Greenwich Mean Time

Greenwich clock with standard measurements
Clock in Kumasi, Ghana, set to GMT.

Mean solar time at the Royal Observatory in Greenwich, London, counted from midnight.

- Greenwich Mean Time
Greenwich clock with standard measurements

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In 1300, the City was still confined within the Roman walls.

London

Capital and largest city of England and the United Kingdom, with a population of just over 9 million.

Capital and largest city of England and the United Kingdom, with a population of just over 9 million.

In 1300, the City was still confined within the Roman walls.
The Lancastrian siege of London in 1471 is attacked by a Yorkist sally.
Westminster Abbey, as seen in this painting (by Canaletto, 1749), is a World Heritage Site and one of London's oldest and most important buildings.
Map of London in 1593. There is only one bridge across the Thames, but parts of Southwark on the south bank of the river have been developed.
The defensive Lines of Communication, planned during the English Civil War, c. 1643, surrounded The City, Westminster, Southwark, Lambeth and related areas (Vertue, 1738).
The Great Fire of London destroyed many parts of the city in 1666.
View to the Royal Exchange in the City of London in 1886
Arms of the Corporation of the City of London: Argent, a cross gules in the first quarter a sword in pale point upwards of the last; Supporters: Two dragons with wings elevated and addorsed argent on each wing a cross gules; Crest: On a dragon's wing displayed sinister a cross gules
Satellite view of London in June 2018
London from Primrose Hill
The Tower of London, a medieval castle, dating in part to 1078
Trafalgar Square and its fountains, with Nelson's Column on the right
Modern styles juxtaposed with historic styles; 30 St Mary Axe, also known as "The Gherkin", towers over St Andrew Undershaft
A fox on Ayres Street, Southwark, South London
Population density map
The City of London, one of the largest financial centres in the world
The London Stock Exchange at Paternoster Square and Temple Bar
A view from Westminster Millennium Pier on the River Thames, December 2018
Journeys in Greater London by mode from 1997 to 2018
Heathrow Airport is the busiest airport in Europe as well as the second busiest in the world for international passenger traffic. (Terminal 5C is pictured.)
The London Underground is the world's oldest and third-longest rapid transit system.
St Pancras International is the main terminal for high-speed Eurostar and High Speed 1 services, as well as commuter suburban Thameslink and inter-city East Midlands Railway services.
A red double-decker bus
Santander Cycle Hire near Victoria in Central London
Imperial College London, a technical research university in South Kensington
King's College London, established by Royal Charter in 1829, is one of the founding colleges of the University of London.
The front façade of the Royal College of Music
Piccadilly Circus
Harrods in Knightsbridge
Scene of the annual Notting Hill Carnival, 2014
Shakespeare's Globe is a modern reconstruction of the Globe Theatre on the south bank of the River Thames.
Sherlock Holmes Museum in Baker Street, bearing the number 221B
Keats House, where Keats wrote his Ode to a Nightingale. The village of Hampstead has historically been a literary centre in London.
Aerial view of Albertopolis. Albert Memorial, Royal Albert Hall, Royal Geographical Society and Royal College of Art are visible near the top; Victoria and Albert Museum and Natural History Museum at the lower end; Imperial College, Royal College of Music, and Science Museum lying in between.
The Royal Albert Hall hosts concerts and musical events.
The Horse Ride is a tree tunnel (route overhung by trees) on the western side of Wimbledon Common.
Aerial view of Queen Elizabeth Olympic Park, home of the 2012 Summer Olympics
Wembley Stadium, home of the England football team, has a seating capacity of 90,000. It is the UK's biggest stadium.
10 Downing Street, official residence of the Prime Minister
The Bank of England, established in 1694, is the model on which most modern central banks are based.
Broadcasting House in central London, headquarters of the BBC
A New Routemaster (which replaced the AEC Routemaster) entered service in 2012. The red double-decker bus is an emblematic symbol of London.
The hackney carriage (black cab) is a common sight on London streets. Although traditionally black, this is not a requirement with some painted in other colours or bearing advertising.
The Royal Albert Hall hosts concerts and musical events, including The Proms which are held every summer.
Abbey Road Studios was given grade II listed status for its "cultural and historical importance" in 2010.
Hyde Park (with Kensington Gardens in foreground) has been a popular public space since it opened in 1637

London has four World Heritage Sites: the Tower of London; Kew Gardens; the combined Palace of Westminster, Westminster Abbey, and St Margaret's Church; and also the historic settlement in Greenwich, where the Royal Observatory, Greenwich defines the Prime Meridian (0° longitude) and Greenwich Mean Time.

