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.

Contents

1 Universal Time and standard time 2 Measurement 3 Versions 4 See also 5 References

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, leading to the widespread use of Greenwich Mean Time in order to set local clocks.

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 time standards is the International Telecommunication Union or "ITU."

The rotation of the Earth is somewhat irregular; also the length of the day 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; thus UT is not a perfect clock time. It was replaced by Ephemeris Time, which has since been replaced by Terrestrial Time (TT). However, because Universal Time is synchronous with night and day, and more perfect clocks drift away from this, UT is still used to produce a correction called leap seconds to atomic time in order to obtain civil clock time. In other words, civil clock time is 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 noon and midnight. Terrestrial Time is TAI + 32.184 s.

Terrestrial Time is still 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 TT 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 appropriate term than Greenwich Mean Time for the basis of an international standard of time. But the term Greenwich Mean Time persisted in common usage, probably because at first the two were the same.

Versions

There are several versions of Universal Time:

• UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources, and also from ranging observations of the Moon and artificial Earth satellites. If 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 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:

<math>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}<math>

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.

See also

• Sidereal Time
• time scale

References

• Federal Standard 1037C and from the Department of Defense Dictionary of Military and Associated Terms and from time scale
• Galison, Peter. Einstein's Clocks, Poincaré's Maps: Empires of Time. New York: W.W. Norton & Company, 2003. ISBN 0393020010. Discusses the history of time standardization.
• Seidelman, P. Kenneth, ed. Explanatory Supplement to the Astronomical Almanac. Mill Valley, California: University Science Books, 1992. ISBN 0935702687.

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