Greenwich Mean Time: Meaning (information, definition, explanation, facts)

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, 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.

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 eventually led to GMT being used world-wide as a reference time independent of location. Although not affecting the local time directly most time zones are based upon this reference, being whole numbers of hours "ahead of GMT" or "behind GMT", there being effectively 24 'hours' in existence around the world at any given moment.

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, Eastern European Time