Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London.[a] Historically used as a world reference time, nowadays it is one of the names for the time zone UTC+00:00. At different times in the past, it has been calculated in different ways, including being calculated from midnight and noon; as a consequence, it cannot be used to specify a particular time unless a context is given.
Some English speakers use GMT as a synonym for Coordinated Universal Time (UTC) but this is incorrect; GMT is a time zone, UTC is not. It is occasionally considered equivalent to UT1 but this is also incorrect for the same reason and in addition GMT, like UTC, can differ from UT1 by up to 0.9s.[b] Thus the term GMT cannot be used for purposes that require precision.
Because of Earth's uneven angular velocity in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Prime Meridian[c] and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described 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".[d]
Originally, astronomers considered a GMT day to start at noon,[e] 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. Today, Universal Time usually refers to UTC or UT1.
The term "GMT" is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, and 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.
As the United Kingdom developed into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention 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. 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." This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted in the Isle of Man in 1883, in Jersey in 1898 and in Guernsey in 1913. Ireland adopted GMT 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 redundant.
The daily rotation of the Earth is irregular (see ?T) and has a slowing trend; therefore atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT as the international civil time standard was superseded by Coordinated Universal Time (UTC), 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 in 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) 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.
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 as "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. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.
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.
During the experiment of 1968 to 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).
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.
Greenwich Mean Time is used as standard time in the following countries and areas, which also advance their clocks one hour (GMT+1) in summer.
Greenwich Mean Time is used as standard time all year round in the following countries and areas:
|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|
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 determined purely by longitude, would consist of the area between meridians 7°30'W and 7°30'E. However, in much of Western and Central Europe, despite lying between those two meridians, UTC+1 is used; similarly, 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: