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Greenwich Mean Time

Greenwich Mean Time

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Greenwich Mean Time (GMT) is a term originally referring to mean solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

 at the Royal Observatory
Royal Observatory, Greenwich
The Royal Observatory, Greenwich , in London, England played a major role in the history of astronomy and navigation, and is best known as the location of the prime meridian...

 in Greenwich
Greenwich
Greenwich is a district of south London, England, located in the London Borough of Greenwich.Greenwich is best known for its maritime history and for giving its name to the Greenwich Meridian and Greenwich Mean Time...

, London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

. It is arguably the same as Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 (UTC) and when this is viewed as a time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

 the name Greenwich Mean Time is especially used by bodies connected with the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, such as the BBC World Service
BBC World Service
The BBC World Service is the world's largest international broadcaster, broadcasting in 27 languages to many parts of the world via analogue and digital shortwave, internet streaming and podcasting, satellite, FM and MW relays...

, the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

, the Met Office
Met Office
The Met Office , is the United Kingdom's national weather service, and a trading fund of the Department for Business, Innovation and Skills...

 and others.

Before the introduction of UTC on 1 January 1972 Greenwich Mean Time (also known as Zulu time) was the same as Universal Time
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

 (UT) which is a standard astronomical concept used in many technical fields. Astronomers no longer use the term "Greenwich Mean Time".

In the United Kingdom, GMT is the official time only during winter; during summer British Summer Time
British Summer Time
Western European Summer Time is a summer daylight saving time scheme, 1 hour ahead of Coordinated Universal Time. It is used in the following places:* the Canary Islands* Portugal * Ireland...

 is used. GMT is the same as Western European Time
Western European Time
Western European Time , defined legally as Greenwich Mean Time in the United Kingdom, is the time zone covering parts of western and northwestern Europe, and includes the following countries and regions:...

.

Noon Greenwich Mean Time is not necessarily the moment when the noon sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 crosses the Greenwich meridian (and reaches its highest point in the sky at Greenwich) because of Earth's uneven speed in its elliptic orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 and its axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

. This event may be up to 16 minutes away from noon GMT (a discrepancy known as the equation of time
Equation of time
The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...

). 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, 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.

History



As the United Kingdom grew into an advanced maritime nation
Maritime nation
A maritime nation is any nation which borders the sea and uses it for any of the following: commerce and transport, war, to define a territorial boundary, or for any maritime activity ....

, British mariners kept at least one chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

 on GMT in order to calculate their longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

 from the Greenwich meridian
Prime Meridian
The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...

, which was by convention considered to have longitude zero degrees (this convention was internationally adopted in the International Meridian Conference
International Meridian Conference
The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States to determine the Prime Meridian of the world. The conference was held at the request of U.S. President Chester A...

 of 1884). Note that the synchronization of the chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

 on GMT did not affect shipboard time itself, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne
Nevil Maskelyne
The Reverend Dr Nevil Maskelyne FRS was the fifth English Astronomer Royal. He held the office from 1765 to 1811.-Biography:...

's method of lunar distance
Lunar distance (navigation)
In celestial navigation, lunar distance is the angle between the Moon and another celestial body. A navigator can use a lunar distance and a nautical almanac to calculate Greenwich time...

s based on observations at Greenwich, eventually 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
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

 by the Railway Clearing House
Railway Clearing House
The British Railway Clearing House was an organisation set up to manage the allocation of revenue collected by numerous pre-grouping railway companies...

 in 1847, and by almost all railway companies by the following year, from which the term "railway time"
Railway time
Railway time was the name given to the standardised time arrangement first applied by the Great Western Railway in England in November 1840. This was the first recorded occasion when a number of different local times were synchronised and a single standard time applied...

 is derived. It was gradually adopted for other purposes, but a legal case
Legal case
A legal case is a dispute between opposing parties resolved by a court, or by some equivalent legal process. A legal case may be either civil or criminal...

 in 1858 held "local mean time
Local mean time
Local mean time is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude...

