Home > Greenwich Mean Time
:For alternate meanings of "GMT", see GMT (disambiguation).Greenwich Mean Time (GMT) is the mean solar time at the Royal Greenwich Observatory in Greenwich near London in England, which by convention is at 0 degrees geographic longitude. Theoretically, noon Greenwich Mean Time is the moment when the Sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich). Because of the Earth's uneven speed in its elliptic orbit, this event may be up to 16 minutes off apparent solar time (this discrepancy is known as the equation of time); but this is averaged out over the year through the use of the mean sun.
Time zones of Europe, Blue UTC WET GMT, Red CET/MET, Green EETEastern European Time (EET) is the time zone 2 hours ahead of UTC. WET or GMT or UTC , Red CET/MET, Green EET, Khaki MSK During summertime, DST ( Daylight Saving Time) is in effect in some countries such as Finland and all other member states of the Europ, Khaki MSKMoscow Time (MSK) is the time zone 3 hours ahead of UTC. WET or GMT or UTC , Red CET/MET, Green EET, Khaki MSK Moscow Summer Time (MSD) is UTC+4. Time zones.
With the growth of Britain as a maritime nation, mariners kept their timepieces on GMT in order to calculate their longtitude "from the Greenwich meridian" although not affecting ship-board time itself which was still solar time. This, combined with mariners from other nations drawing from Nevil MaskelyneNevil Maskelyne ( October 6, 1732 February 9, 1811) was the fifth British Astronomer Royal. He held the office from 1765 to 1811. The solar eclipse of 1748 made a deep impression upon him; and having graduated as seventh wrangler from Trinity College, Cam's method of lunar distanceIn celestial navigation, lunar distance is in the angle of the Moon's centre from the Sun or from the bright stars. It is measured using a sextant. Using the lunar distance and a nautical almanac it is possible to calculate the current time and thus the cs based on observations at Greenwich, eventually led to GMT being used world-wide as a reference time independent of location. Although not affecting the local time directly, most time zoneTime Zone was also an old historical computer game. Time zones are areas of the Earth that have adopted the same standard time. Formerly, people used local solar time (originally apparent and then mean), resulting in time differing slightly from town to ts are based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".
The daily rotation of the Earth is somewhat irregular (see Delta-TDelta-T ephemeris time Universal Time Universal Time is a time scale based on the rotation of the Earth. Because the rotation of the Earth is somewhat irregular, time can not be kept this way with an accuracy better than 1 : 108. Moreover, the Earth slows) and is slowing down. Therefore, GMT is not used as official clock time anymore. Nowadays, the official clock time is measured by atomic clockAn atomic clock is a type of clock that uses an atomic frequency standard as its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on more advanced physics involving caesium beas and is known as Coordinated Universal Time (UTC). By using leap seconds, UTC is kept within 0.9 seconds from GMT.
Hourly time signals from Greenwich Observatory were first broadcast on February 5, 1924.
Civil time, notably the Greenwich Time Signal, in the United Kingdom has now moved to a UTC-based system, though it is still popularly called GMT it is also called Western European Time (WET).
Those countries marked in Dark Blue use European Summer Time and advance their clock one hour in the summer. In the UK this is known as British Summer Time (BST). Those countries marked in light blue keep their clocks on UTC (GMT/WET) all year round.
