Greenwich Mean Time

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Greenwich Mean Time
Time zone
UTC offset
GMT UTC±00:00
Current time
22:22, 29 January 2024 GMT [refresh]
Observance of DST
DST is observed throughout this time zone.
Time in Europe:
Light Blue
Western European Time / Greenwich Mean Time (UTC)
Blue
Western European Time / Greenwich Mean Time (UTC)
Western European Summer Time / British Summer Time / Irish Standard Time (UTC+1)
Red
Central European Time (UTC+1)
Central European Summer Time (UTC+2)
Yellow
Eastern European Time / Kaliningrad Time (UTC+2)
Ochre
Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
Green
Moscow Time / Turkey Time (UTC+3)
Turquoise
Armenia Time / Azerbaijan Time / Georgia Time / Samara Time (UTC+4)
#### Pale colours: Standard time observed all year
### Dark colours: Summer time observed Time zones of Europe, incl. Transcaucasia.svg
Time in Europe:
Light Blue Western European Time / Greenwich Mean Time (UTC)
Blue Western European Time / Greenwich Mean Time (UTC)
Western European Summer Time / British Summer Time / Irish Standard Time (UTC+1)
Red Central European Time (UTC+1)
Central European Summer Time (UTC+2)
Yellow Eastern European Time / Kaliningrad Time (UTC+2)
Ochre Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
Green Moscow Time / Turkey Time (UTC+3)
Turquoise Armenia Time / Azerbaijan Time / Georgia Time / Samara Time (UTC+4)
 Pale colours: Standard time observed all year
 Dark colours: Summer time observed
Time zones of Africa:

UTC-01:00
Cape Verde Time
UTC+-00:00
Greenwich Mean Time
UTC+01:00
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Central European Time
West Africa Time
UTC+02:00
Central Africa Time
Eastern European Time
Egypt Standard Time
South African Standard Time
UTC+03:00
East Africa Time
UTC+04:00
Mauritius Time
Seychelles Time
.mw-parser-output .citation{word-wrap:break-word}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}
The islands of Cape Verde are to the west of the African mainland.

Mauritius and the Seychelles are to the east and north-east of Madagascar respectively. TimeZones-Africa.svg
Time zones of Africa:
  UTC-01:00   Cape Verde Time
  UTC±00:00   Greenwich Mean Time
  UTC+01:00  
  UTC+02:00  
  UTC+03:00   East Africa Time
  UTC+04:00  
The islands of Cape Verde are to the west of the African mainland.
Mauritius and the Seychelles are to the east and north-east of Madagascar respectively.

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] [lower-alpha 1]

Contents

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 [lower-alpha 2] 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". [lower-alpha 3]

Originally, astronomers considered a GMT day to start at noon, [lower-alpha 4] 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.

History

The Shepherd Gate Clock at the gates of the Royal Observatory, Greenwich is permanently kept on Greenwich Mean Time. Greenwich clock.jpg
The Shepherd Gate Clock at the gates of the Royal Observatory, Greenwich is permanently kept on Greenwich Mean Time.

As the United Kingdom developed into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention adopted in the International Meridian Conference of 1884. Synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most time zones were based upon GMT, as an offset of a number of hours (and possibly half or quarter hours) "ahead of GMT" or "behind GMT".

Greenwich Mean Time was adopted across the island of Great Britain by the Railway Clearing House in 1847 and by almost all railway companies by the following year, from which the term railway time is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official time. [10] On 14 May 1880, a letter signed by "Clerk to Justices" appeared in The Times, stating that "Greenwich time is now kept almost throughout England, but it appears that Greenwich time is not legal time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 p.m." [11] [12] This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted in the Isle of Man in 1883, in Jersey in 1898 and in Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time. [13] Hourly time signals from Greenwich Observatory were first broadcast by shortwave radio on 5 February 1924 at 17:30:00 UTC, [14] rendering the time ball at the observatory redundant. [15]

The daily rotation of the Earth is irregular (see ΔT) and has a slowing trend; therefore atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT as the international civil time standard was superseded by Coordinated Universal Time (UTC), maintained by an ensemble of atomic clocks around the world. Universal Time (UT), a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally defined universal day; from 1 January 1956 (as decided by the International Astronomical Union in Dublin in 1955, at the initiative of William Markowitz) this "raw" form of UT was re-labelled UT0 and effectively superseded by refined forms UT1 (UT0 equalised for the effects of polar wandering) [16] and UT2 (UT1 further equalised for annual seasonal variations in Earth rotation rate).

Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by "the centre of the transit instrument at the Observatory at Greenwich". Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world's longitude and time is not strictly defined in material form but from a statistical solution resulting from observations of all time-determination stations which the BIPM takes into account when co-ordinating the world's time signals. Nevertheless, the line in the old observatory's courtyard today differs no more than a few metres from that imaginary line which is now the prime meridian of the world.

