Greenwich Mean Time

Last updated

Greenwich Mean Time
Time zone
UTC offset
GMT UTC±00:00
Current time
12:33, 29 September 2022 GMT [refresh]
Observance of DST
DST is observed throughout this time zone.
Time in Europe:
Western European Time / Greenwich Mean Time (UTC)
Western European Time / Greenwich Mean Time (UTC)
Western European Summer Time / British Summer Time / Irish Standard Time (UTC+1)
Central European Time (UTC+1)
Central European Summer Time (UTC+2)
Eastern European Time / Kaliningrad Time (UTC+2)
Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
Moscow Time / Turkey Time (UTC+3)
Armenia Time / Azerbaijan Time / Georgia Time (UTC+4)
#### pale colours: standard time observed all year;
### dark colours: summer time observed Time Zones of Europe.svg
Time in Europe :
Western European Time / Greenwich Mean Time (UTC)
Western European Time / Greenwich Mean Time (UTC)
Western European Summer Time / British Summer Time / Irish Standard Time (UTC+1)
Central European Time (UTC+1)
Central European Summer Time (UTC+2)
Eastern European Time / Kaliningrad Time (UTC+2)
Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
Moscow Time / Turkey Time (UTC+3)
Armenia Time / Azerbaijan Time / Georgia Time (UTC+4)
 pale colours: standard time observed all year;
 dark colours: summer time observed
Time zones of Africa:

-01:00
Cape Verde Time
+-00:00
Greenwich Mean Time
+01:00
Central European Time
West Africa Time
+02:00
Central Africa Time
Eastern European Time
Egypt Standard Time
South African Standard Time
+03:00
East Africa Time
+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:
  -01:00   Cape Verde Time
  ±00:00   Greenwich Mean Time
  +01:00  
  +02:00  
  +03:00   East Africa Time
  +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 mean solar 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

English speakers often use GMT as a synonym for Coordinated Universal Time (UTC): [4] in modern usage, this is incorrect GMT is now a time zone, not a time standard. [5] [6] 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. [7]

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 to denote GMT as counted from midnight. [9] Today, Universal Time usually refers to UTC or UT1. [6]

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

Greenwich clock with standard measurements Greenwich clock.jpg
Greenwich clock with standard measurements

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 on 5 February 1924, rendering the time ball at the observatory redundant.

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) [14] 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. [15] 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), according to the Interpretation Act 1978, 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 an Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time. [3] Under subsection 23(3), 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. [9] [16]

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. [17] [18] 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. [8]
  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

<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.

The Julian day is the 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.

<span class="mw-page-title-main">Time standard</span> Specification for measuring 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 Greenwich, London, UK

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 passes 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 timekeeping system that astronomers use to locate celestial objects. Using sidereal time, it is possible to easily point a telescope to the proper coordinates in the night sky. In short, sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars", or more correctly, relative to the March equinox.

<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">Time in the United States</span> U.S. time zones

Time in the United States, by law, is divided into nine standard time zones covering the states, territories and other US possessions, with most of the United States 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).

<span class="mw-page-title-main">Standard time</span> Synchronization of clocks within a geographical region

Standard time is the synchronisation of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the region, often near the centre of the region. Historically, standard time was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical regions became time zones. The standard time in each time zone has come to be defined as an offset from Universal Time. A further offset is applied for part of the year in regions with daylight saving time.

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.

Time in New Zealand is divided by law into two standard time zones. The main islands use New Zealand Standard Time (NZST), 12 hours in advance of Coordinated Universal Time (UTC) / military M (Mike), while the outlying Chatham Islands use Chatham Standard Time (CHAST), 12 hours 45 minutes in advance of UTC / military M^ (Mike-Three).

<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 scales designated by civilian authorities or to local time indicated by clocks. Modern civil time is generally 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">South African Standard Time</span> Time zone

South African Standard Time (SAST) is the time zone used by all of South Africa as well as Eswatini and Lesotho. The zone is two hours ahead of UTC (UTC+02:00) and is the same as Central Africa Time. Daylight saving time is not observed in either time zone. Solar noon in this time zone occurs at 30° E in SAST, effectively making Pietermaritzburg at the correct solar noon point, with Johannesburg and Pretoria slightly west at 28° E and Durban slightly east at 31° E. Thus, most of South Africa's population experience true solar noon at approximately 12:00 daily.

<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.

Malaysian Standard Time or Malaysian Time (MYT) is the standard time used in Malaysia. It is 8 hours ahead of Coordinated Universal Time (UTC). The local mean time in Kuala Lumpur was originally GMT+06:46:46. Peninsular Malaysia used this local mean time until 1 January 1901, when they changed to Singapore mean time GMT+06:55:25. Between the end of the Second World War and the formation of Malaysia on 16 September 1963, it was known as British Malayan Standard Time, which was GMT+07:30. At 2330 hrs local time of 31 December 1981, people in Peninsular Malaysia adjusted their clocks and watches ahead by 30 minutes to become 00:00 hours local time of 1 January 1982, to match the time in use in East Malaysia, which is UTC+08:00. SGT (Singapore) followed on and uses the same until now.

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 1 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).

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. "Coordinated Universal Time". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 14 June 2020.
  5. Buckle, Anne. "The Difference Between GMT and UTC". timeanddate.com. Retrieved 27 February 2021.
  6. 1 2 "Astronomical Almanac Online". Her Majesty's Nautical Almanac Office. 2020. "Glossary" s.v. Universal Time.
  7. Hilton & McCarthy 2013, pp. 231–232.
  8. "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.
  9. 1 2 McCarthy & Seidelmann 2009, p. 17.
  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. UT1 as explained on IERS page
  15. Astronomical Supplement to the Astronomical Almanac. University Science Books. 1992. p. 76. ISBN   0-935702-68-7.
  16. Howse 1997, p. 157.
  17. 1 2 Dumortier, Hannelore, & Loncke (n.d.)
  18. Seago, Seidelmann & Allen 2011.
  19. 1 2 "STANDARD TIME ACT, 1968; Section 1". Government of Ireland.
  20. 1 2 "STANDARD TIME (AMENDMENT) ACT, 1971; Section 1". Government of Ireland.

Sources