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universal time (UT), the international time standard common to every place in the world, it nominally reflects the mean solar time along the earth's prime meridian (renumbered to equate to civil time). In 1884, under international agreement, the prime meridian was established as running through the Royal Observatory in Greenwich, England, setting the standard of Greenwich mean time (GMT). In keeping with tradition, the start of a solar day occurred at noon. In 1925 the numbering system for GMT was changed so that the day began at midnight to make it consistent with the civil day. Some confusion in terminology resulted, however, and in 1928 the International Astronomical Union (IAU) changed the designation of the standard time of the prime meridian to universal time. In 1955 the IAU defined several kinds of UT. The initial values of universal time obtained at 75 observatories, denoted UT0, differ slightly because of polar motion. By adding a correction each observatory converts UT0 into UT1, which gives the Earth's rotational position in space. An empirical correction to take account of annual changes in the speed of rotation is then added to convert UT1 to UT2. However, UT2 has since been superseded by atomic time (time as given by atomic clocks). Universal time is also called world time, Z time, and Zulu time.
In 1964 a new timescale, called coordinated universal time (UTC), was internationally adopted. UTC is more uniform and more accurate than the UT2 system because the UTC second is based on atomic time (although the UTC year is still based on the time it takes the earth to complete one orbit). Because the rate of the earth's rotation is gradually slowing, it is occasionally necessary to add an extra second, called the leap second, to the length of the UTC year; synchronization is obtained by making the last minute of June or December contain 61 seconds. About one leap second per year has been inserted since 1972.
universal time(UT) The precise measure of time that forms the basis for all civil timekeeping. It conforms closely to the mean diurnal motion of the Sun.
UT is defined by a mathematical formula relating UT to sidereal time. It is thus determined from observations of the diurnal motions of stars. It is not a uniform timescale, however, due to variations in the Earth's rotation. The timescale determined directly from stellar observations is dependent on the place of observation, and is designated UT0. The timescale independent of location is obtained by correcting UT0 for the variation in the observer's meridian that arises from the irregular varying motion of the Earth's geographical poles; it is designated UT1. This is the timescale used by astronomers. It is counted from 0 hours at midnight, and the unit is the mean solar day.
Coordinated universal time (UTC) is based on International Atomic Time (TAI). Since Jan. 1 1972 the time given by broadcast time signals has been UTC. UTC differs from TAI by an integral number of seconds. UTC is kept within 0.90 seconds of UT1 by the insertion (or deletion) of exactly one second when necessary, usually at the end of December or June; these step adjustments are termed leap seconds. An approximation (DUT1) to the value of UT1 minus UTC is transmitted in code on broadcast time signals. TAI and UTC are recommended for the precise dating of observations. See also dynamical time.
universal time[¦yü·nə¦vər·səl ′tīm]
universal time (UT)
Universal Time(time, standard)
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.