universal time

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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

universal time

[¦yü·nə¦vər·səl ′tīm]

(astronomy)

Greenwich mean time

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

universal time (UT)

A measure of time that conforms, within a close approximation, to the mean diurnal rotation of the earth and serves as the basis of civil timekeeping. Universal Time is based on mean sidereal time as measured in Greenwich. It is approximately equal to the mean solar time from Greenwich. Like most astronomical calculations, eclipse predictions are usually presented in terms of Universal Time. To convert UT into local time, one has to know what zone he or she is in and apply a zonal correction to the UT. The UTC, or Coordinated Universal Time, is synchronized and adjusted to stay within 0.9 s of UT.

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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.

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UTC

(Coordinated Universal Time, Temps Universel Coordonné) The international time standard (formerly Greenwich Mean Time, or GMT). Zero hours UTC is midnight in Greenwich, England, which is located at 0 degrees longitude. Everything east of Greenwich (up to 180 degrees) is later in time; everything west is earlier. There are 42 time authorities around the world that are constantly synchronizing with each other. In the U.S., the time authorities are located at the U.S. Naval Observatory (USNO) and the National Institute of Standards & Technology (NIST). See NTP.

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