Editing Greenwich Mean Time

{{Short description|Time zone of Western Europe, same as WET}}
{{Redirect|GMT}}
{{pp-move-indef}}
{{Use British English | date=July 2014}}
{{Use dmy dates|date=April 2022}}

{{Infobox time zone
|caption = This is the zone marked '0' in the middle of the map, coloured green.
|initials = GMT
|display observance = Yes
}}

{{Time zones of Europe}}
{{Time zones of Africa}}
'''Greenwich Mean Time''' ('''GMT''') is the [[local mean time]] at the [[Royal Observatory, Greenwich|Royal Observatory]] in [[Greenwich]], London, counted from [[midnight]]. At different times in the past, it has been calculated in different ways, including being calculated from [[noon]];<ref>{{cite web |title=Time scales |url=https://www.ucolick.org/~sla/leapsecs/timescales.html#GMT |website=UCO Lick |access-date=28 July 2018}}</ref> as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as [[Western European Time|one of the names]] for the [[time zone]] [[UTC+00:00]] and,<ref name=RMG>{{cite web |title=What is Greenwich Mean Time? | publisher = [[Royal Museums Greenwich]] |date=2021 | access-date=28 October 2021 |url=https://www.rmg.co.uk/stories/topics/greenwich-mean-time-gmt |at=What does GMT stand for?}}</ref> in UK law, is the basis for [[civil time]] in the United Kingdom.<ref name=IA1978/>{{efn|[[British Summer Time]] is defined in law as being one hour in advance of Greenwich Mean Time.}}

Because of Earth's uneven [[angular velocity]] in its [[Elliptic orbit|elliptical orbit]] and its [[axial tilt]], noon (12:00:00) GMT is rarely the exact moment the Sun crosses the [[Prime meridian (Greenwich)|Greenwich Meridian]]{{efn|The 'Prime Meridian', 0°, was  [[International Meridian Conference|originally defined]] as being the [[Greenwich meridian]] but is now the "[[IERS Reference Meridian]]": they are not quite the same.<ref>{{cite web |title=What is the Prime Meridian and why is it in Greenwich? |url=https://www.rmg.co.uk/stories/topics/what-prime-meridian-why-it-greenwich |website=Royal Museums Greeenwich |access-date=13 December 2021 |quote=The IRM is the only meridian that may now be described as the prime meridian of the world, as it defines 0° longitude by international agreement. The IRM passes 102.5 metres to the east of the historic Prime Meridian of the World at the latitude of the Airy Transit Circle here. The entire Observatory and the historic Prime Meridian now lie to the west of the true prime meridian. }}</ref> }} and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the [[equation of time]]. Noon GMT is the annual average (the [[arithmetic mean]]) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".{{efn|There is no such thing as the "Greenwich Mean".}}

Originally, astronomers considered a GMT day to start at noon,{{efn|Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date.}} while for almost everyone else it started at midnight. To avoid confusion, the name [[Universal Time]] was introduced in 1928 to denote GMT as counted from midnight.{{sfn|McCarthy|Seidelmann|2009|page=17}}<ref>{{cite web | title=UTC (Universal Time Coordinated) | website=Greenwich Mean Time| access-date=12 May 2023| url=https://greenwichmeantime.com/articles/history/utc/}}</ref> Today, Universal Time usually refers to [[Coordinated Universal Time]] (UTC) or else to [[Universal Time|UT1]];<ref name="AAO-UT">{{cite web |title= Astronomical Almanac Online |at= "Glossary" s.v. Universal Time |year= 2020 |url= https://asa.hmnao.com/SecM/Glossary.html#ut |publisher= Her Majesty's Nautical Almanac Office |access-date= 27 February 2021 |archive-date= 23 February 2022 |archive-url= https://web.archive.org/web/20220223151156/http://asa.hmnao.com/SecM/Glossary.html#ut |url-status= dead }}</ref> English speakers often use GMT as a synonym for UTC.<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Coordinated_Universal_Time |archive-url=https://web.archive.org/web/20200614012007/https://www.lexico.com/definition/coordinated_universal_time |url-status=dead |archive-date=14 June 2020 |title=Coordinated Universal Time |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref> For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9{{nbsp}}s.{{efn|See [[Leap second]] and [[Coordinated Universal Time#Mechanism]] for the reasons for this.}} The term "GMT" should thus not be used for purposes that require precision.{{sfn|Hilton|McCarthy|2013|pages=231–232}}

