Universal Time

Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, London, UK. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see below).[1] All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.

Universal Time and standard time

Prior to the introduction of standard time, each municipality throughout the civilized world set its official clock, if it had one, according to the local position of the Sun (see solar time). This served adequately until the introduction of rail travel in Britain, which made it possible to travel fast enough over long distances to require continuous re-setting of timepieces as a train progressed in its daily run through several towns. Greenwich Mean Time, where all clocks in Britain were set to the same time, was established to solve this problem. Chronometers or telegraphy were used to synchronize these clocks.[2]

Standard time zones of the world since 2016. The number at the bottom of each timezone specifies the number of hours to add to UTC to convert it to the local time.

Standard time, as originally proposed by Scottish-Canadian Sir Sandford Fleming in 1879, divided the world into twenty-four time zones, each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Greenwich Observatory in Greenwich, England, was adopted as standard on 22 October 1884 at the end of the International Meridian Conference,[3][4] leading to the widespread use of Greenwich Mean Time to set local clocks. This location was chosen because by 1884 two-thirds of all nautical charts and maps already used it as their prime meridian.[5] The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, which was to choose a basis for universal time (as well as a prime meridian).

During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.[6]

In 1935, the term Universal Time was recommended by the International Astronomical Union as a more precise term than Greenwich Mean Time, because GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight.[7] The term Greenwich Mean Time persists, however, in common usage to this day in reference to civil timekeeping.

Other Languages
العربية: توقيت عالمي
čeština: Světový čas
Ελληνικά: Παγκόσμια ώρα
français: Temps universel
한국어: 세계시
Bahasa Indonesia: Waktu Universal
Bahasa Melayu: Waktu Sejagat
日本語: 世界時
slovenčina: Svetový čas
slovenščina: Univerzalni čas
українська: Всесвітній час
中文: 世界时