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


{{Infobox time zone
'''Greenwich Mean Time''' ('''GMT''') is a term originally referring to ] at the ] in ], ]. It is now often used to refer to ] (UTC) when this is viewed as a ], although strictly UTC is an ] ] which only approximates GMT in the old sense. It is also used to refer to ] (UT), which is the ] concept that directly replaced the original GMT. In the ], GMT is the official time only during winter; during summer ] is used.
|caption = This is the zone marked '0' in the middle of the map, coloured green.
|initials = GMT
|display observance = Yes
}}


{{Time zones of Europe}}
GMT is substantially equivalent to ].{{Fact|date=October 2008}}
{{Time zones of Africa}}
'''Greenwich Mean Time''' ('''GMT''') is the ] at the ] in ], ], counted from ]. At different times in the past, it has been calculated in different ways, including being calculated from ];<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 ] for the ] ] and,<ref name=RMG>{{cite web |title=What is Greenwich Mean Time? | publisher = ] |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 ] in the United Kingdom.<ref name=IA1978/>{{efn|] is defined in law as being one hour in advance of Greenwich Mean Time.}}


Because of Earth's uneven ] in its ] and its ], noon (12:00:00) GMT is rarely the exact moment the Sun crosses the ]{{efn|The 'Prime Meridian', 0°, was ] as being the ] but is now the "]": 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 ]. Noon GMT is the annual average (the ]) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".{{efn|There is no such thing as the "Greenwich Mean".}}
Noon Greenwich Mean Time is not necessarily the moment when the ] crosses the ] (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic ] and its ]. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the ]). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of ''mean'' in Greenwich Mean Time.


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 ] 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 ] (UTC) or 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=] UK English Dictionary |publisher=]}}</ref> For navigation, it is considered equivalent to ] (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9{{nbsp}}s. The term "GMT" should thus not be used for purposes that require precision.{{sfn|Hilton|McCarthy|2013|pages=231–232}}
Historically the term GMT has been used with two different conventions for numbering hours. The old astronomical convention (before 1925) was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours. The latter is modern astronomical and civil convention. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

The term "GMT" is especially used by institutional bodies within the ], such as the ], the ], and the ]; and others particularly in Arab countries, such as the ] and ].


== History == == History ==
{{main|History of longitude}}
As the United Kingdom grew into an advanced ], British mariners kept at least one ] on GMT in order to calculate their ] from the ], which was by convention considered to have longitude zero degrees (convention which was internationally adopted in the ] of 1884) . This did not affect shipboard time itself, which was still solar time. This, combined with mariners from other nations drawing from ]'s method of ]s based on observations at Greenwich, eventually led to GMT being used worldwide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".


{{Main|History of longitude}}
Greenwich Mean Time was adopted across the island of ] by the ] in 1847, and by almost all railway companies by the following year from which the term ] is derived. It was gradually adopted for other purposes, but a ] in 1858 held "]" to be the official time. This changed in 1880, when GMT was legally adopted throughout the island of Great Britain. GMT was adopted on the ] in 1883, ] in 1898 and ] in 1913. Ireland adopted Greenwich Mean Time in 1916, supplanting ].<ref name="Myers">Myers, Joseph. (2007). Retrieved 4 January 2008.</ref> Hourly ]s from Greenwich Observatory were first broadcast on 5 February 1924, rendering the ] at the observatory obsolete in the process.


] at the gates of the ] is permanently kept on Greenwich Mean Time.]]
The daily rotation of the Earth is somewhat irregular (see ]) and is slowing down slightly; ]s constitute a much more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by ], maintained by an ensemble of atomic clocks around the world. ], introduced in 1928, represents ] time. ]s are added to or subtracted from UTC to keep it within 0.9 seconds of UT1.


