}}</ref>

The daily rotation of the Earth is irregular (see [[ΔT (timekeeping)|ΔT]]) and has a slowing trend; therefore [[atomic clock]]s constitute a much more stable timebase. On 1 January 1972, GMT as the international civil time standard was superseded by [[Coordinated Universal Time|Coordinated Universal Time (UTC)]], maintained by an ensemble of atomic clocks around the world. [[Universal Time|''Universal Time'' (''UT'')]], a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally defined [[International Meridian Conference|universal day]]; from 1 January 1956 (as decided by the [[International Astronomical Union]] in [[Dublin]] in 1955, at the initiative of [[William Markowitz]]) this "raw" form of UT was re-labelled ''UT0'' and effectively superseded by refined forms UT1 (UT0 equalised for the effects of [[polar wandering]])<ref>{{Cite web |url=https://www.iers.org/IERS/EN/Science/EarthRotation/UT1LOD.html |website=IERS  |title=Universal Time (UT1) and Length of Day (LOD) |access-date=3 May 2024 |archive-date=5 April 2023 |archive-url=https://web.archive.org/web/20230405030706/https://www.iers.org/IERS/EN/Science/EarthRotation/UT1LOD.html |url-status=live }}</ref> and UT2 (UT1 further equalised for annual seasonal variations in Earth rotation rate).

{{Blockquote|Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by "the centre of the transit instrument at the Observatory at Greenwich". Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world's longitude and time is not strictly defined in material form but from a statistical solution resulting from observations of all time-determination stations which the [[International Bureau of Weights and Measures|BIPM]] takes into account when co-ordinating the world's time signals. Nevertheless, the line in the old observatory's courtyard today differs no more than a few metres from that imaginary line which is now the prime meridian of the world. | Howse, D. (1997). ''Greenwich time and the longitude''. London: Philip Wilson. | }}