Howse, D. (1997). Greenwich time and the longitude. London: Philip Wilson.

Ambiguity in the definition of GMT

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

United Kingdom

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

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

Time zone

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.

Clock in Kumasi, Ghana, set to GMT Donald Stewart's Monument- Kumasi.jpg
Clock in Kumasi, Ghana, set to GMT

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

See also

Notes

  1. British Summer Time is defined in law as being one hour in advance of Greenwich Mean Time.
  2. The 'Prime Meridian', 0°, was originally defined as being the Greenwich meridian but is now the "IERS Reference Meridian": they are not quite the same. [4]
  3. There is no such thing as the "Greenwich Mean".
  4. Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date.

Related Research Articles

Δ<i>T</i> (timekeeping) Measure of variation of solar time from atomic time

In precise timekeeping, ΔT is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time. Formally, ΔT is the time difference ΔT = TT − UT between Universal Time and Terrestrial Time. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day which are dealt with separately.

<span class="mw-page-title-main">Time zone</span> Area that observes a uniform standard time

A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

Universal Time is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship

<span class="mw-page-title-main">Royal Observatory, Greenwich</span> Observatory in London, England

The Royal Observatory, Greenwich is an observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north. It played a major role in the history of astronomy and navigation, and because the Prime Meridian passed through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC). The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and the clipper ship Cutty Sark are collectively designated Royal Museums Greenwich.

<span class="mw-page-title-main">Sidereal time</span> Timekeeping system on Earth relative to the celestial sphere

Sidereal time is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars".

<span class="mw-page-title-main">Solar time</span> Calculation of elapsed time by the apparent position of the sun

Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time, and sidereal time, which is based on the apparent motions of stars other than the Sun.

<span class="mw-page-title-main">International Meridian Conference</span> 1884 conference in Washington, D.C., United States

The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use. The conference was held at the request of U.S. President Chester A. Arthur. The subject to discuss was the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world". It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude.

<span class="mw-page-title-main">Prime meridian (Greenwich)</span> Meridian passing through Greenwich, London

The historic prime meridian or Greenwich meridian is a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. The modern IERS Reference Meridian widely used today is based on the Greenwich meridian, but differs slightly from it. This prime meridian was first established by Sir George Airy in 1851, and by 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their charts and maps. In October of that year, at the behest of US President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., United States, for the International Meridian Conference. This conference selected the meridian passing through Greenwich as the world standard prime meridian due to its popularity. However, France abstained from the vote, and French maps continued to use the Paris meridian for several decades. In the 18th century, London lexicographer Malachy Postlethwayt published his African maps showing the "Meridian of London" intersecting the Equator a few degrees west of the later meridian and Accra, Ghana.

<span class="mw-page-title-main">Time in the United States</span> U.S. time zones

In the United States, time is divided into nine standard time zones covering the states, territories and other US possessions, with most of the country observing daylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation, but no single map of those existed until the agency announced intentions to make one in September 2022. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) ; and the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.

<span class="mw-page-title-main">Meridian (geography)</span> Line between the poles with the same longitude

In geography and geodesy, a meridian is the locus connecting points of equal longitude, which is the angle east or west of a given prime meridian. In other words, it is a line of longitude. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. On a Mercator projection or on a Gall-Peters projection, each meridian is perpendicular to all circles of latitude. A meridian is half of a great circle on Earth's surface. The length of a meridian on a modern ellipsoid model of Earth has been estimated as 20,003.93 km (12,429.87 mi).

Washington Mean Time was the time at the meridian through the center of the old dome atop the main building at the old US Naval Observatory at Washington, D.C. This Washington meridian was defined on 28 September 1850 by the United States Congress. The Old Naval Observatory is now on the grounds of the United States Navy Bureau of Medicine and Surgery, southwest of the corner of E and 23rd Streets in Foggy Bottom. Washington Mean Time was sometimes called Washington Meridian Time. It was never used as the basis of any time zone, although it was the local mean time of the city of Washington before the advent of American time zones on 18 November 1883. It was also used to time astronomical events by users of the American Ephemeris and Nautical Almanac, first published for the year 1855.

<span class="mw-page-title-main">Hong Kong Time</span> Official time zone of Hong Kong

Hong Kong Time is the time in Hong Kong, observed at UTC+08:00 all year round. The Hong Kong Observatory is the official timekeeper of the Hong Kong Time. It is indicated as Asia/Hong_Kong in the IANA time zone database.

In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.

<span class="mw-page-title-main">Longitude by chronometer</span>

Longitude by chronometer is a method, in navigation, of determining longitude using a marine chronometer, which was developed by John Harrison during the first half of the eighteenth century. It is an astronomical method of calculating the longitude at which a position line, drawn from a sight by sextant of any celestial body, crosses the observer's assumed latitude. In order to calculate the position line, the time of the sight must be known so that the celestial position i.e. the Greenwich Hour Angle and Declination, of the observed celestial body is known. All that can be derived from a single sight is a single position line, which can be achieved at any time during daylight when both the sea horizon and the sun are visible. To achieve a fix, more than one celestial body and the sea horizon must be visible. This is usually only possible at dawn and dusk.