The term "GMT" is especially used by institutional bodies within the [[United Kingdom]], such as the [[BBC World Service]], the [[Royal Navy]], and the [[Met Office]]; and others particularly in Arab countries, such as the [[Middle East Broadcasting Centre]] and Dubai-based [[OSN]].

== 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. | }}

== Ambiguity in the definition of GMT ==

Historically, GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention, dating from the work of [[Ptolemy]], was to refer to noon as zero hours (see [[Julian day]]). This contrasted with the civil convention of referring to midnight as zero hours dating from the [[Roman Empire]]. The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day. The instant that was designated as "December 31.5 GMT" in 1924 almanacs became "January 1.0 GMT" in 1925 almanacs. The term ''Greenwich Mean Astronomical Time'' (''GMAT'') was introduced to unambiguously refer to the previous noon-based astronomical convention for GMT.<ref>{{cite book
 | title = Astronomical Supplement to the Astronomical Almanac
 | publisher = University Science Books
 | year = 1992
 | isbn = 0-935702-68-7
 | page = 76
}}</ref> The more specific terms ''UT'' and ''UTC'' do not share this ambiguity, always referring to midnight as ''zero hours''.

== GMT in legislation ==

=== United Kingdom ===

Legally, the civil time used in the UK is called "Greenwich mean time" (without capitalisation), with an exception made for those periods when the [[Summer Time Act 1972]] orders an hour's shift for daylight saving. The [[Interpretation Act 1978]], section 9, provides that whenever an expression of time occurs in any Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time.<ref name=IA1978>{{Cite legislation UK |type=act |year= 1978|chapter=30 |act=Interpretation Act 1978 |section=9 |date=20 July 1978 |accessdate=30 October 2021 }}</ref> Under subsection 23, the same rule applies to deeds and other instruments.{{sfn|Myers|2007}}

During the experiment of 1968 to 1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all-year [[British Summer Time]] was called ''British Standard Time'' (''BST'').

In the UK, UTC+00:00 is disseminated to the general public in winter and [[UTC+01:00]] in summer.{{sfn|McCarthy|Seidelmann|2009|page=17}}{{sfn|Howse|1997|page=157}}

BBC radio stations broadcast the "six pips" of the [[Greenwich Time Signal]]. It is named from its original generation at the [[Royal Observatory, Greenwich|Royal Greenwich Observatory]] but is callibrated to UTC. If announced (such as near the start of summer time or of winter time), announcers on domestic channels declare the time as ''GMT'' or ''BST'' as appropriate. As the BBC World Service is broadcast to all time zones, the announcers use the term "Greenwich Mean Time" consistently throughout the year.

=== Other countries ===

Several countries define their local time by reference to Greenwich Mean Time.<ref name="AMANO">Dumortier, Hannelore, & Loncke (n.d.)</ref>{{sfn|Seago|Seidelmann|Allen|2011}} Some examples are:
*[[Belgium]]: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.<ref name="AMANO"/>
* [[Republic of Ireland|Ireland]]: "Standard Time" ({{Langx|ga|Am Caighdeánach}}) is defined as being one hour in advance of GMT.<ref name=IST>{{cite web| title= STANDARD TIME ACT, 1968; Section 1 | publisher =Government of Ireland |url=http://www.irishstatutebook.ie/eli/1968/act/23/enacted/en/print.html }}</ref> "Winter Time" ({{lang|ga|Am Geimhridh}}) is defined as being the same as GMT.<ref name=IWT>{{cite web| title= STANDARD TIME (AMENDMENT) ACT, 1971; Section 1 | publisher =Government of Ireland |url=http://www.irishstatutebook.ie/eli/1971/act/17/enacted/en/index.html }}</ref>{{efn|The net effect is that Ireland and the UK are in the same time zone.}}
*[[Canada]]: Interpretation Act, R.S.C. 1985, c. I-21, section 35(1). This refers to "standard time" for the several provinces, defining each in relation to "Greenwich time", but does not use the expression "Greenwich mean time". Several provinces, such as [[Nova Scotia]] (Time Definition Act. R.S., c. 469, s. 1), have their own legislation which specifically mentions either "Greenwich Mean Time" or "Greenwich mean solar time".

