Greenwich Mean Time (GMT) is the

local mean time Local mean time (LMT) is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude. This measurement of time was used for everyday use during the 19th century before time zones ...

at the Royal Observatory in

Greenwich Greenwich ( , , ) is an List of areas of London, area in south-east London, England, within the Ceremonial counties of England, ceremonial county of Greater London, east-south-east of Charing Cross. Greenwich is notable for its maritime hi ...

, London, counted from

midnight Midnight is the transition time from one day to the next – the moment when the date changes, on the local official clock time for any particular jurisdiction. By clock time, midnight is the opposite of noon, differing from it by 12 hours. ...

. At different times in the past, it has been calculated in different ways, including being calculated from

noon Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour cl ...

; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the

time zone A time zone is an area which observes a uniform standard time for legal, Commerce, commercial and social purposes. Time zones tend to follow the boundaries between Country, countries and their Administrative division, subdivisions instead of ...

UTC+00:00 and, in UK law, is the basis for

civil time In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a UTC offset, fixed offset from Coordinated Universal Time (UTC), possibly adjusted ...

in the United Kingdom. Because of Earth's uneven

angular velocity In physics, angular velocity (symbol or \vec, the lowercase Greek letter omega), also known as the angular frequency vector,(UP1) is a pseudovector representation of how the angular position or orientation of an object changes with time, i ...

in its

elliptical orbit In astrodynamics or celestial mechanics, an elliptical orbit or eccentric orbit is an orbit with an orbital eccentricity, eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. Some or ...

and its

axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbita ...

, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the

Greenwich Meridian The Greenwich meridian is a prime meridian, a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. From 1884 to 1974, the Greenwich meridian was the international standard prime meridian, ...

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 The equation of time describes the discrepancy between two kinds of solar time. The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun ...

. Noon GMT is the annual average (the

arithmetic mean In mathematics and statistics, the arithmetic mean ( ), arithmetic average, or just the ''mean'' or ''average'' is the sum of a collection of numbers divided by the count of numbers in the collection. The collection is often a set of results fr ...

) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time". Originally, astronomers considered a GMT day to start at noon, while for almost everyone else it started at midnight. To avoid confusion, the name

Universal Time Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle wi ...

was introduced in 1928 to denote GMT as counted from midnight. Today, Universal Time usually refers to

Coordinated Universal Time Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communicat ...

(UTC) or else to UT1; English speakers often use GMT as a synonym for UTC. For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9s. The term "GMT" should thus not be used for purposes that require precision. The term "GMT" is especially used by institutional bodies within the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, such as the

BBC World Service The BBC World Service is a British Public broadcasting, public service broadcaster owned and operated by the BBC. It is the world's largest external broadcaster in terms of reception area, language selection and audience reach. It broadcas ...

, the

Royal Navy The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King of the United Kingdom, King. Although warships were used by Kingdom ...

, and the

Met Office The Met Office, until November 2000 officially the Meteorological Office, is the United Kingdom's national weather and climate service. It is an executive agency and trading fund of the Department for Science, Innovation and Technology and ...

; and others particularly in Arab countries, such as the

Middle East Broadcasting Centre MBC Group (), formerly known as Middle East Broadcasting Center (), is a Saudi media conglomerate based in the Riyadh region. Launched in London in 1991, the company moved its headquarters to Dubai in 2002 and to Riyadh in 2022. It is majority ...

and Dubai-based

OSN Orbit Showtime Network, commonly known as OSN, is a Dubai-based Satellite television, satellite TV company, serving the Middle East and North Africa region. OSN mainly broadcasts programming from TV networks owned by Paramount Global, Warner B ...

History

As the

developed into an advanced

maritime nation A maritime nation is any nation that borders the sea and is dependent on its use for the majority of the following state activities: commerce and transport, war, to define a territorial boundary, or for any maritime activity (activities using the ...

