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Greenwich
Greenwich
Mean Time
Time
(GMT) is the mean solar time at the Royal Observatory in Greenwich, London. GMT was formerly used as the international civil time standard, now superseded in that function by Coordinated Universal Time
Coordinated Universal Time
(UTC). Today GMT is considered equivalent to UTC for UK civil purposes (but this is not formalised) and for navigation is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); these two meanings can differ by up to 0.9 s. Consequently, the term GMT should not be used for precise purposes.[1] Because of Earth's uneven speed in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the sun crosses the Greenwich
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 calculated by the equation of time. Noon
Noon
GMT is the annual average (i.e. "mean") moment of this event, which accounts for the word "mean" in " Greenwich
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
Universal Time
was introduced to denote GMT as counted from midnight.[2] Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date. Today Universal Time
Universal Time
usually refers to UTC or UT1.[3] The term "GMT" is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN. It is a term commonly used in the United Kingdom and countries of the Commonwealth, including Australia, New Zealand, South Africa, India, Pakistan, Bangladesh and Malaysia; and in many other countries of the eastern hemisphere. In some countries (Britain for example) Greenwich
Greenwich
Mean Time
Time
is the legal time in the winter and the population uses the term. See GMT in legislation, below, for further explanation.

Contents

1 History 2 Ambiguity in the definition of GMT 3 GMT in legislation

3.1 United Kingdom 3.2 Other countries

4 Time
Time
zone 5 Discrepancies between legal GMT and geographical GMT 6 See also 7 Notes 8 References 9 External links

History[edit] Main article: History of longitude

Greenwich
Greenwich
clock with standard measurements

As the United Kingdom
United Kingdom
grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich
Greenwich
meridian, which was by convention considered to have longitude zero degrees, internationally adopted in the International Meridian Conference
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 possibly a half-hour) "ahead of GMT" or "behind GMT". Greenwich
Greenwich
Mean Time
Time
was adopted across the island of Great Britain
Great Britain
by the Railway Clearing House
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.[4] On 14 May 1880, a letter signed by 'Clerk to Justices' appeared in 'The Times', stating that " Greenwich
Greenwich
time is now kept almost throughout England, but it appears that Greenwich
Greenwich
time is not legal time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 p.m."[5][6] This was changed later in 1880, when Greenwich
Greenwich
Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted on the Isle of Man
Isle of Man
in 1883, Jersey
Jersey
in 1898 and Guernsey
Guernsey
in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time.[7] Hourly time signals from Greenwich
Greenwich
Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory redundant. The daily rotation of the Earth is irregular (see ΔT) and constantly slows; therefore the atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was superseded as the international civil time standard by Coordinated Universal Time, 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 universal day; from 1 January 1956 (as decided by the IAU
IAU
at Dublin, 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)[8] and UT2 (UT1 further equalised for annual seasonal variations in earth rotation rate).

Indeed, even the Greenwich
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
Greenwich
time and the longitude. London: Philip Wilson.

Ambiguity in the definition of GMT[edit] 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 Romans. 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 'December 31.5 GMT' in 1924 almanacs became 'January 1.0 GMT' in 1925 almanacs. The term Greenwich
Greenwich
Mean Astronomical Time
Time
(GMAT) was introduced to unambiguously refer to the previous noon-based astronomical convention for GMT.[9] The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours. GMT in legislation[edit] United Kingdom[edit] Legally, the civil time used in Britain is called still "Greenwich mean time" (without capitalisation), according to the Interpretation Act 1978, with an exception made for those periods when the Summer Time
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 an Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich
Greenwich
mean time. Under subsection 23(3), the same rule applies to deeds and other instruments.[7] During the experiment of 1968-1971, when the British Isles did not revert to Greenwich
Greenwich
Mean Time
Time
during the winter, the all-year British Summer Time
Time
was called British Standard Time
Time
(BST). In Britain, UTC+0 is disseminated to the general public in winter and UTC+1
UTC+1
in summer.[10][2] BBC radio stations broadcast the "six pips" of the Greenwich
Greenwich
Time Signal. It is named from its original generation at the Royal Greenwich
Greenwich
Observatory, and is aligned to either Greenwich
Greenwich
Mean Time
Time
or British Summer Time
British Summer Time
as appropriate for the time of year. A coded radio signal is broadcast in the UK by Time
Time
from NPL. Other countries[edit] Several countries define their local time by reference to Greenwich Mean Time.[11][12] Some examples are:

Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.[11] Ireland: Standard Time
Time
(Amendment) Act, 1971, section 1, and Interpretation Act 2005, part iv, section 18(i). 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
Greenwich
time', but does not use the expression ' Greenwich
Greenwich
mean time'. Several provinces, such as Nova Scotia
Nova Scotia
(Time Definition Act. R.S., c. 469, s. 1), have their own legislation which specifically mentions either " Greenwich
Greenwich
Mean Time" or " Greenwich
Greenwich
mean solar time".

Time
Time
zone[edit] See also: UTC±00:00 Greenwich
Greenwich
Mean Time
Time
is used as standard time in the following countries, which also advance their clock one hour (GMT+1) in summer.

United Kingdom, where the summer time is called British Summer Time (BST) Republic of Ireland, where it is called Irish Standard Time (IST)[13]—officially changing to GMT in winter. Portugal
Portugal
(with the exception of the Azores) Morocco Western Sahara

Country subdivisions or dependent territories:

Canary Islands
Canary Islands
(Spain) Faroe Islands

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

Iceland Burkina Faso The Gambia Ghana Guinea Guinea-Bissau Ivory Coast Liberia Mali Mauritania Senegal Sierra Leone Togo Saint Helena, Ascension and Tristan da Cunha
Saint Helena, Ascension and Tristan da Cunha
(to UK)

Discrepancies between legal GMT and geographical GMT[edit]

Colour Legal time vs local mean time

1 h ± 30 m behind

0 h ± 30 m

1 h ± 30 m ahead

2 h ± 30 m ahead

3 h ± 30 m ahead

European winter

European summer

Since legal, political, social and economic criteria in addition to physical or geographical criteria are used in the drawing of time zones, actual time zones do not precisely adhere to meridian lines. The 'GMT' time zone, were it drawn by purely geographical terms, would consist of 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 use another time zone ( UTC+1
UTC+1
in particular); conversely, there are European areas that use UTC, even though their 'physical' time zone is UTC−1 (e.g., most of Portugal), or UTC−2 (the westernmost part of Iceland). Because the UTC time zone in Europe is 'shifted' to the west, Lowestoft
Lowestoft
in the United Kingdom
United Kingdom
at only 1°45'E is the easternmost settlement in Europe in which UTC is applied. Following is a list of the 'incongruencies':

Countries (or parts thereof) west of 22°30'W ("physical" UTC−2) that use UTC

The westernmost part of Iceland, including the northwest peninsula and its main town of Ísafjörður, which is west of 22°30'W, uses UTC. Bjargtangar, Iceland
Iceland
is the westernmost point in which UTC is applied.

Countries (or parts thereof) west of 7°30'W ("physical" UTC−1) that use UTC

Canary Islands
Canary Islands
(Spain) Most of Portugal, including Lisbon, Porto, Braga, Aveiro, and Coimbra. (Only the easternmost part, including cities such as Bragança and Guarda, lies east of 7°30'W.) Since the Treaty of Windsor in 1386 (the world's oldest diplomatic alliance), Portugal
Portugal
has maintained close ties to Britain, which possibly explains its choice of UTC. Madeira, even further to the west, also employs UTC. A more likely explanation is that during the mid-1970s, when Portugal
Portugal
was on Central European Time
Time
all year round, it did not begin to get light in Lisbon in winter until 08:30. Western part of Ireland, including the cities of Cork, Limerick, and Galway. Westernmost tip of Northern Ireland, including the county town of County Fermanagh, Enniskillen Extreme westerly portion of the Outer Hebrides, west of Scotland; for instance, Vatersay, an inhabited island and the westernmost settlement in Great Britain, lies at 7°54'W. If uninhabited islands or rocks are taken into account St Kilda, west of the Outer Hebrides, at 8°58'W, and Rockall, at 13°41'W, should be included. Westernmost island of the Faroe Islands
Faroe Islands
(autonomous region of the Danish Kingdom), Mykines Iceland, including Reykjavík Northeastern part of Greenland, including Danmarkshavn

This arch that stretches over a highway indicates the prime meridian in Spain.

Countries (mostly) between meridians 7°30'W and 7°30'E ("physical" UTC) that use UTC+1

Spain
Spain
(except for the Canary Islands, which use UTC). Parts of Galicia lie west of 7°30'W ('physical' UTC−1), whereas there is no Spanish territory east of 7°30'E ('physical' UTC+1). Spain's time is the direct result of Franco's presidential order (published in Boletín Oficial del Estado of 8 March 1940)[14] abandoning Greenwich
Greenwich
Mean Time and advancing clocks one hour effective 23:00 16 March 1940. This is an excellent example of political criteria used in the drawing of time zones: the time change was passed "in consideration of the convenience from the national time marching in step according to that of other European countries".[15][16] The presidential order (most likely enacted to be in synchrony with Germany
Germany
and Italy, with which the Franco regime was unofficially allied) included in its 5th article a provision for its future phase out,[16] which never took place. Due to this political decision Spain
Spain
is two hours ahead of its local mean time during the summer, one hour ahead in winter, which possibly explains the notoriously late schedule for which the country is known.[17] In Portugal, which is a mere one hour behind Spain, the timetable is quite different. Most of France, including the cities of Paris, Marseilles
Marseilles
and Lyon. Only small parts of Alsace, Lorraine and Provence
Provence
are east of 7°30'E ("physical" UTC+1). Monaco Andorra Belgium Netherlands Luxembourg

