Greenwich Mean Time

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon;[1] as a consequence, it cannot be used to specify a precise time unless a context is given.

English speakers often use GMT as a synonym for Coordinated Universal Time (UTC).[2] 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 not thus be used for technical purposes.[3]

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 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 GMT is the annual average (i.e. "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 to denote GMT as counted from midnight.[4] Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date. Today Universal Time usually refers to UTC or UT1.[5]

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.

Greenwich Mean Time
World Time Zones Map
This is the zone marked '0' in the middle of the map, coloured green.
UTC offset
GMTUTC±00:00
Current time
04:40, 19 March 2019 GMT [refresh]
Time zones of Europe
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)
golden Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
light green Further-eastern European Time / Moscow Time / Turkey Time (UTC+3)
Light colours indicate where standard time is observed all year; dark colours indicate where a summer time is observed.
UTC hue4map X region Africa
Time zones of Africa:
 -01:00  Cape Verde Time[a]
 ±00:00 
 +01:00 
 +02:00 
 +03:00  East Africa Time
 +04:00 
Light colours indicate where standard time is observed all year; dark colours indicate where daylight saving is observed.
a The islands of Cape Verde are to the west of the African mainland.

History

Greenwich clock
Greenwich clock with standard measurements

As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees, 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 possibly 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.[6] 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."[7][8] This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time.[9] Hourly time signals from 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 (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 at 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)[10] 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 "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.[11] 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), according to the Interpretation Act 1978, 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 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.[9]

During the experiment of 1968-1971, 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+0 is disseminated to the general public in winter and UTC+1 in summer.[12][4]

BBC radio stations broadcast the "six pips" of the Greenwich Time Signal. It is named from its original generation at the Royal Greenwich Observatory, is aligned to Coordinated Universal Time, and called either Greenwich Mean Time or British Summer Time as appropriate for the time of year.

Other countries

Several countries define their local time by reference to Greenwich Mean Time.[13][14] Some examples are:

  • Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.[13]
  • Ireland: Standard 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 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 used as standard time in the following countries, which also advance their clocks one hour (GMT+1) in summer.

Country subdivisions or dependent territories:

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

Discrepancies between legal GMT and geographical GMT

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
Tzdiff-Europe-winter
European winter
Tzdiff-Europe-summer
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 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 in the 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 is the westernmost point in which UTC is applied.
Countries (or parts thereof) west of 7°30'W ("physical" UTC−1) that use UTC
Greenwich mean time line
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 (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)[16] abandoning 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".[17][18] The presidential order (most likely enacted to be in synchrony with Germany and Italy, with which the Franco regime was unofficially allied) included in its 5th article a provision for its future phase out,[18] which never took place. Due to this political decision 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.[19] In Portugal, which is a mere one hour behind Spain, the timetable is quite different.
  • Most of France, including the cities of Paris, Marseille and Lyon. Only small parts of Alsace, Lorraine and Provence are east of 7°30'E ("physical" UTC+1).
  • Monaco
  • Andorra
  • Belgium
  • Netherlands
  • Luxembourg

See also

Notes

  1. ^ "Time scales". UCO Lick. Retrieved 28 July 2018.
  2. ^ "Coordinated Universal Time". Oxford Dictionaries. Oxford University Press.
  3. ^ Hilton and McCarthy 2013, p. 231–2.
  4. ^ a b McCarthy & Seidelmann 2009, p. 17.
  5. ^ Astronomical Almanac Online 2015, Glossary s.v. "Universal Time".
  6. ^ Howse 1997, p. 114.
  7. ^ CLERK TO JUSTICES. "Time, Actual And Legal". Times, London, England, 14 May 1880: 10. The Times Digital Archive. Web. 18 August 2015.
  8. ^ Bartky, Ian R. (2007). One Time Fits All: The Campaigns for Global Uniformity. Stanford University Press. p. 134. ISBN 0804756422. Retrieved 18 August 2015.
  9. ^ a b Myers (2007).
  10. ^ UT1 as explained on IERS page
  11. ^ Astronomical Supplement to the Astronomical Almanac. University Science Books. 1992. p. 76. ISBN 0-935702-68-7.
  12. ^ Howse 1997, p. 157.
  13. ^ a b Dumortier, Hannelore, & Loncke (n.d.)
  14. ^ Seago & Seidelmann (2011).
  15. ^ Standard Time Act, 1968.
  16. ^ "BOE Orden sobre adelanto de la hora legal en 60 minutos". Retrieved 2 December 2008.
  17. ^ "B.O.E. #68 03/08/1940 p.1675". Retrieved 2 December 2008.
  18. ^ a b "B.O.E. #68 03/08/1940 p.1676". Retrieved 2 December 2008.
  19. ^ "Hábitos y horarios españoles". Retrieved 27 November 2008.