<span class="mw-page-title-main">UTC+00:00</span> Identifier for the UTC +0 offset

UTC+00:00 is an identifier for a time offset from UTC of +00:00. This time zone is the basis of UTC and all other time zones are based on it. In ISO 8601, an example of the associated time would be written as 2069-01-01T12:12:34+00:00. It is also known by the following geographical or historical names:

Nautical time is a maritime time standard established in the 1920s to allow ships on high seas to coordinate their local time with other ships, consistent with a long nautical tradition of accurate celestial navigation. Nautical time divides the globe into 24 nautical time zones with hourly clock offsets, spaced at 15 degrees by longitudinal coordinate, with no political deviation.

The Nautical Almanac has been the familiar name for a series of official British almanacs published under various titles since the first issue of The Nautical Almanac and Astronomical Ephemeris, for 1767: this was the first nautical almanac to contain data dedicated to the convenient determination of longitude at sea. It was originally published from the Royal Greenwich Observatory in England. A detailed account of how the publication was produced in its earliest years has been published by the National Maritime Museum.

<span class="mw-page-title-main">Coordinated Universal Time</span> Primary time standard

Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time. It is within about one second of mean solar time at 0° longitude and is not adjusted for daylight saving time. It is effectively a successor to Greenwich Mean Time (GMT).

<span class="mw-page-title-main">Time in the Republic of Ireland</span> Time zone (UTC+1 summer, UTC+0 winter)

Ireland uses Irish Standard Time in the summer months and Greenwich Mean Time in the winter period.

References

  1. "Time scales". UCO Lick. Retrieved 28 July 2018.
  2. "What is Greenwich Mean Time?". Royal Museums Greenwich. 2021. What does GMT stand for?. Retrieved 28 October 2021.
  3. 1 2 "Interpretation Act 1978: Section 9", legislation.gov.uk , The National Archives, 20 July 1978, 1978 c. 30 (s. 9), retrieved 30 October 2021
  4. "What is the Prime Meridian and why is it in Greenwich?". Royal Museums Greeenwich. Retrieved 13 December 2021. The IRM is the only meridian that may now be described as the prime meridian of the world, as it defines 0° longitude by international agreement. The IRM passes 102.5 metres to the east of the historic Prime Meridian of the World at the latitude of the Airy Transit Circle here. The entire Observatory and the historic Prime Meridian now lie to the west of the true prime meridian.
  5. 1 2 McCarthy & Seidelmann 2009, p. 17.
  6. "UTC (Universal Time Coordinated)". Greenwich Mean Time. Retrieved 12 May 2023.
  7. "Astronomical Almanac Online". Her Majesty's Nautical Almanac Office. 2020. "Glossary" s.v. Universal Time. Archived from the original on 23 February 2022. Retrieved 27 February 2021.
  8. "Coordinated Universal Time". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 14 June 2020.
  9. Hilton & McCarthy 2013, pp. 231–232.
  10. Howse 1997, p. 114.
  11. CLERK TO JUSTICES. "Time, Actual And Legal". Times, London, England, 14 May 1880: 10. The Times Digital Archive. Web. 18 August 2015.
  12. Bartky, Ian R. (2007). One Time Fits All: The Campaigns for Global Uniformity. Stanford University Press. p. 134. ISBN   978-0804756426 . Retrieved 18 August 2015.
  13. 1 2 Myers 2007.
  14. "Greenwich Time by Wireless— New Scheme Today". Liverpool Daily Post. 5 February 1924. p. 6. the last four seconds of the preceding minute will be heard as 'clicks' when the signal is about to be given, representing the 55th, 56th, 57th, 58th and 59th second, and the final click, which will be a little louder than the others
  15. Street, Sean (2015). "Greenwich Time Signal". Historical Dictionary of British Radio. Scarecrow Press. p. 156.
  16. "UT1 as explained on IERS page". Archived from the original on 14 May 2013. Retrieved 9 July 2009.
  17. Astronomical Supplement to the Astronomical Almanac. University Science Books. 1992. p. 76. ISBN   0-935702-68-7.
  18. Howse 1997, p. 157.
  19. 1 2 Dumortier, Hannelore, & Loncke (n.d.)
  20. Seago, Seidelmann & Allen 2011.
  21. 1 2 "STANDARD TIME ACT, 1968; Section 1". Government of Ireland.
  22. 1 2 "STANDARD TIME (AMENDMENT) ACT, 1971; Section 1". Government of Ireland.
  23. Office, Office of the Houses of the Oireachtas House Services Directorate Bills. "AN tACHT UM AM CAIGHDEÁNACH (LEASÚ), 1971". www.acts.ie.

Sources