Main Page | See live article | Alphabetical index

Universal Time

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.

Table of contents
1 Universal Time and standard time
2 Measurement
3 Versions
4 Sources
5 External links

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, Sir Sandford Fleming invented standard time to fix the problem.

Standard time divides the world into 24 "timezones", 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.


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).

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 It has been replaced by ephemeris time which has since been replaced by International Atomic Time (TAI).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:

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... .

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


External links