== Time zone ==
Greenwich Mean Time is defined in law as standard time in the following countries and areas, which also advance their clocks one hour (GMT+1) in summer.
* United Kingdom, where the summer time is called ''[[British Summer Time]]'' (''BST'')
* Ireland, where it is called ''Winter Time'',<ref name=IWT /> changing to Standard Time in summer.<ref name=IST />
*[[Portugal]] (with the exception of the [[Azores]])
*[[Canary Islands]]
*[[Faroe Islands]]
[[File:Donald Stewart's Monument- Kumasi.jpg|thumb|Clock in [[Kumasi]], [[Ghana]], set to GMT]]
Greenwich Mean Time is used as standard time all year round in the following countries and areas:
*[[Burkina Faso]]
*[[The Gambia]]
*[[Ghana]]
*[[Guinea]]
*[[Guinea-Bissau]]
*[[Iceland]]
*[[Ivory Coast]]
*[[Liberia]]
*[[Mali]]
*[[Mauritania]]
*[[Sahrawi Arab Democratic Republic]] [[Western Sahara War|(disputed)]]
*[[Saint Helena, Ascension and Tristan da Cunha]]
*[[Senegal]]
*[[Sierra Leone]]
*[[Togo]]

== See also ==

* [[Ruth Belville]] – "the Greenwich Time Lady", daughter of John Henry Belville, who was in the business of daily personal distribution of Greenwich Mean Time via a watch
* {{Annotated link |Coordinated Universal Time}}
* {{Annotated link |Greenwich Time Signal}}
* {{Annotated link |Marine chronometer}}
* {{Annotated link |Radio clock}}
* {{Annotated link |Royal Observatory, Greenwich}}
* {{anli|Time in Europe}}
* {{Annotated link |Time in the United Kingdom}}
* {{Annotated link |Western European Time}}
* {{Annotated link |Western European Summer Time}}
* [[Coordinated Universal Time#Time zones|Zulu Time]]