" 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
Isle of Man
The Isle of Man , otherwise known simply as Mann , is a self-governing British Crown Dependency, located in the Irish Sea between the islands of Great Britain and Ireland, within the British Isles. The head of state is Queen Elizabeth II, who holds the title of Lord of Mann. The Lord of Mann is...

 in 1883, Jersey
Jersey
Jersey, officially the Bailiwick of Jersey is a British Crown Dependency off the coast of Normandy, France. As well as the island of Jersey itself, the bailiwick includes two groups of small islands that are no longer permanently inhabited, the Minquiers and Écréhous, and the Pierres de Lecq and...

 in 1898 and Guernsey
Guernsey
Guernsey, officially the Bailiwick of Guernsey is a British Crown dependency in the English Channel off the coast of Normandy.The Bailiwick, as a governing entity, embraces not only all 10 parishes on the Island of Guernsey, but also the islands of Herm, Jethou, Burhou, and Lihou and their islet...

 in 1913. Ireland adopted Greenwich Mean Time in 1916, supplanting Dublin Mean Time. Hourly time signal
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

s from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball
Time ball
A time ball is a large painted wooden or metal ball that drops at a predetermined time, principally to enable sailors to check their marine chronometers from their boats offshore...

 at the observatory obsolete in the process.

The daily rotation of the Earth is somewhat irregular (see ΔT
Delta T
ΔT, Delta T, delta-T, deltaT, or DT is the time difference obtained by subtracting Universal Time from Terrestrial Time : ΔT=TT−UT....

) and is slowing down slightly; atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s constitute a much more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

, maintained by an ensemble of atomic clocks around the world. Universal Time (UT)
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

, a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally-defined universal day
International Meridian Conference
The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States to determine the Prime Meridian of the world. The conference was held at the request of U.S. President Chester A...

; then from 1 January 1956 (as decided by the IAU
IAU
IAU may refer to:*International Astronomical Union*International American University*International American University College of Medicine*International Association of Universities*International Association of Ultrarunners...

 at Dublin, 1955, at the initiative of William Markowitz
William Markowitz
William Markowitz was an American astronomer, principally known for his work on the standardization of time....

) this 'raw' form of UT was re-labeled UT0 and effectively superseded by refined forms UT1 (UT0 equalized for the effects of polar wandering) and UT2 (UT1 further equalized for annual seasonal variations in earth rotation rate). Leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

s 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 the term GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention dating from the work of Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

, was to refer to noon
Noon
Noon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break...

 as zero hours (see Julian day
Julian day
Julian day is used in the Julian date system of time measurement for scientific use by the astronomy community, presenting the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon...

). This contrasted with the civil convention of referring to midnight
Midnight
Midnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....

 as zero hours dating from the Romans. The latter convention was adopted on and after 1 January 1925 for astronomical purposes as well, 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.

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(1).

Time zone


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
British Summer Time
Western European Summer Time is a summer daylight saving time scheme, 1 hour ahead of Coordinated Universal Time. It is used in the following places:* the Canary Islands* Portugal * Ireland...

 (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

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, and economic criteria (in addition to purely physical or 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
Geography
Geography is the science that studies the lands, features, inhabitants, and phenomena of Earth. A literal translation would be "to describe or write about the Earth". The first person to use the word "geography" was Eratosthenes...

, 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
UTC+1
UTC+01:00 is an identifier for a time offset from UTC of +01:00. In ISO 8601 the associated time would be written as .This time is used in:*Central European Time*West Africa Time*Western European Summer Time**British Summer Time**Irish Standard Time...

 in particular); contrariwise, there are European areas that use UTC, even though their 'physical' time zone is UTC−1
UTC-1
UTC−01 is an identifier for a time offset from UTC of −01. This time is used in:-As standard time :*Greenland**Eastern Greenland *Azores...

