Noon Greenwich Mean Time is rarely the exact moment when the sun crosses the Greenwich meridian and reaches its highest point in the sky there, 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, a discrepancy calculated by the equation of time. Noon is the annual average (i.e. mean) time of this event, prompting the inclusion of "mean" in "Greenwich Mean Time".
Historically the term GMT has been used with two different conventions, sometimes numbering hours starting at midnight and sometimes starting at noon. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours. Astronomers preferred the latter GMT convention to simplify their observational data so that each night was logged under a single calendar date.
As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees, internationally 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 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 Greenwich Mean Time 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 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 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. 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 IAU at Dublin, 1955, at the initiative of William Markowitz) this "raw" form of UT was re-labeled 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). Leap seconds are nowadays 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.
Ambiguity in the definition of GMT
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 Romans. 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 earlier. The instant that was designated '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.
Some authors, such as Howse, assert there are different names for the civil time when summer time (also known as daylight saving time) is not in effect (GMT) versus when it is in effect (summer time, British Summer Time, or BST). Others, such as McCarthy and Seidelmann, assert that GMT can be used as a name for the civil time in the United Kingdom both summer and winter.
Greenwich Mean Time in legislation
Several countries define their local time by 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(1).
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 Standard Time (IST)—officially changing to GMT in winter. Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.
Discrepancies between legal GMT and geographical GMT
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 drawn by purely geographical terms, would consist of 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 use another time zone (UTC+1 in particular); conversely, 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 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
The westernmost part of Iceland, including 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
Most of Portugal, including Lisbon, Porto, Braga, Aveiro, and Coimbra. (Only the easternmost part, including cities such as Bragança and Guarda, lies east of 7°30'W.) Since the Treaty of Windsor in 1386 (the world's oldest diplomatic alliance), Portugal has maintained close ties to Britain, which possibly explains its choice of UTC. Madeira, even further to the west, also employs UTC. A more likely explanation is that during the mid-1970s, when Portugal was on Central European Time all year round, it did not begin to get light in Lisbon in winter until 08:30.
Countries (mostly) between meridians 7°30'W and 7°30'E ("physical" UTC) that use UTC+1
Spain (except for the Canary Islands, which use UTC). Parts of Galicia lie west of 7°30'W ('physical' UTC−1), whereas there is no Spanish territory east of 7°30'E ('physical' UTC+1). Spain's time is the direct result of Franco's Presidential Order (published in Boletín Oficial del Estado of 8 March 1940) abandoning Greenwich UTC time in favour of UTC+1 effective 23:00 16 March 1940. This is an excellent example of political criteria used in the drawing of time zones: the time change was passed "in consideration of the convenience from the national time marching in step according to that of other European countries". The Presidential Order (most likely enacted to be in synchrony with Germany and Italy, with which the Franco regime was unofficially allied) included in its 5th article a provision for its future phase out, which never took place. Due to this political decision Spain is two hours ahead of its local mean time during the summer, one hour ahead in winter, which possibly explains the notoriously late schedule for which the country is known. In Portugal, which is a mere one hour behind Spain, the timetable is quite different.
^Legal time in the UK is still "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), see also the legal section in this article. Mean time at Greenwich was the prototype of Universal Time, as from the late 19th century, see the terms of the International Meridian Conference. UT1 is a form of UT equalized for polar motion. The widely used time signals based on UTC are kept within 0.9 seconds of UT1, see "History" section of this article). UTC+0 is explained elsewhere in this article (links from map caption) as equal to Western European Time (WET).
^Interpretation Act, R.S.C. 1985, c. I-21 – this refers to 'standard time' for the several provinces, defining each in relation to 'Greenwich time', but does not use the expression 'Greenwich mean time'. Several provinces, such as Nova Scotia (Time Definition Act. R.S., c. 469, s. 1), have their own legislation which specifically mentions either "Greenwich Mean Time" or "Greenwich mean solar time".