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The mean time for the meridian at Greenwich, England (0° longitude), which runs through the former site of the Royal Observatory. It is based on the sidereal period of Earth's rotation and is used as a basis for calculating standard clock time throughout most of the world. Also called Greenwich Mean Time. Compare coordinated universal time.
The measure of time obtained from the rotation of the Earth, also known as Greenwich mean time, after the Greenwich Observatory in England. The world's time standard today is Coordinated Universal Time, which is kept by atomic clocks. The two universal times are kept in synchronization by the occasional insertion of leap seconds into the year.
(UT) The mean solar time along the prime meridian (0 longitude) that runs through the Greenwich Observatory outside of London, UK, where the current system originated. UT is tied to the rotation of the Earth in respect to the fictitious "mean Sun".
Greenwich Mean Time (GMT) was measured from Greenwich mean midday until 1925 when the reference point was changed from noon to midnight and the name changed to "Universal Time".
There are three separate definitions, UT0, UT1, and UT2, depending on which corrections have been applied to the Earth's motion. Coordinated Universal Time is kept within 0.9 seconds of UT1, by addition of leap seconds to International Atomic Time.