 (e.g., most of Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...

), or even UTC−2 (the westernmost part of Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

). Actually, because the UTC time zone in Europe is 'shifted' to the west, Lowestoft
Lowestoft
Lowestoft is a town in the English county of Suffolk. The town is on the North Sea coast and is the most easterly point of the United Kingdom. It is north-east of London, north-east of Ipswich and south-east of Norwich...

 in Suffolk
Suffolk
Suffolk is a non-metropolitan county of historic origin in East Anglia, England. It has borders with Norfolk to the north, Cambridgeshire to the west and Essex to the south. The North Sea lies to the east...

, East Anglia
East Anglia
East Anglia is a traditional name for a region of eastern England, named after an ancient Anglo-Saxon kingdom, the Kingdom of the East Angles. The Angles took their name from their homeland Angeln, in northern Germany. East Anglia initially consisted of Norfolk and Suffolk, but upon the marriage of...

, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 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
    Ísafjörður
    Ísafjörður is a town in the north west of Iceland, seat of Ísafjarðarbær municipality.The town draws its name from the fjord and the fjord was given its name simultaneously to the island...

    , which is west of 22°30'W, uses UTC. Bjargtangar
    Bjargtangar
    Bjargtangar is the westernmost point of Iceland and is considered the westernmost point of Europe outside the mid-Atlantic archipelago of Azores . It is located in the county of Vestur-Barðastrandarsýsla. It is the westernmost point in the UTC+0 timezone- External links :*...

    , 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
    Canary Islands
    The Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...

     (Spain
    Spain
    Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

    )
  • Most of Portugal
    Portugal
    Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...

    , incl. Lisbon
    Lisbon
    Lisbon is the capital city and largest city of Portugal with a population of 545,245 within its administrative limits on a land area of . The urban area of Lisbon extends beyond the administrative city limits with a population of 3 million on an area of , making it the 9th most populous urban...

    , Porto
    Porto
    Porto , also known as Oporto in English, is the second largest city in Portugal and one of the major urban areas in the Iberian Peninsula. Its administrative limits include a population of 237,559 inhabitants distributed within 15 civil parishes...

    , Braga
    Braga
    Braga , a city in the Braga Municipality in northwestern Portugal, is the capital of the Braga District, the oldest archdiocese and the third major city of the country. Braga is the oldest Portuguese city and one of the oldest Christian cities in the World...

    , Aveiro, and Coimbra
    Coimbra
    Coimbra is a city in the municipality of Coimbra in Portugal. Although it served as the nation's capital during the High Middle Ages, it is better-known for its university, the University of Coimbra, which is one of the oldest in Europe and the oldest academic institution in the...

    . (Only the easternmost part, incl. cities such as Bragança
    Bragança (Portugal)
    Bragança is a city and municipality in north-eastern Portugal, capital of district of Bragança, in Alto Trás-os-Montes subregion of Portugal. In 2001, the population of the municipality was 34,774, in an area of 1173.57 km².-History:...

     and Guarda
    Guarda, Portugal
    Guarda is a city in Guarda Municipality, Portugal. The city has a population of 32 111Guarda was founded in 1199 by the second king of Portugal, Sancho I....

    , lies east of 7°30'W.). Portugal has since the Treaty of Windsor (1386, world's oldest diplomatic alliance) always had close ties to Britain, which possibly explains its choice of UTC. The Madeira Islands, even further to the west, also employ 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 8.30 A.M.
  • Western part of Ireland
    Republic of Ireland
    Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...

    , incl. the cities of Cork
    Cork (city)
    Cork is the second largest city in the Republic of Ireland and the island of Ireland's third most populous city. It is the principal city and administrative centre of County Cork and the largest city in the province of Munster. Cork has a population of 119,418, while the addition of the suburban...

