Greenwich clock with standard measurements
Time zones of the world
Clock in Kumasi, Ghana, set to GMT.
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 term 'GMT' is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

- Greenwich Mean 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.

- Time zone
Greenwich clock with standard measurements

5 related topics

Alpha

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.

It is commonly used in practice to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, especially by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others, although strictly UTC is an atomic time scale which only approximates GMT with a tolerance of 0.9 second.

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).

Time zones around the world are expressed using positive or negative offsets from UTC, as in the list of time zones by UTC offset.

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.

This measurement of time was used for everyday use during the 19th century before time zones were introduced beginning in the late 19th century; it still has some uses in astronomy and navigation.

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.

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".

Thus the conference did not adopt any time zones, contrary to popular belief.

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).

Despite this early reluctance, railway time rapidly became adopted as the default time across the whole of Great Britain, although it took until 1880 for the government to legislate on the establishment of a single standard time and a single time zone for the country.