# 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.

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): 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 It has been replaced by ephemeris time and nowadays by atomic time. However, because universal time is synchronous with night and day, and more perfect clocks drift away from this, UT is still the basis for civil clock time.

There are several versions of UT, depending on the corrections that are applied:

### UT0

This is the raw time as measured at an observatory. Actually, observations yield the local time at the meridian of the observatory. Iff 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.

### UT1

For this timescale, the raw observations have been corrected for the polar motion: it is the same everywhere on Earth, and defines the true rotation angle of the Earth with respect to a fixed frame of reference.

### UT2

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... . UT2 is not used much anymore.

### UT1R

UT1 where short-term variations with periods up to 35 days are filtered out: so this time scale runs smoother than UT1.

### UTC

Coordinated Universal Time: this is civil clock time. It is measured with atomic clocks, but always kept within 0.9 seconds from UT1. Because UT slows down with respect to TAI, a leap second must be applied about once every one or two years. This is the responsibility of the IERS: see http://www.iers.org/iers/publications/bulletins/bull_c/ for announcements; and http://www.iers.org/iers/earth/rotation/utc/table1.html and http://www.iers.org/iers/earth/rotation/utc/table2.html for previous events.

### Literature

P.K.Seidelmann (ed.): Explanatory Supplement to the Astronomical Almanac. University Science Books, CA, 1992,1997 ; ISBN 0-935702-68-7