As the ] developed into an advanced ], British mariners kept at least one ] on GMT to calculate their ] from the Greenwich meridian,{{efn|For explanation, see ] 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 ] 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 ]'s method of ]s based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most ]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".
<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 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."<ref>Howse 1997, p. 178</ref></blockquote>


Greenwich Mean Time was adopted across the island of ] by the ] in 1847 and by almost all railway companies by the following year, from which the term '']'' is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "]" 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. . 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 ] in 1883, in ] in 1898 and in ] in 1913. ] adopted GMT in 1916, supplanting ].{{sfn|Myers|2007}} Hourly ]s from Greenwich Observatory were first broadcast by ] 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> rendering the ] at the observatory redundant.<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>
== Greenwich Mean Time in legislation ==
Several countries throughout the world legislatively define their local time by explicit reference to Greenwich Mean Time.<ref name="AMANO">{{cite web |url=http://www.e-timing.net/legal%20report%20E-timing%20ICRI%20TS.pdf |title=Legal Aspects of Trusted Time Services in Europe |author=Jos Dumortier, Hannelore Dekeyser, Mieke Loncke |publisher=K.U. Leuven Interdisciplinary Centre for Law & ICT (ICRI) |work=Amano Legal Report |accessdate=2008-08-07}}</ref><ref name="Seago">{{cite web |url=http://www.ucolick.org/~sla/leapsecs/seago.pdf |author=John H. Seago, P. Kenneth Seidelmann |title=National Legal Requirements for Coordinating with Universal Time |accessdate=2008-08-07}}</ref> Some examples are:


The daily rotation of the Earth is irregular (see ]) and has a slowing trend; therefore ]s constitute a much more stable timebase. On 1 January 1972, GMT as the international civil time standard was superseded by ], maintained by an ensemble of atomic clocks around the world. ], a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally defined ]; from 1 January 1956 (as decided by the ] in ] in 1955, at the initiative of ]) this "raw" form of UT was re-labelled ''UT0'' and effectively superseded by refined forms UT1 (UT0 equalised for the effects of ])<ref>{{Cite web |url=https://www.iers.org/IERS/EN/Science/EarthRotation/UT1LOD.html |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).
*United Kingdom: The Interpretation Act 1978, section 9 - provides that whenever an expression of time occurs in an Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time. Under subsection 23(3), the same rule applies to deeds and other instruments.<ref>{{cite web |url=http://www.bailii.org/uk/legis/num_act/1978/1838152.html |title=Interpretation Act |accessdate=2008-08-07}}</ref><ref name="Myers"/>


{{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 ] 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. | }}
*Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.<ref name="AMANO"/>


== Ambiguity in the definition of GMT ==
*Republic of Ireland: Standard Time (Amendment) Act, 1971, section 1,<ref>{{cite web |url=http://www.bailii.org/ie/legis/num_act/1971/0017.html#zza17y1971s1 |title=Standard Time (Amendment) Act, 1971 (Ireland) |accessdate=2008-08-07}}</ref> and Interpretation Act 2005, section 18(i).<ref>{{cite web |url=http://www.bailii.org/ie/legis/num_act/2005/0023.html#partiv-sec18 |title=Interpretation Act (Ireland) |accessdate=2008-08-07}}</ref>


Historically, GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention, dating from the work of ], was to refer to noon as zero hours (see ]). This contrasted with the civil convention of referring to midnight as zero hours dating from the ]. 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
*Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35.<ref>{{cite web| url=http://www.canlii.org/ca/sta/i-21/sec35.html |title=Interpretation Act (Canada) |accessdate=2008-08-07}}</ref>
| 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''.


== Time zone == == GMT in legislation ==
Although ] in the ], e.g., the ], is in practice now based on UTC,<ref>{{cite web |url=http://news.bbc.co.uk/1/hi/sci/tech/271319.stm |title=Six pip salute |date=] |accessdate=2008-08-07 |work=BBC Online Network |quote=‘It’s a little known but interesting fact that the Greenwich Time Signal no longer gives Greenwich Mean Time,’ said the NPL’s John Chambers. ‘Since 1972, all the time signals in the world have been based on atomic time.’}}</ref> the winter time scale, which is equal to UTC, is still popularly called GMT. Civil time in the UK is legally (but not practically) still based on astronomical GMT, not UTC.
Those countries marked in dark blue on the map above use ] and advance their clock one hour in summer. In the United Kingdom, this is known as ] (BST); in the Republic of Ireland it is called Irish Standard Time (IST) - officially changing to GMT in winter. Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.


