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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A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.

Marine chronometer

Precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation.

Precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation.

A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.
The marine "Chronometer" of Jeremy Thacker used gimbals and a vacuum in a bell jar.
Henry Sully (1680-1729) presented a first marine chronometer in 1716
John Harrison's H1 marine chronometer of 1735
Drawings of Harrison's H4 chronometer of 1761, published in The principles of Mr Harrison's time-keeper, 1767.
Ferdinand Berthoud's marine chronometer no.3, 1763
Pierre Le Roy marine chronometer, 1766, photographed at the Musée des Arts et Métiers in Paris
Harrison's Chronometer H5 of 1772, now on display at the Science Museum, London
Ferdinand Berthoud chronometer no. 24 (1782), on display at the Musée des Arts et Métiers, Paris
Einheitschronometer pattern MX6 marine chronometer mass-produced in the Soviet Union after World War II
Mechanical boxed Marine Chronometer used on Queen Victoria's royal yacht, made about 1865
A chronometer mechanism diagrammed (text is in German). Note fusee to transform varying spring tension to a constant force
Einheitschronometer pattern marine chronometer (A. Lange & Söhne, 1948) displaying its second hand advancing in ½ second increments over a 60 seconds marked sub dial for optimal timing of celestial objects angle measurements at the GFZ
Omega 4.19 MHz (4,194,304 = 222 high frequency quartz resonator) Ships Marine Chronometer giving an autonomous accuracy of less than ± 5 seconds per year, French Navy issued,1980. The second hand can advance in ½ second increments for optimal timing of celestial objects angle measurements.

It is used to determine longitude by comparing Greenwich Mean Time (GMT), or in the modern world its successor Coordinated Universal Time (UTC), and the time at the current location found from observations of celestial bodies.

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.

Time zones of the world

Time zone

Area that observes a uniform standard time for legal, commercial and social purposes.

Area that observes a uniform standard time for legal, commercial and social purposes.

Time zones of the world
Time zones of the world
Plaque commemorating the Railway General Time Convention of 1883 in North America
The control panel of the Time Zone Clock in front of Coventry Transport Museum
1913 time zone map of the United States, showing boundaries very different from today
World map of time zones in 1928
Difference between sun time and clock time during daylight saving time:

The Royal Observatory, Greenwich, founded in 1675, established Greenwich Mean Time (GMT), the mean solar time at that location, as an aid to mariners to determine longitude at sea, providing a standard reference time while each location in England kept a different time.

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.

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.

Flamsteed House in 1824

Royal Observatory, Greenwich

Observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north.

Observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north.

Flamsteed House in 1824
Royal Observatory, Greenwich c. 1902 as depicted on a postcard
Greenwich Observatory (Latinized as "Observatorium Anglicanum Hoc Grenovici prope Londinum"), as illustrated in Johann Gabriel Doppelmayr's map of the southern celestial hemisphere, ca. 1730
Royal Observatory, Greenwich
Dome of the Greenwich 28 inch refractor telescope and tree
The Airy Transit Circle, used for over a century (1851–1953) as the reference point when charting the heavens and determining times, thus earning for it the epithet "the centre of time and space"
The building housing the origin of the Greenwich Prime Meridian
Laser projected from the observatory marking the Prime Meridian line
Laser at night
Shepherd Gate Clock at Royal Greenwich Observatory
One of the hyper-accurate timekeepers at the observatory
The time ball is the red ball on a post – when it drops a certain time is signalled. This allowed clocks to be set from afar with great accuracy, particularly the chronometers of ships on the River Thames below, prior to sailing. The observatory would first determine the time by stellar observations.
Dome of the Great Equatorial Building overlooking Greenwich Park
21st-century view of the Altazimuth Pavilion
Standard lengths on the wall of the Royal Observatory, Greenwich, London – 1 yard (3 feet), 2 feet, 1 foot, 6 inches (1/2-foot), and 3 inches. The separation of the inside faces of the marks is exact at an ambient temperature of 60 °F and a rod of the correct measure, resting on the pins, will fit snugly between them.
Aerial view of the Royal Greenwich Observatory, Herstmonceux site in East Sussex; the dome that formerly housed the Isaac Newton Telescope is the single dome to the right. The telescope was moved to La Palma in the Canary Isles in 1979.
Former Royal Greenwich Observatory, Herstmonceux, East Sussex
Greenwich House at Cambridge
The Queen's House (centre left) at Greenwich, with the Royal Observatory on the skyline behind, in 2017.
The Magnetic Pavilion, 1900
Tourists flock to the Observatory museum, 2009
The centuries-old Flamsteed House overlooking Greenwich Park in London. The statue at left is of Major General James Wolfe, who died capturing Quebec in 1759, and was buried in St Alfege Church, Greenwich.

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, the precursor to today's Coordinated Universal Time (UTC).

Old Royal Naval College and University of Greenwich buildings on the bank of the River Thames

Greenwich

Town in southeast London, located in the historic county of Kent and the ceremonial county of Greater London.

Town in southeast London, located in the historic county of Kent and the ceremonial county of Greater London.

Old Royal Naval College and University of Greenwich buildings on the bank of the River Thames
Prehistoric burial mounds in Greenwich Park
Adriaen van Stalbemt's A View of Greenwich, c. 1632, showing King Charles I (in the black hat) and his family. Royal Collection, London.
In the 1880s, if this place is so cut into three: east, central and west zones of about 30,000 inhabitants each, the central one had less than 10% recognisable poverty, the minimum of London's map above, but the others (east and west) more than 40%.
The former Greenwich Town Hall, now known as "Meridian House"
A map showing the wards of Greenwich Metropolitan Borough as they appeared in 1916.
Boats at Greenwich at the end of the Great River Race
The Royal Navy Type 45 destroyer moored on the riverfront at Greenwich in 2015
Trinity Hospital, Greenwich
Spiral staircase and lantern at the Queen's House in Greenwich
Town centre
Greenwich Market
Millennium Leisure Park
Royal Observatory with the time ball atop the Octagon Room
Pepys Building
Greenwich station

Greenwich is notable for its maritime history and for giving its name to the Greenwich Meridian (0° longitude) and Greenwich Mean Time.

World map of current time zones

Coordinated Universal Time

Primary time standard by which the world regulates clocks and time.

Primary time standard by which the world regulates clocks and time.

World map of current time zones
Graph showing the difference DUT1 between UT1 and UTC (in seconds). Vertical segments correspond to leap seconds.

It is effectively a successor to Greenwich Mean Time (GMT).

Difference between legal time and local mean solar time in Europe during the winter

Western European Time

Difference between legal time and local mean solar time in Europe during the winter

Western European Time (WET, UTC±00:00) is a time zone covering parts of western Europe and consists of countries using UTC±00:00 (also known as Greenwich Mean Time, shortly called GMT).

Standard time zones of the world (February 2021). The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.

Universal Time

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

Standard time zones of the world (February 2021). The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.
An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time

Starting in 1847, Britain established Greenwich Mean Time, the mean solar time on the Prime Meridian at Greenwich, England, to solve this problem: all clocks in Britain were set to this time regardless of local solar noon.