Universal Time

Universal Time (UT) is a time scale 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 counted from 0 hours at midnight, with unit of duration the mean solar day, defined to be as uniform as possible despite variations in the rotation of the Earth.

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.. 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. It varies from uniformity because of the irregularities in the Earth's rotation.
Coordinated Universal Time (UTC) differs from TAI by an integral number of seconds. UTC 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, Universal Time can be used to mean Coordinated Universal Time (UTC).

Universal Time is the official civil time of the United Kingdom, formerly called Greenwich Mean Time.

In celestial navigation applications, Universal Time is obtained from UTC by applying increments determined by the U.S. Naval Observatory.