The Astronomer, 1668, by Johannes Vermeer

Julian day

Continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events .

Continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events .

The Astronomer, 1668, by Johannes Vermeer

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.

The equation of time — above the axis a sundial will appear fast relative to a clock showing local mean time, and below the axis a sundial will appear slow.

Local mean time

Form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude.

Form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude.

The equation of time — above the axis a sundial will appear fast relative to a clock showing local mean time, and below the axis a sundial will appear slow.

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.

These automatic signal clocks were synchronized by telegraphy in 1905 before the widespread use of radio

Time signal

Visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.

Visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.

These automatic signal clocks were synchronized by telegraphy in 1905 before the widespread use of radio
The time ball on the roof of Greenwich Observatory, London
Advertisement for a telegraph time signal service (1900)
A modern LF Radio clock
A low cost LF radio clock receiver, antenna left, receiver right.

Greenwich Mean Time was distributed by telegraph from the Greenwich Observatory.

Clock on The Exchange, Bristol, showing two minute hands, one for London time (GMT) and one for Bristol time (GMT +11 minutes).

Railway time

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 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.

Clock on The Exchange, Bristol, showing two minute hands, one for London time (GMT) and one for Bristol time (GMT +11 minutes).

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).

Time (Ireland) Act 1916

Time in the Republic of Ireland

Time (Ireland) Act 1916

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.

BBC World Service

International broadcaster owned and operated by the BBC, with funding from the British Government through the Foreign Secretary's office.

International broadcaster owned and operated by the BBC, with funding from the British Government through the Foreign Secretary's office.

Bush House in London was home to the World Service between 1941 and 2012.
BBC World Service logo used from 2008 to 2019
BBC World Service logo used from 2019 to 2022
The BBC World Service is located in Broadcasting House, London.
Steve Titherington - BBC World Questions broadcasting from Budapest

However, the building was vacated in 2012 as a result of the Broadcasting House redevelopment 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.

Clocks at Leeds City Station; the left hand one being at British Summer Time and the right hand one being at Greenwich Mean Time.

British Summer Time

Clocks at Leeds City Station; the left hand one being at British Summer Time and the right hand one being at Greenwich Mean 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 evenings one hour more.

Railway Clearing House map of Scotland, 1920

Railway Clearing House

Organisation set up to manage the allocation of revenue collected by pre-grouping railway companies for the conveyance of passengers and goods over the lines (or using the rolling stock) of other companies.

Organisation set up to manage the allocation of revenue collected by pre-grouping railway companies for the conveyance of passengers and goods over the lines (or using the rolling stock) of other companies.

Railway Clearing House map of Scotland, 1920
The RCH jumper cable of a BR Mark 1 coach

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

Saint Helena, Ascension and Tristan da Cunha

British Overseas Territory located in the South Atlantic and consisting of the island of Saint Helena, Ascension Island and the archipelago of Tristan da Cunha including Gough Island.

British Overseas Territory located in the South Atlantic and consisting of the island of Saint Helena, Ascension Island and the archipelago of Tristan da Cunha including Gough Island.

A View of the Town and Island of St Helena in the Atlantic Ocean belonging to the English East India Company (engraving c. 1790)
Gough and Inaccessible Islands were declared World Heritage Site by UNESCO in 1995.
Edinburgh of the Seven Seas, Tristan da Cunha
Tristan da Cunha on 6 February 2013, as seen from the International Space Station
frameless
Elizabeth II is the head of state of Saint Helena, Ascension and Tristan da Cunha
Ascension Government House
The flag of Saint Helena
The flag of Ascension Island
The flag of Tristan da Cunha
The airport of Saint Helena

The three territories lie in the Western Hemisphere and have the same time zone: Greenwich Mean Time.