Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. 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. In the UK, GMT is the official time only during winter; during summer it's British Summer Time.
GMT is substantially equivalent to Western European Time.
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 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.
As the United Kingdom grew into an advanced maritime nation
, British mariners kept at least one chronometer
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 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. 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, rendering the time ball at the observatory obsolete in the process.
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 in 1928, represents earth rotation time. Leap seconds are added to or subtracted from UTC to keep it within 0.9 seconds of UT1.
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.
Greenwich Mean Time in legislation
Several countries throughout the world legislatively define their local time by explicit reference to Greenwich Mean Time. Some examples are:
- United Kingdom: 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.
- Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.
- Republic of Ireland: Standard Time (Amendment) Act, 1971, section 1, and Interpretation Act 2005, section 18(i).
- Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35.
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.
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
, East Anglia
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, incl. 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
- Canary Islands (Spain)
- Most of Portugal, incl. Lisbon, Porto, Braga, Aveiro, and Coimbra. (Only the easternmost part, incl. cities such as Bragança 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 (autonomous region of the Danish Kingdom), Mykines
- Iceland, including Reykjavík
Major metropolitan areas
- Aberdeen, Scotland, United Kingdom
- Accra, Ghana
- Belfast, Northern Ireland, United Kingdom
- Birmingham, England, United Kingdom
- Bristol, England, United Kingdom
- Cardiff, Wales, United Kingdom
- Casablanca, Morocco
- Dakar, Senegal
- Dublin, Ireland
- Edinburgh, Scotland, United Kingdom
- Glasgow, Scotland, United Kingdom
- Las Palmas de Gran Canaria, Canary Islands, Spain
- Leeds, England, United Kingdom
- Lisbon, Portugal
- Liverpool, England, United Kingdom
- London, England, United Kingdom
- Manchester, England, United Kingdom
- Porto, Portugal
- Reykjavík, Iceland
- Santa Cruz de Tenerife, Canary Islands, Spain
- Sheffield, England, United Kingdom
- Swansea, Wales, United kingdom