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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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.

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.

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.

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).

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.

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).

The vertical red line left of the middle is the Greenwich meridian.

International Meridian Conference

Conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use.

Conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use.

The vertical red line left of the middle is the Greenwich meridian.
Local times in major towns across the United States, as published in 1857 when, as the document states, 'There is no "Standard Railroad Time" in the United States or Canada'.

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".

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).

Finding Greenwich time while at sea using a lunar distance. The lunar distance is the angle between the Moon and a star (or the Sun). The altitudes of the two bodies are used to make corrections and determine the time.

Lunar distance (navigation)

Angular distance between the Moon and another celestial body.

Angular distance between the Moon and another celestial body.

Finding Greenwich time while at sea using a lunar distance. The lunar distance is the angle between the Moon and a star (or the Sun). The altitudes of the two bodies are used to make corrections and determine the time.

The lunar distances method uses this angle, also called a lunar, and a nautical almanac to calculate Greenwich time if so desired, or by extension any other time.

Canary Islands

The Canary Islands (Canarias, ), also known informally as the Canaries, are a Spanish region and archipelago in the Atlantic Ocean, in Macaronesia.

The Canary Islands (Canarias, ), also known informally as the Canaries, are a Spanish region and archipelago in the Atlantic Ocean, in Macaronesia.

Map of the Canary Islands
Hacha Grande, a mountain in the south of Lanzarote, viewed from the road to the Playa de Papagayo
Panoramic view of Gran Canaria, with Roque Nublo at the left and Roque Bentayga at the center
Humid laurel forest in La Gomera
Mount Teide, the highest mountain in Spain, is also one of the most visited National Parks in the world.
Caldera de Taburiente National Park (La Palma)
The Province of Las Palmas
The Province of Santa Cruz de Tenerife
Municipalities in the Las Palmas Province
Municipalities in the Santa Cruz de Tenerife Province
Guanche mummy of a woman (830 AD). Museo de la Naturaleza y el Hombre, Santa Cruz de Tenerife.
A selection of artefacts unearthed from the Lomo de los Gatos site on Gran Canaria
Reconstruction of a Guanche settlement of Tenerife
Alonso Fernández de Lugo presenting the captured native Guanche kings of Tenerife to the Catholic Monarchs
Casa de Colón (Las Palmas de Gran Canaria), which Christopher Columbus visited during his first trip
Maps of the Canary Islands drawn by William Dampier during his voyage to New Holland in 1699
Coat of arms of the Castilian and Spanish Realm of Canary Islands
Slave-driving in order to sell into enforced labour
Amaro Pargo (1678–1741), corsair and merchant from Tenerife who participated in the Spanish treasure fleet (the Spanish-American trade route)
Coast El Golfo, El Hierro
The port of Las Palmas in 1912
Auditorio de Tenerife by Santiago Calatrava, and an icon of contemporary architecture in the Canary Islands, (Santa Cruz de Tenerife)
Map of the European Union in the world with overseas countries and territories and outermost regions (pre 31 January 2020)
Overview of Las Palmas de Gran Canaria
View of Santa Cruz de Tenerife
Basilica of the Virgin of Candelaria (Patroness of the Canary Islands) in Candelaria, Tenerife
View of Fataga, Gran Canaria
San Cristóbal de La Laguna in 1880 (Tenerife)
The dunes of Maspalomas in Gran Canaria is one of the tourist attractions.
Bus Station—Estación de Guaguas also known as El Hoyo (The hole), on the left, out of the image—at San Telmo Park, Las Palmas de Gran Canaria
Tenerife Tram
Port of Las Palmas, the largest port in the Canary Islands
Canary Island spurge in Fuerteventura
Skull of Tenerife giant rat (Canariomys bravoi). It was an endemic species that is now extinct.
A loggerhead sea turtle, by far the most common species of marine turtle in the Canary Islands
The Dance of the Dwarves is one of the most important acts of the Lustral Festivities of the Bajada de la Virgen de las Nieves in Santa Cruz de La Palma.
Dancers with typical costume in El Tamaduste (El Hierro)
Band of Agaete in the Traída del Agua (Gran Canaria)
Parade in the Carnival of Santa Cruz de Tenerife
Gran Canaria space tracking station
Heliodoro Rodríguez López Stadium in Tenerife, the stadium with the largest field area in the Canary Islands
Gran Canaria Stadium, the biggest sports venue in the Canary Islands
Serinus canaria
Phoenix canariensis
El Hierro
La Palma
La Gomera
Tenerife
Gran Canaria
Fuerteventura
Lanzarote
La Graciosa
Arbutus canariensis
Argyranthemum frutescens
Bosea yervamora
Canarina canariensis
Digitalis canariensis
Echium wildpretii
Euphorbia canariensis
Gonospermum elegans
Lavatera acerifolia var. acerifolia
Lavatera phoenicea
Lotus berthelotii
Pericallis webbii
Persea indica
Phoenix canariensis
Sonchus palmensis
Spartocytisus supranubius

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