Greenwich Mean Time

GMTGMT+4UTC±00:00Greenwich Timelocal time+7UTC+0UTC(GMT)British time
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight.wikipedia
6,129 Related Articles

Greenwich

Greenwich, LondonGreenwich, EnglandEast Greenwich
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight.
Greenwich is notable for its maritime history and for giving its name to the Greenwich Meridian (0° longitude) and Greenwich Mean Time.

Royal Observatory, Greenwich

Royal Greenwich ObservatoryRoyal ObservatoryGreenwich Observatory
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight.
It played a major role in the history of astronomy and navigation, and because the prime meridian passes through it, it gave its name to Greenwich Mean Time.

London

London, EnglandLondon, United KingdomLondon, UK
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight.
London contains four World Heritage Sites: the Tower of London; Kew Gardens; the site comprising the Palace of Westminster, Westminster Abbey, and St Margaret's Church; and the historic settlement in Greenwich where the Royal Observatory, Greenwich defines the Prime Meridian, 0° longitude, and Greenwich Mean Time.

Coordinated Universal Time

UTCUTC-3UTC-4
English speakers often use GMT as a synonym for Coordinated Universal Time (UTC).
In some countries, the term Greenwich Mean Time is used.

Universal Time

UTUT1UTC
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.
Greenwich Mean Time, the mean solar time on the Prime Meridian at Greenwich, England, was established to solve this problem: all clocks in Britain were set to this time irrespective of local solar noon.

Marine chronometer

chronometerchronometersChronoscope
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.
A marine chronometer is a timepiece 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 accurately measuring the time of a known fixed location, for example Greenwich Mean Time (GMT) and the time at the current location.

Time zone

Sri Lanka Standard Time Zonetime zoneslocal time
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 (GMT) was established in 1675, when the Royal Observatory was built, as an aid to mariners to determine longitude at sea, providing a standard reference time while each city in England kept a different local time.

BBC World Service

World ServiceBBCBBC Overseas Service
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.
However, the building was vacated in 2012 as a result of the Broadcasting House changes and the end of the building's lease that year; the first service to move was the Burmese Service on 11 March 2012 and the final broadcast from Bush House was a news bulletin broadcast at 11.00GMT on 12 July 2012.

International Meridian Conference

1884 prime meridian systemabandonedcontemporaneously
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.
In Britain, the Great Western Railway had standardised time by 1840 and in 1847 the Railway Clearing Union decreed that "GMT be adopted at all stations as soon as the General Post Office permitted it".

Railway time

Railway time in IndiaRailway time of IndiaStandard railway time
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.
The schedules by which trains were organised and the time station clocks displayed were brought in line with the local mean time for London or "London Time", the time set at Greenwich by the Royal Observatory, which was already widely known as Greenwich Mean Time (GMT).

Lunar distance (navigation)

lunar distance methodlunar distancemethod of lunar distances
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.
The lunar distances method uses this angle, also called a lunar, and a nautical almanac to calculate Greenwich time.

Time in the Republic of Ireland

ISTIrish Standard TimeDublin Mean Time
Ireland adopted GMT in 1916, supplanting Dublin Mean Time.
Ireland uses Irish Standard Time (IST, UTC+01:00; Am Caighdeánach Éireannach) in the summer months and Greenwich Mean Time (UTC+00:00; Meán-Am Greenwich) in the winter period.

Local mean time

mean timemeanLMT
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight.
Standard time means that the same time is used throughout some region—usually, it was either offset from Greenwich Mean Time or was the local mean time of the capital of the region.

UTC±00:00

UTC+0UTCUTC±0
In the UK, UTC+00:00 is disseminated to the general public in winter and UTC+01:00 in summer.

British Summer Time

BSTBritish Double Summer TimeUTC+1
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).
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 evenings have more daylight and mornings have fewer daylight hours.

Julian day

JDJulian DateJulian day number
The long-standing astronomical convention, dating from the work of Ptolemy, was to refer to noon as zero hours (see Julian day).
Historically, Julian dates were recorded relative to Greenwich Mean Time (GMT) (later, Ephemeris Time), but since 1997 the International Astronomical Union has recommended that Julian dates be specified in Terrestrial Time.

Time signal

time serviceradio time signaltime
Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory redundant.
Greenwich Mean Time was distributed by telegraph from the Greenwich Observatory.

Prime meridian (Greenwich)

Greenwich MeridianPrime MeridianGreenwich
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 Greenwich meridian and reaches its highest point in the sky there.

United Kingdom

BritishUKBritain
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. 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.

Saint Helena, Ascension and Tristan da Cunha

Saint Helena and DependenciesSt Helena, Ascension and Tristan da CunhaSaint Helena
The three territories lie in the Western Hemisphere and have the same time zone: Greenwich Mean Time.

Mykines, Faroe Islands

MykinesMykines Island
However, Mykines uses Greenwich Mean Time like the rest of the Faroes.

Railway Clearing House

Clearing HouseRCH
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.
On 22 September 1847, the RCH recommended that Greenwich Mean Time be adopted as the standard time for all railways in the United Kingdom.

Canary Islands

Canary IslandCanariesCanarian
Canarian time is Western European Time (WET) (or GMT; in summer one hour ahead of GMT).

Bailiwick of Guernsey

GuernseyBailiwickThe Bailiwick of Guernsey
GMT was adopted in the Isle of Man in 1883, in Jersey in 1898 and in Guernsey in 1913.

Great Britain

BritishBritainGBR
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.