References

External links

23 November 2006 Sadr City bombings

The 2006 Sadr City bombings were a series of car bombs and mortar attacks in Iraq that occurred on 23 November at 15:10 Baghdad time (12:10 Greenwich Mean Time) and ended at 15:55 (12:55 UTC). Six car bombs and two mortar rounds were used in the attack on the Shia slum in Sadr City.

Atlantic Time Zone

The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC−04:00. During part of the year, some portions of the zone observe daylight saving time, referred to as Atlantic Daylight Time (ADT), by moving their clocks forward one hour to result in UTC−03:00. The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.

In Canada, the provinces of New Brunswick, Nova Scotia, and Prince Edward Island are in this zone, though legally they calculate time specifically as an offset of four hours from Greenwich Mean Time (GMT–4) rather than from UTC. Small portions of Quebec (eastern Côte-Nord and the Magdalen Islands) also observe Atlantic Time. Officially, the entirety of Newfoundland and Labrador observes Newfoundland Standard Time, but in practice Atlantic Time is used in most of Labrador.

No portion of the continental United States currently uses Atlantic Time, although it is used by the territories of Puerto Rico and the U.S. Virgin Islands. A number of New England states are considering a regional change to Atlantic Standard Time year-round (with no observance of daylight saving time), even though only a small portion of Maine lies to the east of the 67.5°W theoretical extent of this zone. Florida is in the process of enacting a similar change; in both cases any changes will need to be approved by the United States Department of Transportation and the United States Congress.

Central European Summer Time

Central European Summer Time (CEST), sometime referred also as Central European Daylight Time (CEDT), is the standard clock time observed during the period of summer daylight-saving in those European countries which observe Central European Time (UTC+01:00) during the other part of the year. It corresponds to UTC+02:00, which makes it the same as Central Africa Time, South African Standard Time and Kaliningrad Time in Russia.

Civil time

In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC) or from Greenwich Mean Time (GMT), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks, and was adopted in 1972. Older systems use telescope observations.

In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight. HM Nautical Almanac Office in the United Kingdom used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity, whereas the Nautical Almanac Office at the United States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the International Astronomical Union introduced the term Universal Time for GMT beginning at midnight, but the two Nautical Almanac Offices did not accept it until 1952.

Madras Time

Madras Time was a time zone established in 1802 by John Goldingham, the first official astronomer of the British East India Company in India when he determined the longitude of Madras as 5 hours, 21 minutes and 14 seconds ahead of Greenwich Mean Time. It has been described as 9 minutes from UTC+05:30 and 33 minutes and 20 seconds behind Calcutta time which puts it at (UTC+05:21). Before India's independence, it was the closest precursor to Indian Standard Time which is derived from the location of the observatory at 82.5°E longitude in Shankargarh Allahabad in Uttar Pradesh.After Bombay time and Calcutta time were set up as the two official time zones of British India in 1884, railway companies in India began to use Madras time as an intermediate time zone between the two zones. This led to Madras time also being known as "Railway time of India".

Time in Denmark

Denmark, including the dependencies Faroe Islands and Greenland, uses six different time zones.

Time in Malaysia

Malaysian Standard Time (MST; Malay: Waktu Piawai Malaysia, WPM) or Malaysian Time (MYT) is a standard time used in Malaysia. It is 8 hours ahead of Greenwich Mean Time and Coordinated Universal Time. The local mean time in Kuala Lumpur was originally UTC+06:46:46. Peninsular Malaysia used this local mean time until 1 January 1901, when they changed to Singapore mean time UTC+06:55:25. Between the end of the Second World War and the formation of Malaysia on 16 September 1963, it was known as British Malayan Standard Time, which was UTC+07:30 hours ahead of Greenwich Mean Time. At 2330 hrs local time of 31 December 1981, people in Peninsular Malaysia adjusted their clocks and watches ahead by 30 minutes to become 00:00 hours local time of 1 January 1982, to match the time in use in East Malaysia, which is UTC+08:00. SGT (Singapore) as follow on the same until now.

Time in Spain

Spain has two time zones and observes daylight saving time. Spain mainly uses Central European Time (GMT+01:00) and Central European Summer Time (GMT+02:00) in Peninsular Spain, the Balearic Islands, Ceuta, Melilla and plazas de soberanía. In the Canary Islands, the time zone is Western European Time (GMT±00:00) and Western European Summer Time (GMT+01:00). Daylight saving time is observed from the last Sunday in March (01:00 GMT) to the last Sunday in October (01:00 GMT) throughout Spain.

Spain used Greenwich Mean Time (UTC±00:00) before the Second World War (except for the Canary Islands which used GMT−01:00 before this date). However, the time zone was changed to Central European Time in 1940 and has remained so since then, meaning that Spain does not use its "natural" time zone under the coordinated time zone system. Some observers believe that this time zone shift plays a role in the country's relatively unusual daily schedule (late meals and sleep times).