    , Limerick
    Limerick
    Limerick is the third largest city in the Republic of Ireland, and the principal city of County Limerick and Ireland's Mid-West Region. It is the fifth most populous city in all of Ireland. When taking the extra-municipal suburbs into account, Limerick is the third largest conurbation in the...

    , and Galway
    Galway
    Galway or City of Galway is a city in County Galway, Republic of Ireland. It is the sixth largest and the fastest-growing city in Ireland. It is also the third largest city within the Republic and the only city in the Province of Connacht. Located on the west coast of Ireland, it sits on the...

    . If uninhabited islands or rocks, or both, are to be taken into account then Rockall
    Rockall
    Rockall is an extremely small, uninhabited, remote rocky islet in the North Atlantic Ocean. It gives its name to one of the sea areas named in the shipping forecast provided by the British Meteorological Office....

     (Disputed island), at 13°41'W, should also be included.
  • Westernmost tip of Northern Ireland
    Northern Ireland
    Northern Ireland is one of the four countries of the United Kingdom. Situated in the north-east of the island of Ireland, it shares a border with the Republic of Ireland to the south and west...

    , incl. the capital of County Fermanagh
    County Fermanagh
    Fermanagh District Council is the only one of the 26 district councils in Northern Ireland that contains all of the county it is named after. The district council also contains a small section of County Tyrone in the Dromore and Kilskeery road areas....

    , Enniskillen
    Enniskillen
    Enniskillen is a town in County Fermanagh, Northern Ireland. It is located almost exactly in the centre of the county between the Upper and Lower sections of Lough Erne. It had a population of 13,599 in the 2001 Census...

  • Extreme westerly portion of the Outer Hebrides
    Outer Hebrides
    The Outer Hebrides also known as the Western Isles and the Long Island, is an island chain off the west coast of Scotland. The islands are geographically contiguous with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland...

    , west of Scotland
    Scotland
    Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

    ; for instance, Vatersay
    Vatersay
    Vatersay is an inhabited island in the Outer Hebrides of Scotland. Vatersay is also the name of the only village on the island.-Location:The westernmost permanently inhabited place in Scotland, Vatersay is linked to Barra by a causeway completed in 1991...

    , an inhabited island in the Outer Hebrides
    Outer Hebrides
    The Outer Hebrides also known as the Western Isles and the Long Island, is an island chain off the west coast of Scotland. The islands are geographically contiguous with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland...

     and the westernmost settlement in the whole of Great Britain
    Great Britain
    Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

    , lies at 7°54'W. If uninhabited islands or rocks, or both, are to be taken into account then St Kilda, west of the Outer Hebrides
    Outer Hebrides
    The Outer Hebrides also known as the Western Isles and the Long Island, is an island chain off the west coast of Scotland. The islands are geographically contiguous with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland...

    , at 8°58'W, and Rockall
    Rockall
    Rockall is an extremely small, uninhabited, remote rocky islet in the North Atlantic Ocean. It gives its name to one of the sea areas named in the shipping forecast provided by the British Meteorological Office....

     (Disputed island), at 13°41'W, should also be included.
  • Westernmost island of the Faroe Islands
    Faroe Islands
    The Faroe Islands are an island group situated between the Norwegian Sea and the North Atlantic Ocean, approximately halfway between Scotland and Iceland. The Faroe Islands are a self-governing territory within the Kingdom of Denmark, along with Denmark proper and Greenland...

     (autonomous region of the Danish Kingdom
    Denmark
    Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

    ), Mykines
    Mykines
    Mykines is the western-most of the main 18 islands in the Faroe Islands. See also the only settlement on the island, Mykines.Only 11 people live in village Mykines all year around. The oldest inhabitant is 75 and the youngest is 6 years old....

  • Iceland
    Iceland
    Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

    , including Reykjavík
    Reykjavík
    Reykjavík is the capital and largest city in Iceland.Its latitude at 64°08' N makes it the world's northernmost capital of a sovereign state. It is located in southwestern Iceland, on the southern shore of Faxaflói Bay...