== Anomalies == === United Kingdom ===
Since political, in addition to purely geographical, criteria are used in the drawing of time zones, it follows that actual time zones do not precisely adhere to meridian lines. The GMT time zone, were it drawn by purely ], would consist of exactly the area between meridians 7°30'W and 7°30'E. As a result, there are European locales that despite lying in an area with a 'physical' UTC time, actually use another time zone (] in particular); contrariwise, there are European areas that use UTC, even though their 'physical' time zone is ] (e.g., most of ]), or even UTC−2 (the westernmost part of ]). Actually, because the UTC time zone in Europe is 'shifted' to the west, ] in ], ], ] at only 1°45'E is the easternmost settlement in Europe in which UTC is applied. Following is a list of the 'incongruencies':


Legally, the civil time used in the UK is called "Greenwich mean time" (without capitalisation), with an exception made for those periods when the ] orders an hour's shift for daylight saving. The ], 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}}
;Countries (or parts thereof) west of 22°30'W ('physical' UTC-2) that use UTC
* The westernmost part of Iceland, incl. the northwest peninsula and its main town of ], which is west of 22°30'W, uses UTC. ], Iceland is the westernmost point in which UTC is applied.


During the experiment of 1968 to 1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all-year ] was called ''British Standard Time'' (''BST'').
;Countries (or parts thereof) west of 7°30'W ('physical' UTC-1) that use UTC
* ] (])
* Most of ], incl. ], ], ], ], and ]. (Only the easternmost part, incl. cities such as ] and ], lies east of 7°30'W.) The ], even further to the west, also employ UTC.
* Western part of the ], incl. the cities of ], ], and ]
* Westernmost tip of ], incl. the capital of ], ]
* Extreme westerly portion of the ], west of ]; for instance, ], an inhabited island in the ] and the westernmost settlement in the whole of ], lies at 7°54'W. If ]s and/or rocks are to be taken into account then ], west of the ], at 8°58'W, and ], at 13°41'W, should also be included.
* Westernmost island of the ] (autonomous region of the ]), ]
* ], including ]


In the UK, UTC+00:00 is disseminated to the general public in winter and ] in summer.{{sfn|McCarthy|Seidelmann|2009|page=17}}{{sfn|Howse|1997|page=157}}
;Countries (mostly) between meridians 7°30'W and 7°30'E ('physical' UTC) that use UTC+1

* ] (except for the ] which use UTC). In fact parts of ] lie west of 7°30'W ('physical' UTC-1), whereas there is no Spanish territory east of 7°30'E ('physical' UTC+1). ]'s Presidential Order published in ] of March 8th, 1940, abandoned Greenwich UTC time in favor of UTC+1 effective 23:00 March 16th 1940, in consideration of the "convenience from the national time marching in step according to that of other European countries"<ref>{{cite web |url=http://horariosenespana.com/publicaciones/espana-en-hora/habitos-y-horarios-espanoles.php#1subir |title=Hábitos y horarios españoles |accessdate=2008-11-27 }}</ref> (notably Nazi Germany).
BBC radio stations broadcast the "six pips" of the ]. It is named from its original generation at the ]. 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 ===
* Most of ], incl. the cities of ], ] and ]. Only small parts of ], ] and ] are east of 7°30'E ('physical' UTC+1).