Time in the Czech Republic

Time in the Czech Republic is Central European Time (Středoevropský čas, SEČ: UTC+01:00) and Central European Summer Time (Středoevropský letní čas, SELČ: UTC+02:00). Daylight saving time is observed from the last Sunday in March (2:00 CET) to the last Sunday in October (3:00 CEST). The Czech Republic has observed Central European Time since 1979. Until 1993 when Czechoslovakia was separated into the Czech Republic and Slovakia, they also had Central European Time and Central European Summer Time. After the summer months, time in the Czech Republic is shifted back by one hour to Central European Time. Like most states in Europe, Summer time (daylight saving time) is observed in the Czech Republic, when time is shifted forward by one hour, two hours ahead of Greenwich Mean Time.

Time in the United Kingdom

The United Kingdom uses Greenwich Mean Time or Western European Time (UTC) and British Summer Time or Western European Summer Time (UTC+01:00).

Universal Time

Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, England. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see § Versions). All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.

West Africa Time

West Africa Time, or WAT, is a time zone used in west-central Africa; with countries west of Benin instead using Greenwich Mean Time (GMT; equivalent to UTC with no offset). West Africa Time is one hour ahead of Coordinated Universal Time (UTC+01), which makes it the same as Central European Time (CET) during winter, or Western European Summer Time (WEST) and British Summer Time (BST) during the summer.

As most of this time zone is in the tropical region, there is little change in day length throughout the year, so daylight saving time is not observed.

West Africa Time is used by the following countries:

Algeria (as Central European Time)

Angola

Benin

Chad

Cameroon

Central African Republic

Democratic Republic of the Congo (western side only)

Equatorial Guinea

Gabon

Morocco (as Greenwich Mean Time + 1 hour)

Niger

Nigeria

Republic of the Congo

Tunisia (as Central European Time)

Western European Summer Time

Western European Summer Time (WEST) is a summer daylight saving time scheme, 1 hour ahead of Greenwich Mean Time and Coordinated Universal Time. It is used in:

the Canary Islands

Portugal (including Madeira but not the Azores)

Ireland

the United Kingdom

the British Crown dependencies

the Faroe IslandsWestern European Summer Time is known in the countries concerned as:

British Summer Time (BST) in the United Kingdom.

Irish Standard Time (IST) (Am Caighdeánach na hÉireann (ACÉ)) in Ireland. Also sometimes erroneously referred to as "Irish Summer Time" (Am Samhraidh na hÉireann).The scheme runs from the last Sunday in March to the last Sunday in October each year. At both the start and end of the schemes, clock changes take place at 01:00 UTC+00:00. During the winter, Western European Time (WET, GMT+0 or UTC±00:00) is used.

The start and end dates of the scheme are asymmetrical in terms of daylight hours: the vernal time of year with a similar amount of daylight to late October is mid-February, well before the start of summer time. The asymmetry reflects temperature more than the length of daylight.

Ireland observes Irish Standard Time during the summer months and changes to UTC±00:00 in winter. As Ireland's winter time period begins on the last Sunday in October and finishes on the last Sunday in March, the result is the same as if it observed summer time.

Western European Time

Western European Time (WET, UTC±00:00) is a time zone covering parts of western and northwestern Europe. The following countries and regions use WET in winter months:

Canary Islands, since 1946 (rest of Spain is CET, UTC+01:00)

Faroe Islands, since 1908

North Eastern Greenland (Danmarkshavn and surrounding area)

Iceland, since 1968, without summer time changes

Portugal, since 1912 with pauses (except Azores, UTC−01:00)

Madeira islands, since 1912 with pauses

Ireland (legally known as Greenwich Mean Time), since 1916, except between 1968 and 1971

United Kingdom and Crown dependencies (legally known as Greenwich Mean Time), since 1847 in England, Scotland, Wales, the Channel Islands, and the Isle of Man, and since 1916 in Northern Ireland, with pausesAll the above countries except Iceland implement daylight saving time in summer (from the last Sunday in March to the last Sunday in October each year), switching to Western European Summer Time (WEST, UTC+01:00), which is one hour ahead of WET. WEST is called British Summer Time in the UK and is officially known as Irish Standard Time in Ireland.

The nominal span of the time zone is 7.5°E to 7.5°W (0° ± 7.5°), but the WET zone does not include the Netherlands, Belgium, Luxembourg, France, Gibraltar or Spain which use Central European Time (CET), although these are mostly (France) or completely (the rest) west of 7.5°E. Conversely, Iceland and eastern Greenland are included although both are west of 7.5°W. In September 2013, a Spanish parliamentary committee recommended switching to WET.

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