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. Technically, GMT no longer exists, although the term is still used as a
synonym for UTC.
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 Service (IERS) External link: http://www.iers.org/
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 It has been replaced by ephemeris time which has since been replaced by International Atomic Time (TAI).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.
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:
- UT2 = UT1 + 0.0220*sin(2*pi*t) - 0.0120*cos(2*pi*t) - 0.0060*sin(4*pi*t) + 0.0070*cos(4*pi*t) seconds
- 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 for civil time. 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
- P.K.Seidelmann (ed.), Explanatory Supplement to the Astronomical Almanac. University Science Books, CA, 1992,1997 ; ISBN 0-935702-68-7