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:
*]: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.<ref name="AMANO"/>
* ]: "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><ref>{{Cite web|url=http://www.acts.ie/ga.act.1971.0017.1.html|title=AN tACHT UM AM CAIGHDEÁNACH (LEASÚ), 1971|first=Office of the Houses of the Oireachtas House Services Directorate Bills|last=Office|website=www.acts.ie}}</ref>
*]: 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 ] (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".
*]: The term GMT is still in use when it comes to electronics such as cellular phones.{{cn|date=June 2024}}{{clarify|reason=Android phones are cell phones, so disproved straight away. Do iPhones say GMT or UTC? Something else? DECT phones maybe?|date=June 2024}} Android phones use "UTC" but keypad phones{{which|date=June 2024}} use "GMT" to define any time zone around the world.{{cn|date=June 2024}}

== 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 '']'' (''BST'')
* Ireland, where it is called ''Winter Time'',<ref name=IWT /> changing to Standard Time in summer.<ref name=IST />
*] (with the exception of the ])
*]
*]
], ], set to GMT]]
Greenwich Mean Time is used as standard time all year round in the following countries and areas:
*]
*]
*]
*]
*]
*]
*]
*]
*]
*]
*] ]
*]
*]
*]
*]


== See also == == See also ==
*]&nbsp;&mdash; United States' time signal service
*]&nbsp;&mdash; Canada's time signal service
*]&nbsp;&mdash; China's national time signal service
*]&nbsp;&mdash; Germany's time signal service
*]&nbsp;&mdash; Australia's national time signal service
*]&nbsp;&mdash; synchronised with GMT, and used by ships to calculate their longitude
*]&nbsp;&mdash; 24-hour wristwatch
*]&nbsp;&mdash; alternative, decimal measure of time
*]
*]


* ] – "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
== Notes ==
* {{Annotated link |Coordinated Universal Time}}
{{reflist}}
* {{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}}
* ]

==Notes==
{{notelist}}


== References == == References ==
*Howse, Derek. (1997). ''Greenwich time and the longitude''. London: Philip Wilson.
*Myers, Joseph. (2007). Retrieved 4 January 2008.


{{Reflist}}
== External links ==

*
== 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 }} ]. (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 ])
* {{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 }} (])
* {{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 }} ]
* {{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==
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* {{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=] | access-date=20 October 2009}}


{{Time measurement and standards}} {{Time measurement and standards}}


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Latest revision as of 00:32, 17 December 2024

Time zone of Western Europe, same as WET "GMT" redirects here. For other uses, see GMT (disambiguation).

Greenwich Mean Time
Time zone
UTC offset
GMTUTC+00:00
Current time
01:08, 13 January 2025 GMT
Observance of DST
DST is observed throughout this time zone.
Time in Europe:
Light Blue Western European Time / Greenwich Mean Time (UTC)
Blue Western European Time / Greenwich Mean Time (UTC)
Western European Summer Time / British Summer Time / Irish Standard Time (UTC+1)
Red Central European Time (UTC+1)
Central European Summer Time (UTC+2)
Yellow Eastern European Time / Kaliningrad Time (UTC+2)
Ochre Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
Green Moscow Time / Turkey Time (UTC+3)
Turquoise Armenia Time / Azerbaijan Time / Georgia Time / Samara Time (UTC+4)
▉▉▉▉ Pale colours: Standard time observed all year
▉▉▉ Dark colours: Summer time observed
Time zones of Africa:
Light Blue Cape Verde Time (UTC−1)
Blue Greenwich Mean Time (UTC)
Red (UTC+1)
Ochre (UTC+2)
Green East Africa Time (UTC+3)
Turquoise (UTC+4)
The islands of Cape Verde and Canary Islands are to the west of the African mainland.
Mauritius and the Seychelles are to the east and north-east of Madagascar respectively.

Greenwich Mean Time (GMT) is the local mean time at the 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; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

Because of Earth's uneven angular velocity in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Greenwich Meridian 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".

Originally, astronomers considered a GMT day to start at noon, 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. Today, Universal Time usually refers to Coordinated Universal Time (UTC) or UT1; English speakers often use GMT as a synonym for UTC. 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 s. The term "GMT" should thus not be used for purposes that require precision.

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 OSN.

History

Main article: History of longitude
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 chronometer on GMT to calculate their longitude from the Greenwich meridian, 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 distances based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most time zones 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. 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." 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 Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time. Hourly time signals from Greenwich Observatory were first broadcast by shortwave radio on 5 February 1924 at 17:30:00 UTC, rendering the time ball at the observatory redundant.

