Katana VentraIP

Greenwich Mean Time

Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon;[1] as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and,[2] in UK law, is the basis for civil time in the United Kingdom.[3][a]

"GMT" redirects here. For other uses, see GMT (disambiguation).

Greenwich Mean Time

Because of Earth's uneven angular velocity in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Greenwich Meridian[b] and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy described by the equation of time. Noon GMT is the annual average (the arithmetic mean) moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".[c]


Originally, astronomers considered a GMT day to start at noon,[d] while for almost everyone else it started at midnight. To avoid confusion, the name Universal Time was introduced in 1928 to denote GMT as counted from midnight.[5][6] Today, Universal Time usually refers to Coordinated Universal Time (UTC) or UT1;[7] English speakers often use GMT as a synonym for UTC.[8] For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9 s. The term "GMT" should thus not be used for purposes that require precision.[9]


The term "GMT" is especially used by United Kingdom bodies, such as the BBC World Service, the Royal Navy, and the Met Office; and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN.

Ambiguity in the definition of GMT[edit]

Historically, GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention, dating from the work of Ptolemy, was to refer to noon as zero hours (see Julian day). This contrasted with the civil convention of referring to midnight as zero hours dating from the Roman Empire. The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day. The instant that was designated as "December 31.5 GMT" in 1924 almanacs became "January 1.0 GMT" in 1925 almanacs. The term Greenwich Mean Astronomical Time (GMAT) was introduced to unambiguously refer to the previous noon-based astronomical convention for GMT.[17] The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

GMT in legislation[edit]

United Kingdom[edit]

Legally, the civil time used in the UK is called "Greenwich mean time" (without capitalisation), with an exception made for those periods when the Summer Time Act 1972 orders an hour's shift for daylight saving. The Interpretation Act 1978, section 9, provides that whenever an expression of time occurs in any Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time.[3] Under subsection 23, the same rule applies to deeds and other instruments.[13]


During the experiment of 1968 to 1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all-year British Summer Time was called British Standard Time (BST).


In the UK, UTC+00:00 is disseminated to the general public in winter and UTC+01:00 in summer.[5][18]


BBC radio stations broadcast the "six pips" of the Greenwich Time Signal. It is named from its original generation at the Royal Greenwich Observatory. If announced (such as near the start of summer time or of winter time), announcers on domestic channels declare the time as GMT or BST as appropriate. As the BBC World Service is broadcast to all time zones, the announcers use the term "Greenwich Mean Time" consistently throughout the year.

Other countries[edit]

Several countries define their local time by reference to Greenwich Mean Time.[19][20] Some examples are:

United Kingdom, where the summer time is called (BST)

British Summer Time

Ireland, where it is called Winter Time, changing to Standard Time in summer.[21]

[22]

(with the exception of the Azores)

Portugal

Canary Islands

Faroe Islands

Greenwich Mean Time is defined in law as standard time in the following countries and areas, which also advance their clocks one hour (GMT+1) in summer.


Greenwich Mean Time is used as standard time all year round in the following countries and areas:

– "the Greenwich Time Lady", daughter of John Henry Belville, who was in the business of daily personal distribution of Greenwich Mean Time via a watch

Ruth Belville

 – Primary time standard

Coordinated Universal Time

 – Series of six pips broadcast by the BBC

Greenwich Time Signal

 – Clock used on ships to aid in navigation

Marine chronometer

 – Type of clock which self-synchronizes its time using dedicated radio transmitters

Radio clock

 – Observatory in London, England

Royal Observatory, Greenwich

 – Time zones in Europe

Time in Europe

Time in the United Kingdom

 – Time zone in Europe: UTC±00:00

Western European Time

 – Time zone (UTC+01:00)

Western European Summer Time

Zulu Time

International Earth Rotation and Reference Systems Service

The original BBC World Service GMT time signal in MP3 format

Rodgers, Lucy (20 October 2009). . BBC News. Retrieved 20 October 2009.

"At the centre of time"