Greenwich Mean Time

Time in Africa
    UTC−01:00 Cape Verde Time
    UTC±00:00 Greenwich Mean Time
    UTC±00:00
UTC+01:00
Greenwich Mean Time
Moroccan Summer Time
    UTC+01:00 West Africa Time /
Central European 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 GMT was formerly used as the international civil time standard, now superseded in that function by Coordinated Universal Time (UTC).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.
Other Languages
беларуская (тарашкевіца)‎: GMT
bosanski: GMT
català: GMT
eesti: Maailmaaeg
Esperanto: GMT
føroyskt: GMT
hrvatski: GMT
Bahasa Indonesia: Waktu Greenwich
Kurdî: GMT
latviešu: Griničas laiks
lietuvių: Grinvičo laikas
Bahasa Melayu: Waktu Min Greenwich
norsk nynorsk: Greenwich middeltid
occitan: Efemerida
oʻzbekcha/ўзбекча: GMT
Gagana Samoa: Kerenise Uiga Taimi
Simple English: Greenwich Mean Time
удмурт: Гринвич дыр
Tiếng Việt: GMT