Universal Time

time, standard (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.

(2001-08-02)