- 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.
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." This did not affect ship-board time itself, which was still solar time. This, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances 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 zones 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 ΔT) and is slowing down. Therefore, GMT is not used as an 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. It is also called Western European Time (WET).
Those countries marked in dark blue on the map at right 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.