, British mariners kept at least one chronometer on GMT to calculate their

longitude Longitude (, ) is a geographic coordinate that specifies the east- west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lett ...

from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention adopted in the

International Meridian Conference The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use. The conference was held at the request of President of the United State ...

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 Nevil Maskelyne (; 6 October 1732 – 9 February 1811) was the fifth British Astronomer Royal. He held the office from 1765 to 1811. He was the first person to scientifically measure the mass of the planet Earth. He created '' The Nautical Al ...

's method of

lunar distance The instantaneous Earth–Moon distance, or distance to the Moon, is the distance from the center of Earth to the center of the Moon. In contrast, the Lunar distance (LD or \Delta_), or Earth–Moon characteristic distance, is a unit of measure ...

s based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most

s were based upon GMT, as an offset of a number of hours (and occasionally half or quarter hours) "ahead of GMT" or "behind GMT". Greenwich Mean Time was adopted across the island of

Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...

by the

Railway Clearing House The Railway Clearing House (RCH) was an organisation set up to manage the allocation of revenue collected by Railways Act 1921, pre-grouping railway companies for the conveyance of passengers and goods over the lines (or using the rolling stock ...

in 1847 and by almost all railway companies by the following year, from which the term ''

railway time Railway time was the standardised time arrangement first applied by the Great Western Railway in England in November 1840, the first recorded occasion when different local mean times were synchronised and a single standard time applied. The key ...

'' is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "

" 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 The Isle of Man ( , also ), or Mann ( ), is a self-governing British Crown Dependency in the Irish Sea, between Great Britain and Ireland. As head of state, Charles III holds the title Lord of Mann and is represented by a Lieutenant Govern ...

in 1883, in

Jersey Jersey ( ; ), officially the Bailiwick of Jersey, is an autonomous and self-governing island territory of the British Islands. Although as a British Crown Dependency it is not a sovereign state, it has its own distinguishing civil and gov ...

in 1898 and in

Guernsey Guernsey ( ; Guernésiais: ''Guernési''; ) is the second-largest island in the Channel Islands, located west of the Cotentin Peninsula, Normandy. It is the largest island in the Bailiwick of Guernsey, which includes five other inhabited isl ...

in 1913.

Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...

adopted GMT in 1916, supplanting

Dublin Mean Time Dublin is the capital and largest city of Republic of Ireland, Ireland. Situated on Dublin Bay at the mouth of the River Liffey, it is in the Provinces of Ireland, province of Leinster, and is bordered on the south by the Dublin Mountains, pa ...

. Hourly

time signal A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, au ...

s from Greenwich Observatory were first broadcast by

shortwave radio Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the High frequency, high frequency band (HF), which extends from 3 to 30& ...

on 5 February 1924 at 17:30:00 UTC, providing a rival accurate time-source to the

time ball A time ball or timeball is a time-signalling device. It consists of a large, painted wooden or metal ball that is dropped at a predetermined time, principally to enable navigators aboard ships offshore to verify the setting of their marine chron ...

at the Greenwich Observatory. The daily rotation of the Earth is irregular (see ΔT) and has a slowing trend; therefore

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

s 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 The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

Dublin Dublin is the capital and largest city of Republic of Ireland, Ireland. Situated on Dublin Bay at the mouth of the River Liffey, it is in the Provinces of Ireland, province of Leinster, and is bordered on the south by the Dublin Mountains, pa ...

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).

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 Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...

, was to refer to noon as zero hours (see

Julian day The Julian day is a continuous count of days from the beginning of the Julian period; it is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g., food production date and sell by date). Th ...

). This contrasted with the civil convention of referring to midnight as zero hours dating from the

Roman Empire The Roman Empire ruled the Mediterranean and much of Europe, Western Asia and North Africa. The Roman people, Romans conquered most of this during the Roman Republic, Republic, and it was ruled by emperors following Octavian's assumption of ...

. 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''.

