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 (known as the
analemma); 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 broadcasted on February 5, 1924.
See also sidereal time, solar time