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Greenwich Mean Time (GMT) is the mean solar
time at the Royal Greenwich Observatory
in Greenwich near London, 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.
The daily rotation of the Earth is somewhat irregular (see Delta-T) and is slowing
down. Therefore, GMT is not used as official clock time anymore. Nowadays, the official clock time is measured by atomic clocks 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.
See also sidereal time, solar time, BPM, CHU, VNG, WWV
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