GMT in legislation

United Kingdom

Legally, the civil time used in the UK is called "Greenwich mean time" (without capitalisation), 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 The Interpretation Act 1978 (c. 30) is an act of the Parliament of the United Kingdom. The act makes provision for the interpretation of acts of Parliament, Measures of the General Synod of the Church of England, Measures of the Church Asse ...

, section 9, provides that whenever an expression of time occurs in any Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time. Under subsection 23, 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 During British Summer Time (BST), civil time in the United Kingdom is advanced one hour forward of Greenwich Mean Time (GMT), in effect changing the time zone from UTC+00:00 to UTC+01:00, so that mornings have one hour less daylight, and eve ...

was called ''British Standard Time'' (''BST''). In the UK, UTC+00:00 is disseminated to the general public in winter and UTC+01:00 in summer. BBC radio stations broadcast the "six pips" of the

Greenwich Time Signal The Greenwich Time Signal (GTS), popularly known as the pips, is a series of six short tones (or "pips") broadcast at one-second intervals by many BBC Radio stations to mark the precise start of each hour. The pips were introduced in 1924, gene ...

. It is named from its original generation at the

Royal Greenwich Observatory The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in G ...

but is calibrated to UTC. If announced (such as near the start of summer time or of winter time), announcers on domestic channels declare the time as ''GMT'' or ''BST'' as appropriate. As the BBC World Service is broadcast to all time zones, the announcers use the term "Greenwich Mean Time" consistently throughout the year.

Other countries

Several countries define their local time by reference to Greenwich Mean Time.Dumortier, Hannelore, & Loncke (n.d.) Some examples are: *

Belgium Belgium, officially the Kingdom of Belgium, is a country in Northwestern Europe. Situated in a coastal lowland region known as the Low Countries, it is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeas ...

: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT. *

: "Standard Time" () is defined as being one hour in advance of GMT. "Winter Time" () is defined as being the same as GMT. *

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

: Interpretation Act, R.S.C. 1985, c. I-21, section 35(1). 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 Nova Scotia is a Provinces and territories of Canada, province of Canada, located on its east coast. It is one of the three Maritime Canada, Maritime provinces and Population of Canada by province and territory, most populous province in Atlan ...

(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".

Time zone

Greenwich Mean Time is defined in law as standard time in the following countries and areas, which also advance their clocks one hour (GMT+1) in summer. * United Kingdom, where the summer time is called ''

'' (''BST'') * Ireland, where it is called ''Winter Time'', changing to Standard Time in summer. *

Portugal Portugal, officially the Portuguese Republic, is a country on the Iberian Peninsula in Southwestern Europe. Featuring Cabo da Roca, the westernmost point in continental Europe, Portugal borders Spain to its north and east, with which it share ...

(with the exception of the

Azores The Azores ( , , ; , ), officially the Autonomous Region of the Azores (), is one of the two autonomous regions of Portugal (along with Madeira). It is an archipelago composed of nine volcanic islands in the Macaronesia region of the North Atl ...

) *

Canary Islands The Canary Islands (; ) or Canaries are an archipelago in the Atlantic Ocean and the southernmost Autonomous communities of Spain, Autonomous Community of Spain. They are located in the northwest of Africa, with the closest point to the cont ...

Faroe Islands The Faroe Islands ( ) (alt. the Faroes) are an archipelago in the North Atlantic Ocean and an autonomous territory of the Danish Realm, Kingdom of Denmark. Located between Iceland, Norway, and the United Kingdom, the islands have a populat ...

Greenwich Mean Time is used as standard time all year round in the following countries and areas: *

Burkina Faso Burkina Faso is a landlocked country in West Africa, bordered by Mali to the northwest, Niger to the northeast, Benin to the southeast, Togo and Ghana to the south, and Ivory Coast to the southwest. It covers an area of 274,223 km2 (105,87 ...

The Gambia The Gambia, officially the Republic of The Gambia, is a country in West Africa. Geographically, The Gambia is the List of African countries by area, smallest country in continental Africa; it is surrounded by Senegal on all sides except for ...

