Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, England, which is the conventional zero meridian for geographic longitude. GMT is sometimes used as a synonym for UTC. The old GMT has been split, in effect, into UTC and UT1.

Measurement
There are several versions of Universal Time:
where t is the time as fraction of the Besselian year.

UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon and artificial Earth satellites. It is uncorrected for the displacement of Earth's geographic pole from its rotational pole. This displacement, called polar motion, causes the geographic position of any place on Earth to vary by several metres, and different observatories will find a different value for UT0 at the same moment. It is thus not, strictly speaking, Universal.
UT1 is the principal form of Universal Time. It is computed from the raw observed UT0 by correcting UT0 for the effect of polar motion on the longitude of the observing site. UT1 is the same everywhere on Earth, and is proportional to 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. The ratio of UT1 to mean sidereal time is defined to be 0.997269566329084 − 5.8684×10 Notes

Federal Standard 1037C and from the Department of Defense Dictionary of Military and Associated Terms and from time scale
Galison, Peter. Einstein's clocks, Poincaré's maps: Empires of time. New York: W.W. Norton & Company, 2003. ISBN 0-393-02001-0. Discusses the history of time standardization.
O'Malley, Michael. Keeping watch: A history of American time. Washington: Smithsonian, 1996. ISBN 1-56098-672-7
Seidelmann, P. Kenneth, ed. Explanatory supplement to the Astronomical Almanac. Mill Valley, California: University Science Books, 1992. ISBN 0-935702-68-7.
Dennis D. McCarthy. Astronomical Time. Proceedings of the IEEE, Vol. 79, No. 7, July 1991, pp. 915-920.