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. |
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. |
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 |
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 |
Please combine this with the information in the UTC entry. --[[Robert Merkel |
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 It has been replaced by ephemeris time and nowadays by atomic time.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:
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.
P.K.Seidelmann (ed.): Explanatory Supplement to the Astronomical Almanac. University Science Books, CA, 1992,1997 ; ISBN 0-935702-68-7 (amazon.com, search)