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Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of the Greenwich
Mean Time (GMT), i.e. the mean solar time on the meridian of Greenwich,
England, which is the conventional 0-meridian for geographic longitude. GMT is sometimes used, incorrectly, as a synonym for UTC.
Universal Time and standard time
Prior to the introduction of standard time, every municipality set their clock, if they had one, by the local
position of the sun. This served well until the introduction of the train, when it became
possible to travel fast enough to require almost constant re-setting of clocks. After
missing a train for just this reason in 1878, Canadian railway engineer Sir Sandford Fleming
invented standard time to fix the problem.
Standard time divides the world into 24 "time zones", each one covering, in
theory at least, 15 degrees. All clocks within each of these zones would be set to the same time. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard,
leading to the widespread use of Greenwich Mean Time in order
to set local clocks. The standard time system used today, called Universal Standard Time, is standard time based
on Universal Time.
Measurement
One can measure time based on the rotation of the Earth by observing celestial bodies cross the meridian every day. Astronomers have preferred observing meridian
crossings of stars over observations of the Sun,
because these are more accurate. Nowadays, UT in relation to TAI is determined by VLBI observations of distant quasars, which has an accuracy of micro-seconds.
The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service
(IERS).
Because the rotation of the Earth is somewhat irregular and the length of the day increases due to tidal acceleration, UT is not a perfect clock time. It has been
replaced by ephemeris time which has since been replaced by International Atomic Time (TAI). However, because
universal time is synchronous with night and day, and more perfect clocks drift away from this, UT is still used as a correction
to atomic time in order to obtain civil clock time.
Versions
There are several versions of Universal Time:
- UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of
stars or extraterrestrial radio sources, and also from ranging observations of the Moon and artificial Earth satellites. If the
geographic longitude of the observatory with respect to Greenwich is known, a simple subtraction yields UT0. However, because of
polar motion, the geographic
position of any place on Earth varies, and different observatories will find a different value for UT0 at the same moment. UT0
was kept by pendulum clocks but there are errors in UT0 due to polar motion. When UT0 is corrected for the shift in longitude of
the observing station caused by polar motion, the time scale UT1 is obtained.
- UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. UT1
is the same everywhere on Earth, and defines the true rotation angle of the Earth with respect to a fixed frame of reference.
Since the rotational speed of the earth is not uniform, UT1 has an uncertainty of plus or minus 3 milliseconds per day.
- UT1R is a filtered UT1, in which short-term variations with periods up to 35 days are filtered out so UT1R scale runs
smoother than UT1.
- UT2 is rarely used anymore and is mostly of historic interest. It is a smoothed version of UT1. UT1 has
irregular as well as periodic variations. There are seasonal effects, and these can be mostly removed by applying a conventional
correction:
-
-
- t is the time as fraction of the Besselian year; pi is the circular constant π = 3.14159... .
- UTC (Coordinated Universal
Time) is the international standard on which civil time is based, according to time
zone. It is measured with atomic clocks, and is kept within 0.9 seconds of UT1 by the introduction of one-second steps to
UTC, the "leap second." To date these steps have always been positive. When
an accuracy better than one second is not required, UT1 can be used as an
approximation of UTC.
In celestial navigation applications, Universal Time is
obtained from UTC by applying increments
determined by the U.S. Naval
Observatory.
- See also: Coordinated Universal
Time, time scale
Sources
- P.K.Seidelmann (ed.), Explanatory Supplement to the Astronomical Almanac. University Science Books, CA, 1992,1997 ;
ISBN 0-935702-68-7
- On the history of time standardization, see Peter Galison, Einstein's Clocks, Poincaré's Maps: Empires of Time,
Norton: New York, 2003.
External links
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