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Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is now sometimes used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, although strictly UTC is an atomic time scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is the astronomical concept that directly replaced the original GMT. In the UK, GMT is the official time only during winter; during summer British Summer Time is used.

GMT is substantially equivalent to Western European Time.

Noon Greenwich Mean Time is not necessarily the moment when the sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

Historically the term GMT has been used with two different conventions for numbering hours. The old astronomical convention (before 1925) was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours. The latter is modern astronomical and civil convention. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

History

As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees (this convention was internationally adopted in the International Meridian Conference of 1884). Note that the synchronization of the chronometer on GMT did not affect shipboard time itself, 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, eventually led to GMT being used worldwide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-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. This changed in 1880, when GMT 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 Greenwich Mean Time in 1916, supplanting Dublin Mean Time. Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory obsolete in the process.

The daily rotation of the Earth is somewhat irregular (see ΔT) and is slowing down slightly; atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world. UT1, introduced in 1928, represents earth rotation time. Leap seconds are added to or subtracted from UTC to keep it within 0.9 seconds of UT1.

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."

Greenwich Mean Time in legislation

Several countries throughout the world legislatively define their local time by explicit reference to Greenwich Mean Time. Some examples are:

• United Kingdom: 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.

• Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.

• Republic of Ireland: Standard Time (Amendment) Act, 1971, section 1, and Interpretation Act 2005, section 18(i).

• Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35.

Time zone

Although civil time in the United Kingdom, e.g., the Greenwich Time Signal, is in practice now based on UTC, the winter time scale, which is equal to UTC, is still popularly called GMT. Civil time in the UK is legally (but not practically) still based on astronomical GMT, not UTC. Those countries marked in dark blue on the map above use Western European Summer Time and advance their clock one hour in summer. In the United Kingdom, this is known as British Summer Time (BST); in the Republic of Ireland it is called Irish Standard Time (IST) - officially changing to GMT in winter. Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.

Anomalies

Since political, in addition to purely geographical, criteria are used in the drawing of time zones, it follows that actual time zones do not precisely adhere to meridian lines. The GMT time zone, were it drawn by purely geographical terms, would consist of exactly 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, actually use another time zone (UTC+1 in particular); contrariwise, there are European areas that use UTC, even though their 'physical' time zone is UTC-1 (e.g., most of Portugal), or even UTC−2 (the westernmost part of Iceland). Actually, because the UTC time zone in Europe is 'shifted' to the west, Lowestoft in Suffolk, East Anglia, England 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

• The westernmost part of Iceland, incl. the northwest peninsula and its main town of Ísafjörður, which is west of 22°30'W, uses UTC. Bjargtangar, Iceland is the westernmost point in which UTC is applied.

Countries (or parts thereof) west of 7°30'W ('physical' UTC-1) that use UTC

• Canary Islands (Spain)
• Most of Portugal, incl. Lisbon, Porto, Braga, Aveiro, and Coimbra. (Only the easternmost part, incl. cities such as Bragança and Guarda, lies east of 7°30'W.). Portugal has since the Treaty of Windsor (1386, world's oldest diplomatic alliance) always had close ties to Britain, which explains its choice of UTC. The Madeira Islands, even further to the west, also employ UTC.
• Western part of the Republic of Ireland, incl. the cities of Cork, Limerick, and Galway
• Westernmost tip of Northern Ireland, incl. the capital of County Fermanagh, Enniskillen
• Extreme westerly portion of the Outer Hebrides, west of Scotland; for instance, Vatersay, an inhabited island in the Outer Hebrides and the westernmost settlement in the whole of Great Britain, lies at 7°54'W. If uninhabited islands and/or rocks are to be taken into account then St Kilda, west of the Outer Hebrides, at 8°58'W, and Rockall, at 13°41'W, should also be included.
• Westernmost island of the Faroe Islands (autonomous region of the Danish Kingdom), Mykines
• Iceland, including Reykjavík

Countries (mostly) between meridians 7°30'W and 7°30'E ('physical' UTC) that use UTC+1

