Greenwich Mean Time

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"Greenwich Mean Time" (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, although strictly UTC is an atomic time scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is the astronomical concept that directly replaced the original GMT.

Noon Greenwich Mean Time is not necessarily the moment when the Sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the equation of time). 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.

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 and more modern convention is to refer to midnight as zero hours. The more specific terms "UT" and "UTC" do not suffer this ambiguity, always referring to midnight as zero hours.

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[edit] History

As the United Kingdom grew into an advanced maritime nation, British mariners kept their timepieces on GMT in order to calculate their longitude "from the Greenwich meridian", which was by convention considered to have longitude zero degrees. This did not affect shipboard time itself, which was still solar time. This combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used world-wide 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".

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. It was gradually adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official time. This changed in 1880, when GMT was legally adopted throughout Great Britain. GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted Greenwich Mean Time in 1916, supplanting Dublin Mean Time.<ref>History of legal time in Britain</ref>

Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924.

The daily rotation of the Earth is somewhat irregular (see ΔT) and is slowing down slightly. Atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world. UT1, introduced earlier, represents "earth rotation time". Leap seconds are added to or subtracted from UTC to keep it within 0.9 seconds of UT1.

The international prime meridian is no longer precisely the Greenwich meridian, but remains close to it (5.31"E).

[edit] Time zone

Although civil time, e.g., the Greenwich Time Signal in the United Kingdom, is now based on UTC, it is still popularly called GMT. It is also called Western European Time (WET).

Those countries marked in dark blue on the map above use European Summer Time and advance their clock one hour in summer. In the United Kingdom, this is known as British Summer Time (BST); in the Republic of Ireland it is called Irish Summer Time (IST). Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.

[edit] Anomalies

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 geographical terms, 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 (UTC+1 in particular); contrariwise, there are European areas that use UTC, even though their 'physical' time zone is UTC-1 (e.g. most of Portugal), or even UTC−2 (the westernmost part of Iceland). Actually, because the UTC time zone in Europe is 'shifted' to the west, Lowestoft in Suffolk, East Anglia, England at only 1°45'E is the easternmost settlement in Europe in which UTC is applied. Following is a list of the 'incongruencies':

[edit] Countries (or parts thereof) west of 22°30'W ('physical' UTC-2) that use UTC

  • Westernmost part of Iceland, incl. the northwest peninsula and its main town of Ísafjörður. Actually, Bjargtangar, Iceland is the westernmost point in which UTC is applied (and the westernmost point of political Europe for that matter)

[edit] Countries (or parts thereof) west of 7°30'W ('physical' UTC-1) that use UTC

[edit] Countries (or parts thereof) between 7°30'W and 7°30'E ('physical' UTC) that use UTC+1

[edit] Major metropolitan areas

[edit] Notes

<references />

[edit] See also

[edit] External links

es:Greenwich Mean Time eo:GMT fr:Greenwich Mean Time ko:그리니치 평균시 id:GMT it:Greenwich Mean Time he:שעון גריניץ' nl:Greenwich Mean Time ja:グリニッジ標準時 no:Greenwich Mean Time pl:Czas uniwersalny pt:GMT ru:Среднее время по Гринвичу sl:GMT sv:GMT th:GMT tr:GMT zh:格林尼治標準時間

Greenwich Mean Time

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