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GMT is the abbreviation of Greenwich Mean Time.
GMT is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. GMT 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. GMT is also used to refer to universal time (UT), which is the astronomical concept that directly replaced the original GMT. GMT is short for Greenwich Mean Time.
Noon GMT 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 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.
History of GMT.
As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one timepiece 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".
GMT 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 the island of 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. Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924.
The daily rotation of the Earth is somewhat irregular (see ?T) 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 in 1928, 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).
GMT: Time zone.
Although civil time in the United Kingdom, e.g., the Greenwich Time Signal, is in practice now based on UTC, 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 Western 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.
Anomalies with GMT.
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 7º30'W and 7º30'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 1º45'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 22º30'W ('physical' UTC-2) that use UTC.
Countries (or parts thereof) west of 7º30'W ('physical' UTC-1) that use UTC.
Major metropolitan areas of GMT.
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