World map of current time zones
Greenwich clock with standard measurements
Graph showing the difference DUT1 between UT1 and UTC (in seconds). Vertical segments correspond to leap seconds.
Clock in Kumasi, Ghana, set to GMT.

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

- Coordinated Universal Time

English speakers often use GMT as a synonym for Coordinated Universal Time (UTC): in modern usage, this is incorrect – GMT is now a time zone, not a time standard.

- Greenwich Mean Time
World map of current time zones

4 related topics with Alpha

Overall

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

Greenwich

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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.

Greenwich Mean Time was at one time based on the time observations made at the Royal Greenwich Observatory, before being superseded by the closely-related Coordinated Universal Time (UTC).

Time zones of the world

Time zone

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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:

All time zones are defined as offsets from Coordinated Universal Time (UTC), ranging from UTC−12:00 to UTC+14:00.

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.

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

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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.

UT0 and UT2 soon became irrelevant due to the introduction of Coordinated Universal Time (UTC).

Julian day

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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 .

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.

Seidelmann indicates that Julian dates may be used with International Atomic Time (TAI), Terrestrial Time (TT), Barycentric Coordinate Time (TCB), or Coordinated Universal Time (UTC) and that the scale should be indicated when the difference is significant.