universal time

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universal time (UT), the international time standard common to every place in the world, it nominally reflects the mean solar time along the earth's prime meridian (renumbered to equate to civil time ). In 1884, under international agreement, the prime meridian was established as running through the Royal Observatory in Greenwich, England, setting the standard of Greenwich mean time (GMT). In keeping with tradition, the start of a solar day occurred at noon. In 1925 the numbering system for GMT was changed so that the day began at midnight to make it consistent with the civil day. Some confusion in terminology resulted, however, and in 1928 the International Astronomical Union (IAU) changed the designation of the standard time of the prime meridian to universal time. In 1955 the IAU defined several kinds of UT. The initial values of universal time obtained at 75 observatories, denoted UT0, differ slightly because of polar motion. By adding a correction each observatory converts UT0 into UT1, which gives the Earth's rotational position in space. An empirical correction to take account of annual changes in the speed of rotation is then added to convert UT1 to UT2. However, UT2 has since been superseded by atomic time (time as given by atomic clocks ). Universal time is also called world time, Z time, and Zulu time.

In 1964 a new timescale, called coordinated universal time (UTC), was internationally adopted. UTC is more uniform and more accurate than the UT2 system because the UTC second is based on atomic time (although the UTC year is still based on the time it takes the earth to complete one orbit). Because the rate of the earth's rotation is gradually slowing, it is occasionally necessary to add an extra second, called the leap second, to the length of the UTC year; synchronization is obtained by making the last minute of June or December contain 61 seconds. About one leap second per year has been inserted since 1972.

Universal Time (UT) A world-wide standard time-scale, the same as Greenwich Mean Time. Universal Time is the mean solar time on the meridian of Greenwich. It is defined as the Greenwich hour angle of the mean sun plus 12 hours, so that the day begins at midnight rather than noon. It is closely linked to Greenwich Mean Sidereal Time (GMST), since the mean sidereal day is a precisely known fraction of the mean solar day. In practice, UT is determined by a formula from GMST, which in turn is derived from the observations of the meridian transits of stars. The version of UT derived directly from such observations is designated UT0, which is slightly dependent on the observing site. When UT0 is corrected for the variation in longitude due to the Chandler wobble, a version of Universal Time, UT1, is derived which has genuine world-wide application. When UT1 is compared with International Atomic Time (TAI), it is found to be losing approximately a second a year against TAI. Broadcast time signals use the time-scale known as Coordinated Universal Time (UTC). This is TAI with an offset of a whole number of seconds. The offset is adjusted when necessary by the introduction of a leap second, and UTC is always kept within 0.9 s of UT1.

Universal Time a measure of time that conforms, within a close approximation, to the mean diurnal rotation of the earth and serves as the basis of civil timekeeping. Universal Time (UT1) is determined from observations of the stars, radio sources, and also from ranging observations of the moon and artificial earth satellites. The scale determined directly from such observations is designated Universal Time Observed (UTO); it is slightly dependent on the place of observation. When UTO is corrected for the shift in longitude of the observing station caused by polar motion, the time scale UT1 is obtained. Also called Zulu time. Formerly called Greenwich Mean Time.

universal time System of time reckoning based on the mean solar day, the average interval between two successive transits of the Sun across the Greenwich (0°) meridian.