Greenwich Mean Time
Learn more about Greenwich Mean Time
"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.
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>
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).
 Time zone
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.
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':
 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)
 Countries (or parts thereof) west of 7°30'W ('physical' UTC-1) that use UTC
- Canary Islands (Spain)
- Most of Portugal, incl. Lisbon, Porto, Braga, Aveiro, and Coimbra. (Only the easternmost part, incl. cities such as Braganca and Guarda, lies east of 7°30'W). The Madeira Islands, even further to the west, also employ UTC
- Western part of the Republic of Ireland, incl. the cities of Cork, Limerick, and Galway
- Westernmost tip of Northern Ireland, incl. the capital of County Fermanagh, Enniskillen
- Extreme westerly portion of the Outer Hebrides, west of Scotland; for instance Vatersay, an inhabited island in the Outer Hebrides and the westernmost settlement in the whole of Great Britain lies at 7°54'W. If uninhabited islands and/or rocks are to be taken into account then St Kilda, west of the Outer Hebrides, at 8°58'W, and Rockall, at 13°41'W should also be included
- Westernmost island of the Faroe Islands (Danish jurisdiction), Mykines
- Most of Iceland, including Reykjavík
 Countries (or parts thereof) between 7°30'W and 7°30'E ('physical' UTC) that use UTC+1
- France, except for the extreme easternmost part, in Alsace, incl. Strasbourg
- Extreme westernmost part of Germany, incl. the cities of Saarbrücken, Düsseldorf, Aachen, and Trier
- Extreme westernmost part of Italy, incl. the cities of Aosta in Aosta Valley and Cuneo in Piedmont
- The Netherlands
- Westernmost part of Norway, incl. the cities of Bergen and Stavanger
- Spain, except for the westernmost part, which actually is west of 7°30'W
- Part of Switzerland west of Bern (inclusive), also incl. cities such as Geneva, Lausanne, and Fribourg
 Major metropolitan areas
- Birmingham, United Kingdom
- Dublin, Ireland
- Edinburgh, UK
- Glasgow, UK
- Liverpool, England
- Lisbon, Portugal
- London, England
- Manchester, England
- Reykjavík, Iceland
 See also
 External links
- Greenwich Mean Time
- NIST - World Time Scales
- International Earth Rotation and Reference Systems Service
- World Clock - Current Local Timesde:Greenwich Mean Time
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