UTC is a discontinuous time scale composed from segments that are linear transformations of atomic time, the discontinuities being arranged so that UTC approximated UT2 until the end of 1971, and UT1 thereafter.

This will require the addition of an ever greater number of leap seconds to UTC as long as UTC is kept within one second of UT1.

One such time scale is UT1, a form of universal time.

The difference deltaT, though somewhat unpredictable in fine detail, is expected to continue to increase, with UT1 becoming steadily ( but irregularly ) further behind TT in the future.

The more current refined version of UT, UT1, is still in reality mean time at Greenwich.

But Coordinated Universal Time ( UTC ) ( an atomic-based time scale which is always kept within 0. 9 second of UT1 ) is in common actual use in the UK, and the name GMT is often inaccurately used to refer to it.

* UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site.

UTC is kept within 0. 9 second of UT1 by the introduction of one-second steps to UTC, the " leap second ".

In fact, the expression " Universal Time " is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1 ( see below ).

* UT1 is the principal form of Universal Time.

UT1 is required to follow the relationship

* UT1R is a smoothed version of UT1, filtering out periodic variations due to tides.

* UT2 is a smoothed version of UT1, filtering out periodic seasonal variations.

* UT2R is a smoothed version of UT1, incorporating both the seasonal corrections of UT2 and the tidal corrections of UT1R.

* UTC ( Coordinated Universal Time ) is an atomic timescale that approximates UT1.

It usually has 86, 400 SI seconds per day, but is kept within 0. 9 seconds of UT1 by the introduction of occasional intercalary leap seconds.

When an accuracy better than one second is not required, UTC can be used as an approximation of UT1.

The difference between UT1 and UTC is known as DUT1.

GMT is now often called Universal Time, which is now calculated from observations of extra-galactic radio sources, and then converted into several forms, including UT0 ( UT at the remote observatory ), UT1 ( UT corrected for polar motion ), and UTC ( UT in discrete SI seconds within 0. 9 s of UT1 ).

Most standard coordinates in use today refer to 2000 Jan 1. 5 TT ( i. e. to 12h on the Terrestrial Time scale on 2000 Jan 1 ), which occurred about 64 seconds sooner than noon UT1 on the same date ( see ΔT ).

DeltaT was measured at + 65. 7768 seconds ( TT ahead of UT1 ) at 0h UTC on 1 January 2009 ; and by retrospective calculation, deltaT was close to zero around the year 1900.

In practice, the difference is kept track of by the difference UTC – UT1, which is measured by radio telescopes and kept on file and available to the public at the IERS and at the United States Naval Observatory.

UT1 is the non-uniform time defined based on the Earth's rotation.

Nataliya Dmytruk (, alternate transliteration: Natalya Dmytruk ) ( born 1957 or 1958 ) is a former sign language interpreter on the Ukrainian state-run channel UT1 news broadcasts.

She was a barely known sign language interpreter at the Ukrainian state-run channel UT1 but became instantly famous during the live news broadcast on November 24, 2004, when she refused to translate the official script that announced Viktor Yanukovych as the winner of the presidential election.

Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service ( IERS ), is seconds of mean solar time ( UT1 ) Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is seconds of mean solar time ( UT1 ) Thus the sidereal day is shorter than the stellar day by about 8. 4 ms.

Universal Time ( UT ), a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally-defined universal day ; then from 1 January 1956 ( as decided by the IAU at Dublin, 1955, at the initiative of William Markowitz ) this " raw " form of UT was re-labeled UT0 and effectively superseded by refined forms UT1 ( UT0 equalized for the effects of polar wandering ) and UT2 ( UT1 further equalized for annual seasonal variations in earth rotation rate ).

At the International Astronomical Union ( IAU ) meeting in Dublin in 1955, he had proposed the system of distinguishing between variants of Universal Time, as UT0 ( UT as directly observed ), UT1 ( reduced to invariable meridian by correcting to remove effect of polar motion ) and UT2 ( further corrected to remove ( extrapolated ) seasonal variation in earth rotation rate ), a system which remains in some use today.

The time correction DUT1 ( sometimes also written DUT ) is the difference between Universal Time ( UT1 ), which is defined by Earth's rotation, and Coordinated Universal Time ( UTC ), which is defined by a network of precision clocks.

Greenwich Mean Sidereal Time ( GMST ) and UT1 differ from each other in rate, with the second of sidereal time a little shorter than that of UT1, so that ( as at 2000 January 1 noon ) 1. 002737909350795 second of mean sidereal time was equal to 1 second of Universal Time ( UT1 ).

Between 10 and 20 minutes past the hour, RWM transmits a pulse of carrier every second, with the difference between UT1 and UTC in units of one-fiftieth of a second encoded onto the once-per-second pulses.

* Institute of Mathematics of Toulouse – IMT ( in partnership with UPS, UT1, UT2 and CNRS )

In an astronomical context, " GMT " is a deprecated name for Universal Time, or sometimes more specifically for UT1.

Historical Julian dates were recorded relative to GMT or Ephemeris Time, but the International Astronomical Union now recommends that Julian Dates be specified in Terrestrial Time, and that when necessary to specify Julian Dates using a different time scale, that the time scale used be indicated when required, such as JD ( UT1 ).

where D is the interval, in UT1 days including any fraction of a day, since 2000 January 1, at 12h UT ( interval counted positive if forwards to a later time than the 2000 reference instant ), and the result is freed from any integer multiples of 24 hours to reduce it to a value in the range 0-24.

* Between one and sixteen seconds past the minute ( except at the top of the hour ), CHU transmits the difference between UT1 and Coordinated Universal Time ( UTC ) by using split tones.

* UT1 ( Ukraine ), television station

This was a compromise arrangement for a broadcast time scale: a linear transformation of the BIH's atomic time meant that the time scale was stable and internationally synchronised, while approximating UT1 means that tasks such as navigation which require a source of Universal Time continue to be well served by public time broadcasts.

Leap seconds are nowadays added to or subtracted from UTC to keep it within 0. 9 seconds of UT1.

* TT runs a little ahead of UT1 ( a refined measure of mean solar time at Greenwich ) by an amount known as deltaT = TT-UT1.

Most sources of time and celestial coordinate system standards use UT1 as the default meaning of UT, though occasionally UTC may be implied.

( Julian UT1 date-2451545. 0 )