The daily rotation of the Earth is irregular (see ΔT) and has a slowing trend; therefore atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT as the international civil time standard was superseded by Coordinated Universal Time (UTC), maintained by an ensemble of atomic clocks around the world. 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 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) and UT2 (UT1 further equalised for annual seasonal variations in Earth rotation rate).

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 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. 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. Under subsection 23, the same rule applies to deeds and other instruments.

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.

BBC radio stations broadcast the "six pips" of the Greenwich Time Signal. It is named from its original generation at the Royal Greenwich Observatory. 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. Some examples are:

  • Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.
  • Ireland: "Standard Time" (Irish: Am Caighdeánach) is defined as being one hour in advance of GMT. "Winter Time" (Am Geimhridh) is defined as being the same as GMT.
  • 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".
  • Philippines: The term GMT is still in use when it comes to electronics such as cellular phones. Android phones use "UTC" but keypad phones use "GMT" to define any time zone around the world.

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.

Clock in Kumasi, Ghana, set to GMT

Greenwich Mean Time is used as standard time all year round in the following countries and areas:

See also

Notes

  1. British Summer Time is defined in law as being one hour in advance of Greenwich Mean Time.
  2. The 'Prime Meridian', 0°, was originally defined as being the Greenwich meridian but is now the "IERS Reference Meridian": they are not quite the same.
  3. There is no such thing as the "Greenwich Mean".
  4. Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date.
  5. 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.

References

  1. "Time scales". UCO Lick. Retrieved 28 July 2018.
  2. "What is Greenwich Mean Time?". Royal Museums Greenwich. 2021. What does GMT stand for?. Retrieved 28 October 2021.
  3. ^ "Interpretation Act 1978: Section 9", legislation.gov.uk, The National Archives, 20 July 1978, 1978 c. 30 (s. 9), retrieved 30 October 2021
  4. "What is the Prime Meridian and why is it in Greenwich?". Royal Museums Greeenwich. Retrieved 13 December 2021. 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.
  5. ^ McCarthy & Seidelmann 2009, p. 17.
  6. "UTC (Universal Time Coordinated)". Greenwich Mean Time. Retrieved 12 May 2023.
  7. "Astronomical Almanac Online". Her Majesty's Nautical Almanac Office. 2020. "Glossary" s.v. Universal Time. Archived from the original on 23 February 2022. Retrieved 27 February 2021.
  8. "Coordinated Universal Time". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 14 June 2020.
  9. Hilton & McCarthy 2013, pp. 231–232.
  10. Howse 1997, p. 114.
  11. CLERK TO JUSTICES. "Time, Actual And Legal". Times, London, England, 14 May 1880: 10. The Times Digital Archive. Web. 18 August 2015.
  12. Bartky, Ian R. (2007). One Time Fits All: The Campaigns for Global Uniformity. Stanford University Press. p. 134. ISBN 978-0804756426. Retrieved 18 August 2015.
  13. ^ Myers 2007.
  14. "Greenwich Time by Wireless— New Scheme Today". Liverpool Daily Post. 5 February 1924. p. 6. 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
  15. Street, Sean (2015). "Greenwich Time Signal". Historical Dictionary of British Radio. Scarecrow Press. p. 156.
  16. "Universal Time (UT1) and Length of Day (LOD)". Archived from the original on 5 April 2023. Retrieved 3 May 2024.
  17. Astronomical Supplement to the Astronomical Almanac. University Science Books. 1992. p. 76. ISBN 0-935702-68-7.
  18. Howse 1997, p. 157.
  19. ^ Dumortier, Hannelore, & Loncke (n.d.)
  20. Seago, Seidelmann & Allen 2011.
  21. ^ "STANDARD TIME ACT, 1968; Section 1". Government of Ireland.
  22. ^ "STANDARD TIME (AMENDMENT) ACT, 1971; Section 1". Government of Ireland.
  23. Office, Office of the Houses of the Oireachtas House Services Directorate Bills. "AN tACHT UM AM CAIGHDEÁNACH (LEASÚ), 1971". www.acts.ie.

Sources

External links

Time measurement and standards
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