Greenwich Mean Time

Time in Europe:
light blue Western European Time / Greenwich Mean Time (UTC)
blue Western European Time / Greenwich Mean Time (UTC)
Western European Summer Time (UTC+1)
red Central European Time (UTC+1)
Central European Summer Time (UTC+2)
yellow Eastern European Time / Kaliningrad Time (UTC+2)
golden Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
light green Further-eastern European Time / Moscow Time / Turkey Time (UTC+3)
Light colours indicate where standard time is observed all year; dark colours indicate where a summer time is observed.
Time in Africa
    UTC−01:00 Cape Verde Time
    UTC±00:00 Greenwich Mean Time
    UTC±00:00
UTC+01:00		 Greenwich Mean Time
Greenwich Mean Time+1
    UTC+01:00 West Africa Time /
Central European Time
    UTC+01:00
UTC+02:00		 West Africa Time
West Africa Summer Time
    UTC+02:00 Central Africa Time /
South African Standard Time /
Eastern European Time
    UTC+03:00 East Africa Time
    UTC+04:00 Mauritius Time / Seychelles Time
Light colors indicate where standard time is observed all year; dark colors indicate where daylight savings is observed.

Note: The islands of Cape Verde are to the west of the African mainland.

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. GMT was formerly used as the international civil time standard, now superseded in that function by Coordinated Universal Time (UTC). Today GMT is considered equivalent to UTC for UK civil purposes (but this is not formalised) and for navigation is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); these two meanings can differ by up to 0.9 s. Consequently, the term GMT should not be used for precise purposes.[1]

Due to Earth's uneven speed in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the sun crosses the Greenwich meridian and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy calculated by the equation of time. Noon GMT is the annual average (i.e. "mean") moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".

Originally, astronomers considered a GMT day to start at noon 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.[2] Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date. Today Universal Time usually refers to UTC or UT1.[3]

The term "GMT" is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN. It is a term commonly used in the United Kingdom and countries of the Commonwealth, including Australia, New Zealand, South Africa, India, Pakistan and Malaysia; and in many other countries of the eastern hemisphere. In some countries (Britain for example) Greenwich Mean Time is the legal time in the winter and the population uses the term. See GMT in legislation, below, for further explanation.

History

Greenwich clock with standard measurements

As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees, internationally 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 a half-hour) "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.[4] 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 PM.'[5][6] This was changed later in 1880, when Greenwich Mean Time was legally adopted throughout the island of Great Britain. GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted GMT in 1916, supplanting Dublin Mean Time.[7] Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory redundant in the process.

The daily rotation of the Earth is irregular (see ΔT) and constantly slows; therefore the atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was superseded as the international civil time standard by Coordinated Universal Time, 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 IAU at Dublin, 1955, at the initiative of William Markowitz) this "raw" form of UT was re-labeled UT0 and effectively superseded by refined forms UT1 (UT0 equalised for the effects of polar wandering)[8] 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 Romans. 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 earlier. The instant that was designated '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.[9] 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 Britain is called still "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. Under subsection 23(3), the same rule applies to deeds and other instruments.[7]

During the experiment of 1968-1971, 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 Britain, UTC+0 is disseminated to the general public in winter and UTC+1 in summer.[10][2]

BBC radio stations broadcast the "six pips" of the Greenwich Time Signal. It is named from its original generation at the Royal Greenwich Observatory, and is aligned to either Greenwich Mean Time or British Summer Time as appropriate for the time of year.

A coded radio signal is broadcast in the UK by Time from NPL.

Other countries

Several countries define their local time by reference to Greenwich Mean Time.[11][12] Some examples are:

Time zone

Those countries marked in dark blue on the map above use BST/Western European Summer Time and advance their clock one hour in summer. In the United Kingdom, this is British Summer Time (BST); in the Republic of Ireland it is called Irish Standard Time (IST)[13]—officially changing to GMT in winter. Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.

Colour Legal time vs local mean time
1 h ± 30 m behind
0 h ± 30 m
1 h ± 30 m ahead
2 h ± 30 m ahead
3 h ± 30 m ahead
European winter
European summer

Since legal, political, social and economic criteria in addition to physical or geographical criteria are used in the drawing of time zones, actual time zones do not precisely adhere to meridian lines. The 'GMT' time zone, were it drawn by purely geographical terms, would consist of the area between meridians 7°30'W and 7°30'E. As a result, there are European locales that despite lying in an area with a 'physical' UTC time use another time zone (UTC+1 in particular); conversely, there are European areas that use UTC, even though their 'physical' time zone is UTC−1 (e.g., most of Portugal), or UTC−2 (the westernmost part of Iceland). Because the UTC time zone in Europe is 'shifted' to the west, Lowestoft in the United Kingdom at only 1°45'E is the easternmost settlement in Europe in which UTC is applied. Following is a list of the 'incongruencies':

Countries (or parts thereof) west of 22°30'W ("physical" UTC−2) that use UTC
Countries (or parts thereof) west of 7°30'W ("physical" UTC−1) that use UTC
This arch that stretches over a highway indicates the prime meridian in Spain.
Countries (mostly) between meridians 7°30'W and 7°30'E ("physical" UTC) that use UTC+1

See also

Notes

  1. ↑ Hilton and McCarthy 2013, p. 231–2)
  2. 1 2 McCarthy & Seidelmann 2009, p. 17.
  3. ↑ Astronomical Almanac Online 2015, Glossary s.v. "Universal Time"
  4. ↑ Howse 1997, p. 114
  5. ↑ CLERK TO JUSTICES. "Time, Actual And Legal". Times, London, England, 14 May 1880: 10. The Times Digital Archive. Web. 18 Aug. 2015.
  6. ↑ Bartky, Ian R. (2007). One Time Fits All: The Campaigns for Global Uniformity. Stanford University Press. p. 134. ISBN 0804756422. Retrieved 18 August 2015.
  7. 1 2 Myers (2007).
  8. ↑ UT1 as explained on IERS page
  9. ↑ Astronomical Supplement to the Astronomical Almanac. University Science Books. 1992. p. 76. ISBN 0-935702-68-7.
  10. ↑ Howse 1997, p. 157.
  11. 1 2 Dumortier, Hannelore, & Loncke (n.d.)
  12. ↑ Seago & Seidelmann (c. 2001)
  13. ↑ Standard Time Act, 1968.
  14. ↑ "BOE Orden sobre adelanto de la hora legal en 60 minutos". Retrieved 2 December 2008.
  15. ↑ "B.O.E. #68 03/08/1940 p.1675". Retrieved 2 December 2008.
  16. 1 2 "B.O.E. #68 03/08/1940 p.1676". Retrieved 2 December 2008.
  17. ↑ "Hábitos y horarios españoles". Retrieved 27 November 2008.

References

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