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Universal Time

(Redirected from Standard 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.

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Universal Time and standard time

On November 2, 1868 New Zealand officially adopted a standard time to be observed nationally, and was perhaps the first country to do so. It was based on the longitude 172° 30' East of Greenwich, that is 11 hours 30 minutes ahead of Greenwich Mean Time. This standard was known as New Zealand Mean Time.

Prior to the introduction of standard time, every municipality set its clock, if it had one, by the local position of the sun. This served well until the introduction of the train, when it became possible to travel fast enough to require almost constant re-setting of clocks. Standard time, where all clocks in a region use the same time, was invented to solve this problem.

Standard time divides the world into a number "time zones", each one covering, in theory at least, 15 degrees. All clocks within each of these zones would be set to the same time as the others, but so as to differ by one hour from those in the neighbouring zones. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time in order to set local clocks. This location was chosen because by 1884 two-thirds of all charts and maps already used it as their prime meridian.

Measurement

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 International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars, which method has an accuracy of micro-seconds. Most sources of time and celestial coordinate system standards use UT1 as the default meaning of UT, though occasionally UTC may be implied.

The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service (IERS). The International Astronomical Union is also involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union or "ITU."

The rotation of the Earth is somewhat irregular; also the length of the day very gradually increases due to tidal acceleration. Furthermore, the length of the second is based on its conventional length as determined from observations of the Moon between 1750 and 1890. This also causes the mean solar day, on the average, to now extend longer than the nominal 86,400 SI seconds. As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT). However, because Universal Time is synchronous with night and day, and more precise atomic-frequency standards drift away from this, UT is still used to produce a correction called leap seconds to atomic time to obtain a broadcast form of civil time that carries atomic frequency. Thus, civil broadcast standards for time and frequency are a compromise that usually follows, with an offset found from the total of all leap seconds, International Atomic Time (TAI), but occasionally jumps in order to prevent it from drifting too far from mean solar time. Terrestrial Time is TAI + 32.184 s.

Barycentric Dynamical Time (TDB), a form of atomic time, is now used in the construction of the ephemerides of the planets and other solar system objects, for two main reasons. For one thing, these ephemerides are tied to optical and radar observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion, with corrections for general relativity, are followed. For another, the time scales based on Earth's rotation are not uniform, so are not suitable for predicting the motion of solar system objects.

In 1928 the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because the GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. However, the term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.

Versions

There are several versions of Universal Time:

• UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extraterrestrial 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.
• 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:
$UT2 = UT1 + 0.0220\cdot\sin(2\pi t) - 0.0120\cdot\cos(2\pi t) - 0.0060\cdot\sin(4\pi t) + 0.0070\cdot\cos(4\pi t)\;\mbox{seconds}$
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 one-second steps to UTC, the "leap second." 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.