Countries (mostly) between meridians 7°30'W and 7°30'E ('physical' UTC) that use UTC+1
  • Spain
    Spain
    Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

     (except for the Canary Islands
    Canary Islands
    The Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...

     which use UTC). Parts of Galicia in fact 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
    Francisco Franco
    Francisco Franco y Bahamonde was a Spanish general, dictator and head of state of Spain from October 1936 , and de facto regent of the nominally restored Kingdom of Spain from 1947 until his death in November, 1975...

    's Presidential Order (published in Boletín Oficial del Estado
    Boletín Oficial del Estado
    The Boletín Oficial del Estado , Spanish for Official Bulletin of the State, is the official gazette of the Government of Spain. It publishes the laws of the Cortes Generales and the dispositions of the Autonomous Communities...

     of 8 March 1940) abandoning Greenwich UTC time in favour of UTC+1 effective 23:00 16 March 1940. This is indeed an excellent example of the aforementioned 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 allies Nazi Germany
    Nazi Germany
    Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...

     and Fascist Italy
    Italian Fascism
    Italian Fascism also known as Fascism with a capital "F" refers to the original fascist ideology in Italy. This ideology is associated with the National Fascist Party which under Benito Mussolini ruled the Kingdom of Italy from 1922 until 1943, the Republican Fascist Party which ruled the Italian...

    , 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
    Local mean time
    Local mean time is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude...

     during the summer (one hour ahead in winter), which possibly explains the notoriously late schedule for which the country is known. However, in Portugal, which is a mere one hour behind Spain, the timetable is quite different.
  • Most of France
    France
    The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

    , including the cities of Paris
    Paris
    Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

    , Marseilles and Lyon
    Lyon
    Lyon , is a city in east-central France in the Rhône-Alpes region, situated between Paris and Marseille. Lyon is located at from Paris, from Marseille, from Geneva, from Turin, and from Barcelona. The residents of the city are called Lyonnais....

    . Only small parts of Alsace
    Alsace
    Alsace is the fifth-smallest of the 27 regions of France in land area , and the smallest in metropolitan France. It is also the seventh-most densely populated region in France and third most densely populated region in metropolitan France, with ca. 220 inhabitants per km²...

    , Lorraine
    Lorraine (région)
    Lorraine is one of the 27 régions of France. The administrative region has two cities of equal importance, Metz and Nancy. Metz is considered to be the official capital since that is where the regional parliament is situated...

     and Provence
    Provence
    Provence ; Provençal: Provença in classical norm or Prouvènço in Mistralian norm) is a region of south eastern France on the Mediterranean adjacent to Italy. It is part of the administrative région of Provence-Alpes-Côte d'Azur...

     are east of 7°30'E ('physical' UTC+1).
  • Belgium
    Belgium
    Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...

  • Netherlands
    Netherlands
    The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

  • Luxembourg
    Luxembourg
    Luxembourg , officially the Grand Duchy of Luxembourg , is a landlocked country in western Europe, bordered by Belgium, France, and Germany. It has two principal regions: the Oesling in the North as part of the Ardennes massif, and the Gutland in the south...


See also

  • 24-hour watch — 24-hour wristwatch
  • Radio clock
    Radio clock
    A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

  • Marine chronometer
    Marine chronometer
    A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

     — synchronised with GMT, and used by ships to calculate their longitude
  • Sandringham Time
    Sandringham Time
    Sandringham time is the name given to the idiosyncratic alterations that King Edward VII made to the timekeeping at the royal estate of Sandringham...

  • Swatch Internet Time
    Swatch Internet Time
    Swatch Internet Time is a decimal time concept introduced in 1998 and marketed by the Swatch corporation as an alternative, decimal measure of time...

     — alternative, decimal measure of time
  • Western European Summer Time

External links