See also[edit]

Coordinated Universal Time Greenwich
Greenwich
Time
Time
Signal Ruth Belville
Ruth Belville
- the Greenwich
Greenwich
Time
Time
Lady, daughter of John Henry Belville and personal distribution of Greenwich
Greenwich
Mean Time
Time
via a watch. 24-hour watch—24-hour wristwatch Radio clock Marine chronometer—synchronised with GMT, and used by ships to calculate their longitude Time
Time
in the United Kingdom Swatch Internet Time—alternative, decimal measure of time Western European Summer Time

Notes[edit]

^ Hilton and McCarthy 2013, p. 231–2. ^ a b McCarthy & Seidelmann 2009, p. 17. ^ Astronomical Almanac Online 2015, Glossary s.v. "Universal Time". ^ Howse 1997, p. 114. ^ CLERK TO JUSTICES. "Time, Actual And Legal". Times, London, England, 14 May 1880: 10. The Times Digital Archive. Web. 18 Aug. 2015. ^ Bartky, Ian R. (2007). One Time
Time
Fits All: The Campaigns for Global Uniformity. Stanford University Press. p. 134. ISBN 0804756422. Retrieved 18 August 2015.  ^ a b Myers (2007). ^ UT1 as explained on IERS page ^ Astronomical Supplement to the Astronomical Almanac. University Science Books. 1992. p. 76. ISBN 0-935702-68-7.  ^ Howse 1997, p. 157. ^ a b Dumortier, Hannelore, & Loncke (n.d.) ^ Seago & Seidelmann (2011). ^ Standard Time
Time
Act, 1968. ^ "BOE Orden sobre adelanto de la hora legal en 60 minutos". Retrieved 2 December 2008.  ^ "B.O.E. #68 03/08/1940 p.1675". Retrieved 2 December 2008.  ^ a b "B.O.E. #68 03/08/1940 p.1676". Retrieved 2 December 2008.  ^ "Hábitos y horarios españoles". Retrieved 27 November 2008. 

References[edit]

Astronomical Almanac Online. (2015). United States Naval Observatory and Her Majesty's Nautical Almanac Office. Dumortier, J, Hannelore, D, & Loncke, M. (n.d.). "Legal Aspects of Trusted Time
Time
services in Europe". AMANO. Retrieved 8 July 2009. Guinot, Bernard (August 2011). "Solar time, legal time, time in use". Metrologica 48 (4): S181–185. Bibcode: 2011Metro..48S.181G. doi:10.1088/0026-1394/48/4/S08. Hilton, James L and McCarthy, Dennis D.. (2013). "Precession, Nutation, Polar Motion, and Earth Rotation." In Sean Urban and P. Kenneth Seidelmann (Eds.), Explanatory Supplement to the Astronomical Almanac 3rd ed. Mill Valley CA: University Science Books. Howse, D. (1997). Greenwich
Greenwich
time and the longitude. London: Philip Wilson. Interpretation Act, R.S.C. 1985, c. I-21. (2005). CanLII. (Canadian statute) Interpretation Act 1978. UK Law Statute Database. (UK statute) Interpretation Act 2005. British and Irish Legal Information Institute. (Irish statute) McCarthy, D., and Seidelmann, P. K. (2009). TIME—From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH. Myers, J. (2007). History of legal time in Britain. Retrieved 4 January 2008. Seago, J.H., & Seidelmann, P. K. (2011). National Legal Requirements for Coordinating with Universal Time. Steve Allen of University of California
University of California
Observatories. Retrieved 19 January 2018. "Six pip salute". BBC News. Retrieved 9 July 2009. Standard Time
Time
Act, 1968. Irish Statute Book. Office of the Attorney General. (Irish statute) Standard Time
Time
(Amendment) Act, 1971. British and Irish Legal Information Institute. (Irish statute)

External links[edit]

Greenwich
Greenwich
Mean Time Interactive World Clock
Clock
Map in Flash International Earth Rotation and Reference Systems Service Royal Observatory, Greenwich The original BBC World Service
BBC World Service
GMT time signal in mp3 format Rodgers, Lucy (20 October 2009). "At the centre of time". BBC News. Retrieved 20 October 2009. 

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