Ghana Ghana, officially the Republic of Ghana, is a country in West Africa. It is situated along the Gulf of Guinea and the Atlantic Ocean to the south, and shares borders with Côte d’Ivoire to the west, Burkina Faso to the north, and Togo to t ...

Guinea Guinea, officially the Republic of Guinea, is a coastal country in West Africa. It borders the Atlantic Ocean to the west, Guinea-Bissau to the northwest, Senegal to the north, Mali to the northeast, Côte d'Ivoire to the southeast, and Sier ...

Guinea-Bissau Guinea-Bissau, officially the Republic of Guinea-Bissau, is a country in West Africa that covers with an estimated population of 2,026,778. It borders Senegal to Guinea-Bissau–Senegal border, its north and Guinea to Guinea–Guinea-Bissau b ...

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

Ivory Coast Ivory Coast, also known as Côte d'Ivoire and officially the Republic of Côte d'Ivoire, is a country on the southern coast of West Africa. Its capital city of Yamoussoukro is located in the centre of the country, while its largest List of ci ...

Liberia Liberia, officially the Republic of Liberia, is a country on the West African coast. It is bordered by Sierra Leone to Liberia–Sierra Leone border, its northwest, Guinea to Guinea–Liberia border, its north, Ivory Coast to Ivory Coast–Lib ...

Mali Mali, officially the Republic of Mali, is a landlocked country in West Africa. It is the List of African countries by area, eighth-largest country in Africa, with an area of over . The country is bordered to the north by Algeria, to the east b ...

Mauritania Mauritania, officially the Islamic Republic of Mauritania, is a sovereign country in Maghreb, Northwest Africa. It is bordered by the Atlantic Ocean to the west, Western Sahara to Mauritania–Western Sahara border, the north and northwest, ...

Sahrawi Arab Democratic Republic The Sahrawi Arab Democratic Republic (SADR), also known as the Sahrawi Republic and Western Sahara, is a partially recognized state in the western Maghreb, which claims the non-self-governing territory of Western Sahara, but controls only ...

(disputed) *

Saint Helena, Ascension and Tristan da Cunha Saint Helena, Ascension and Tristan da Cunha is a British Overseas Territories, British Overseas Territory located in the Atlantic Ocean, South Atlantic and consisting of the island of Saint Helena, Ascension Island, and the archipelago of Tri ...

Senegal Senegal, officially the Republic of Senegal, is the westernmost country in West Africa, situated on the Atlantic Ocean coastline. It borders Mauritania to Mauritania–Senegal border, the north, Mali to Mali–Senegal border, the east, Guinea t ...

Sierra Leone Sierra Leone, officially the Republic of Sierra Leone, is a country on the southwest coast of West Africa. It is bordered to the southeast by Liberia and by Guinea to the north. Sierra Leone's land area is . It has a tropical climate and envi ...

Togo Togo, officially the Togolese Republic, is a country in West Africa. It is bordered by Ghana to Ghana–Togo border, the west, Benin to Benin–Togo border, the east and Burkina Faso to Burkina Faso–Togo border, the north. It is one of the le ...

Notes

References

Sources

* * * . * * *

CanLII The Canadian Legal Information Institute (CanLII; ) is a non-profit organization created and funded by the Federation of Law Societies of Canada in 2001 on behalf of its 14 member societies. CanLII is a member of the Free Access to Law Movement, ...

. (Canadian statute) * (UK statute, see also

) * (

Irish Statute Book The Irish Statute Book, also known as the electronic Irish Statute Book (eISB), is a database produced by the Office of the Attorney General of Ireland. It contains copies of Acts of the Oireachtas and statutory instruments.

) * * * * *

* (Irish statute)

External links

International Earth Rotation and Reference Systems Service

The original BBC World Service GMT time signal in MP3 format
* {{Time measurement and standards 1884 introductions Geography of the Royal Borough of Greenwich Time in the United Kingdom Time zones