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. GMT is sometimes used, incorrectly, as a synonym for UTC. The old GMT has been split, in effect into UTC and UT1.

Versions

There are several versions of Universal Time:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• 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 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 + 0.0220cdotsin(2pi t) - 0.0120cdotcos(2pi t) - 0.0060cdotsin(4pi t) + 0.0070cdotcos(4pi t);mbox{seconds}

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

::where t is the time as fraction of the Besselian year.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• UTC (Coordinated Universal Time) is the international standard on which civil time is based. It is measured with atomic clocks, and is kept within 0.9 seconds of UT1 by the introduction of occasional one-second steps called "leap seconds" to UTC. To date these steps have always been positive. When an accuracy better than one second is not required, UTC can be used as an approximation of UT1.