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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 arguably the same as Coordinated Universal Time (UTC) and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others.
Before the introduction of UTC on 1 January 1972 Greenwich Mean Time (also known as Zulu time) was the same as Universal Time (UT) which is a standard astronomical concept used in many technical fields. Astronomers no longer use the term "Greenwich Mean Time".
In the UK, GMT is the official time only during winter; during summer British Summer Time is used. GMT is the same as Western European Time.
Noon Greenwich Mean Time is not necessarily the moment when the noon 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 (a discrepancy 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 1 January 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 convention is modern practice (on and after 1 January 1925) for astronomical as well as civil purposes. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.
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. 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; then 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 equalized for the effects of polar wandering) and UT2 (UT1 further equalized for annual seasonal variations in earth rotation rate). Leap seconds are nowadays 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."
The origin of the discrepancy dates back to when surveys were being carried out for the Global Positioning System. There is more than one transit instrument at Greenwich, and unfortunately the surveyors picked the wrong one as their baseline.
;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
;Countries (mostly) between meridians 7°30'W and 7°30'E ('physical' UTC) that use UTC+1 in Spain]] 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 8 March 1940) abandoning Greenwich UTC time in favour of UTC+1 effective 23:00 16 March 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 However, in Portugal, which is a mere one hour behind Spain, the timetable is quite different.
Category:Time Category:Time scales Category:Time zones Category:Time in the United Kingdom Category:Geography of Greenwich
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