• Spain (except for the Canary Islands which use UTC). Parts of Galicia in fact lie west of 7°30'W ('physical' UTC-1), whereas there is no Spanish territory east of 7°30'E ('physical' UTC+1). Spain's time is the direct result of Franco's Presidential Order (published in Boletín Oficial del Estado of March 8th, 1940) abandoning Greenwich UTC time in favor of UTC+1 effective 23:00 March 16th 1940. This is indeed an excellent example of the aforementioned political criteria used in the drawing of time zones: the time change was passed in consideration of the "convenience from the national time marching in step according to that of other European countries". The Presidential Order, most likely enacted to be in synchrony with allies nazi Germany and fascist Italy, included in its 5th article a provision for its future phase out which never took place. Due to this political decision Spain is two hours ahead of its local mean time during the summer (one hour ahead in winter), which probably explains the notoriously late schedule for which the country is known.
• Belgium
• Netherlands
• Most of France, incl. the cities of Paris, Marseilles and Lyon. Only small parts of Alsace, Lorraine and Provence are east of 7°30'E ('physical' UTC+1).

See also

• WWV (radio station) — United States' time signal service
• CHU (radio station) — Canada's time signal service
• BPM (time service) — China's national time signal service
• DCF77 — Germany's time signal service
• VNG — Australia's national time signal service
• Marine chronometer — synchronised with GMT, and used by ships to calculate their longitude
• 24-hour watch — 24-hour wristwatch
• Swatch Internet Time — alternative, decimal measure of time
• Sandringham Time
• Rolex GMT Master II

Notes

1. ^ Myers, Joseph. (2007). History of legal time in Britain. Retrieved 4 January 2008.
2. Howse 1997, p. 178
3. ^ Jos Dumortier, Hannelore Dekeyser, Mieke Loncke. "Legal Aspects of Trusted Time Services in Europe" (PDF). Amano Legal Report. K.U. Leuven Interdisciplinary Centre for Law & ICT (ICRI). Retrieved 2008-08-07.{{cite web}}: CS1 maint: multiple names: authors list (link)
4. John H. Seago, P. Kenneth Seidelmann. "National Legal Requirements for Coordinating with Universal Time" (PDF). Retrieved 2008-08-07.
5. "Interpretation Act". Retrieved 2008-08-07.
6. "Standard Time (Amendment) Act, 1971 (Ireland)". Retrieved 2008-08-07.
7. "Interpretation Act (Ireland)". Retrieved 2008-08-07.
8. "Interpretation Act (Canada)". Retrieved 2008-08-07.
9. "Six pip salute". BBC Online Network. 1999-02-05. Retrieved 2008-08-07. 'It's a little known but interesting fact that the Greenwich Time Signal no longer gives Greenwich Mean Time,' said the NPL's John Chambers. 'Since 1972, all the time signals in the world have been based on atomic time.' {{cite web}}: Check date values in: |date= (help)
10. "B.O.E. Orden sobre adelanto de la hora legal en 60 minutos". Retrieved 2008-12-02.
11. "B.O.E. #68 03/08/1940 p.1675". Retrieved 2008-12-02.
12. "B.O.E. #68 03/08/1940 p.1675". Retrieved 2008-12-02.
13. "Hábitos y horarios españoles". Retrieved 2008-11-27.

References

• Howse, Derek. (1997). Greenwich time and the longitude. London: Philip Wilson.
• Myers, Joseph. (2007). History of legal time in Britain. Retrieved 4 January 2008.

External links

• Greenwich Mean Time
• World Time Zone Map with current time
• The World Clock - Local times around the world by city
• NIST — World Time Scales
• International Earth Rotation and Reference Systems Service
• World Clock — Current Local Times
• Royal Observatory Greenwich
• The original BBC World Service GMT time signal in mp3 format

v t e Time measurement and standards
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Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian
Time in physics	Absolute space and time Spacetime Chronon Continuous signal Coordinate time Cosmological decade Discrete time and continuous time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain Time-translation symmetry T-symmetry
Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch stopwatch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema
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