Greenwich Mean Time

Greenwich Mean Time (GMT) is the <a href="/facts/Local_mean_time/zxvl3ssj">local mean time</a> at the <a href="/facts/Royal_Observatory%2c_Greenwich/gDxv9o65">Royal Observatory</a> in <a href="/facts/Greenwich/ognrRWm2">Greenwich</a>, London, counted from <a href="/facts/Midnight/hSw4Jbn3">midnight</a>. At different times in the past, it has been calculated in different ways, including being calculated from <a href="/facts/Noon/bEe082CA">noon</a>; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as <a href="/facts/Western_European_Time/btYMK6hV">one of the names</a> for the <a href="/facts/Time_zone/x5JzdlX8">time zone</a> <a href="/facts/UTC%252B00%3a00/HvCbyH6w">UTC+00:00</a> and, in UK law, is the basis for <a href="/facts/Civil_time/DehyR1AL">civil time</a> in the United Kingdom.
Because of Earth's uneven <a href="/facts/Angular_velocity/2n8JljAT">angular velocity</a> in its <a href="/facts/Elliptic_orbit/SfuXuATN">elliptical orbit</a> and its <a href="/facts/Axial_tilt/kt7KwL3v">axial tilt</a>, noon (12:00:00) GMT is rarely the exact moment the Sun crosses the <a href="/facts/Prime_meridian_(Greenwich)/5dhcewNL">Greenwich Meridian</a> 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 <a href="/facts/Equation_of_time/9vxzoWDz">equation of time</a>. Noon GMT is the annual average (the <a href="/facts/Arithmetic_mean/L3Pv333o">arithmetic mean</a>) 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 <a href="/facts/Universal_Time/l1FIFGKn">Universal Time</a> was introduced in 1928 to denote GMT as counted from midnight. Today, Universal Time usually refers to <a href="/facts/Coordinated_Universal_Time/2EnjfSyu">Coordinated Universal Time</a> (UTC) or else to <a href="/facts/Universal_Time/l1FIFGKn">UT1</a>; English speakers often use GMT as a synonym for UTC. 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.
The term "GMT" is especially used by institutional bodies within the <a href="/facts/United_Kingdom/kuPAILt2">United Kingdom</a>, such as the <a href="/facts/BBC_World_Service/XdhGb57h">BBC World Service</a>, the <a href="/facts/Royal_Navy/Xdy8YCH7">Royal Navy</a>, and the <a href="/facts/Met_Office/V0reszST">Met Office</a>; and others particularly in Arab countries, such as the <a href="/facts/Middle_East_Broadcasting_Centre/rUD70uPK">Middle East Broadcasting Centre</a> and Dubai-based <a href="/facts/OSN/kPUlnNq3">OSN</a>.

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Greenwich Mean Time