==Notes==
{{notelist}}

== References ==

{{Reflist}}

== Sources ==
{{refbegin |30em |indent=yes}}
* {{cite web |url=http://asa.usno.navy.mil/index.html |title=Astronomical Almanac Online |date=2015 |publisher=United States Naval Observatory and Her Majesty's Nautical Almanac Office |access-date=7 June 2015 |archive-date=20 April 2015 |archive-url=https://web.archive.org/web/20150420225915/http://asa.usno.navy.mil/index.html |url-status=dead }}
* {{cite web |last1=Dumortier |first1=J |last2=Hannelore |first2=D |last3=Loncke |first3=M. |date=n.d. |title=Legal Aspects of Trusted Time services in Europe |url=http://www.e-timing.net/legal%20report%20E-timing%20ICRI%20TS.pdf |archive-url=https://web.archive.org/web/20081003105251/http://www.e-timing.net/legal%20report%20E-timing%20ICRI%20TS.pdf |publisher=AMANO |access-date=8 July 2009 |archive-date=3 October 2008 }}
* {{cite journal |last=Guinot |first=Bernard |date=August 2011 |title=Solar time, legal time, time in use |journal=Metrologia |volume=48 |number=4 |page=S181–185 |bibcode=2011Metro..48S.181G |doi=10.1088/0026-1394/48/4/S08|s2cid=121852011 }}.
* {{cite book |last1=Hilton |first1=James L |last2=McCarthy |first2=Dennis D. |author-link2=Dennis McCarthy (scientist) |date=2013 |chapter=Precession, Nutation, Polar Motion, and Earth Rotation |editor1=Sean Urban |editor2= P. Kenneth Seidelmann  |title=Explanatory Supplement to the Astronomical Almanac |edition= 3rd |location=Mill Valley CA |publisher=University Science Books}}
* {{cite book |title= Greenwich time and the longitude |first=Derek |last=Howse |location=London |publisher=Philip Wilson |date=1997 |isbn=9780856674686 }}
* {{cite web |url=http://www.canlii.org/en/ca/laws/stat/rsc-1985-c-i-21/latest/rsc-1985-c-i-21.html |title=Interpretation Act, R.S.C. 1985, c. I-21. |date=2005 |publisher=Parliament of Canada |archive-url=https://web.archive.org/web/20090705082900/http://www.canlii.org/en/ca/laws/stat/rsc-1985-c-i-21/latest/rsc-1985-c-i-21.html |archive-date=5 July 2009 }} [[CanLII]]. (Canadian statute)
* {{cite web |title=Interpretation Act 1978 |url=http://www.legislation.gov.uk/ukpga/1978/30/contents |publisher=Parliament of the United Kingdom }} (UK statute, see also [[Interpretation Act 1978]])
* {{Cite web |url=http://www.irishstatutebook.ie/eli/2005/act/23/enacted/en/print.html |title=Interpretation Act 2005 |website=Electronic Irish Statute Book |language=en |access-date=2 January 2018 }} ([[Irish Statute Book]])
* {{cite book |last1=McCarthy |first1=Denis D |last2=Seidelmann |first2=P. Kenneth |date=2009 |title=TIME—From Earth Rotation to Atomic Physics |location= Weinheim |publisher=Wiley-VCH |isbn=9783527407804}}
* {{cite web |last1=Myers |first1=J. |date=2007 |title=History of legal time in Britain |url=https://www.polyomino.org.uk/british-time/ |access-date=2 September 2021 }} <!-- original url redirects here. Self-published source! -->
* {{cite journal |last1=Seago |first1=J.H. |last2=Seidelmann |first2=P. K. |last3=Allen |first3=Steve |url=http://www.ucolick.org/~sla/leapsecs/seago.pdf |archive-url=https://web.archive.org/web/20050204162503/http://www.ucolick.org/~sla/leapsecs/seago.pdf |archive-date=2005-02-04 |url-status=live |title=LEGISLATIVE SPECIFICATIONS FOR COORDINATING WITH UNIVERSAL TIME |journal=American Astronomical Society Publishing |date=2011 }} <!-- original url redirects here, paper has been updated -->
* {{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/271319.stm |title=Six pip salute |date=5 February 1999 |work=BBC News |access-date=9 July 2009 }}
* {{cite web |url=http://www.irishstatutebook.ie/1968/en/act/pub/0023/sec0001.html |title=Standard Time Act, 1968 |publisher=Office of the Attorney General |date=1968 }} [[Irish Statute Book]]
* {{cite web |url=http://www.irishstatutebook.ie/eli/1971/act/17/enacted/en/html |title=Standard Time (Amendment) Act, 1971 |publisher=Office of the Attorney General |date=1971 }} (Irish statute)
{{refend}}

==External links==
<!-- Please do not add commercial links, they will be deleted -->
* [https://hpiers.obspm.fr/ International Earth Rotation and Reference Systems Service]
* [http://www.ominous-valve.com/sounds/bbc0.mp3 The original BBC World Service GMT time signal in MP3 format]
* {{cite news | url=http://news.bbc.co.uk/1/hi/magazine/8266883.stm | title=At the centre of time | last=Rodgers | first=Lucy | date=20 October 2009 | work=[[BBC News]] | access-date=20 October 2009}}

{{Time measurement and standards}}

[[Category:1884 introductions]]
[[Category:Geography of the Royal Borough of Greenwich]]
[[Category:Time in the United Kingdom]]
[[Category:Time zones]]