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universal time

 
Dictionary: universal time
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n. (Abbr. UT)

The mean solar time for the meridian at Greenwich, England, used as a basis for calculating time throughout most of the world. Also called Greenwich time, Greenwich Mean Time, Also called Zulu time.


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Modern Science: universal time
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universal time

The measure of time obtained from the rotation of the earth, also known as Greenwich mean time, after the Greenwich Observatory in England. The world's time standard today is Coordinated Universal Time, which is kept by atomic clock s. The two universal times are kept in synchronization by the occasional insertion of leap seconds into the year.

US Military Dictionary: Universal Time
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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.

See the Introduction, Abbreviations and Pronunciation for further details.

Measures and Units: Universal Time
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UT, also UTC and distinctly UT0, UT1, UT2

time The name adopted in 1972 for the long-familiar Greenwich Mean Time (GMT), i.e. the time scheme set to have 12:00 noon when the Sun is nominally overhead on the Greenwich meridian, but precisely synchronized, at the level of the second, with the atomic-based TAI. (As discussed under day, the elapsed time from solar noon to noon is not consistent throughout the year; being overhead relates to an average position.)

Atomic clocks were first brought into practical use in the 1950s. The ‘atomic’ second as the essential unit of time was defined in 1967, and an international time scheme based thereon, labelled TAI (Temps Atomique International), was adopted in 1970. TAI builds an ‘atomic’ day of 24 ‘atomic’ hours of 60 ‘atomic’ minutes of 60 such seconds, i.e. a ‘day’ is precisely 86 400 ‘atomic’ seconds. However, just as a year that is a regular multiple of days does not stay in tune with the seasons, the regular day of TAI does not stay in tune with the diurnal pattern. The day varies considerably in length compared with the year, rhythmically over the year by more than 30 s (see equation of time), but its overall mean value also varies, irregularly and with an overall progressive slowing down. To avert the Sun's zenith creeping progressively from its familiar place, UT incorporates ‘leap seconds’ as appropriate. All adjustments are made at the close of a month, preferably of December else of June, but possibly others, as necessary to keep the discrepancy between TAI and reality (in the form of UT1 shown below) less than 0.9 s. To date, because of the progressive slowing down (the mean solar day being now about 86 400.0028 s long), it has always meant an insertion, but it could require removal of a second, and either way it could be multiple. Starting in 1972, one leap second was added in 22 of the 28 years up to the end of 1998, putting UT 32 s behind TAI (hence 64.184 s behind TDT at the start of 1999). The following three years were devoid of leap seconds.
[For latest information see ftp://tycho.usno.navy.mil/pub/series/ or http://tycho.usno.navy.mil/leap.html]

Universal Time exists in a superfine gradation of forms. The above is distinguished as Co-ordinated Universal Time (UTC). UT0 is the rotational time scale based on observation and standard longitude, UT1 is UT0 adjusted for the effect on functional longitude of the polar wobble caused by crust/mantle movements within Earth, and which results in the angle of tilt varying from its nominal value (currently nearly 23½ degrees, but also subject to periodic variation, nutation, over 41 000 years of about 1 degree either side of this value). UT2 is UT1 adjusted for the seasonal variations in Earth's rotation. The differences between the three versions are utterly trivial for everyday living.

UT is routinely used by satellites and related equipment that need to be mutually synchronized. However, to avoid the complication of leap seconds, each separate system is set permanently to the UTC at the epoch of its initialization. Thus the OMEGA system, its epoch being 01 January 1972, is only 10 s behind TAI, while the Global Positioning System, its epoch being 06 January 1980, is 19 s behind TAI for the computation of global position (the displayed time being full UTC).

Britannica Concise Encyclopedia: Universal Time
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Mean (solar) time of the Greenwich meridian (0° longitude). Universal Time replaced the designation Greenwich Mean Time in 1928; since 1972 it has been based on international atomic time, a uniform time derived from the frequences of certain atomic transitions and measured by an atomic clock. For most purposes, it is now identical to Greenwich Mean Time.

For more information on Universal Time, visit Britannica.com.

 
Columbia Encyclopedia: 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.

Military Dictionary: Universal Time
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(DOD) 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. When an accuracy better than one second is not required, Universal Time can be used to mean Coordinated Universal Time. Also called ZULU time. Formerly called Greenwich Mean Time.

Wikipedia: Universal Time
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"UT1" redirects here. For other uses, see UT1 (disambiguation).

Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, and GMT is sometimes used loosely as a synonym for UTC. 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). All of these versions of UT are based on sidereal time, but with a scaling factor and other adjustments to make them closer to solar time.

Contents

Universal Time and standard time

Prior to the introduction of standard time, every municipality around the civilized world set its official clock, if it had one, according to the local position of the sun (see solar time). This served adequately until the introduction of the steam engine, the telegraph, and rail travel, which made it possible to travel fast enough over long distances to require almost constant re-setting of timepieces, as a train progressed in its daily run through several towns. Standard time, where all clocks in a large region are set to the same time, was established to solve this problem. Chronometers or telegraphy were used to synchronize these clocks.

Standard time zones of the world as of June 2008. Instruction for converting UTC to or from local times are on the bottom.

Standard time, as originally proposed by Sir Sandford Fleming in 1879, divided the world into twenty-four time zones, each one covering exactly 15 degrees of longitude. All clocks within each of these zones would be set to the same time as the others, but differed 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 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. The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, to choose a prime meridian. Nevertheless, by 1929 all major countries had adopted standard time zones. Political considerations have now increased the number of standard time zones to 40.

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.

Measurement

One can measure time based on the rotation of the Earth by observing celestial bodies crossing 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, a method which has an accuracy of microseconds. 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.[1]

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.

Versions

There are several versions of Universal Time:

  • UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic 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. It is thus not, strictly speaking, Universal.
  • UT1 is the principal form of Universal Time. It is computed from the raw observed UT0 by correcting UT0 for the effect of polar motion on the longitude of the observing site. UT1 is the same everywhere on Earth, and is proportional to 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. The ratio of UT1 to mean sidereal time is defined to be 0.997269566329084 − 5.8684×10−11T + 5.9×10−15T², where T is the number of Julian centuries of 36525 days each that have elapsed since JD 2451545.0 (J2000).[2]
  • UT1R is a smoothed version of UT1, filtering out periodic variations due to tides. It includes 62 smoothing terms, with periods ranging from 5.6 days to 18.6 years.[3]
  • UT2 is a smoothed version of UT1, filtering out periodic seasonal variations. It is mostly of historic interest and rarely used anymore. It is defined by the equation:
UT2 = UT1 + 0.022\cdot\sin(2\pi t) - 0.012\cdot\cos(2\pi t) - 0.006\cdot\sin(4\pi t) + 0.007\cdot\cos(4\pi t)\;\mbox{seconds} where t is the time as fraction of the Besselian year. [4]
  • UT2R is a smoothed version of UT1, incorporating both the seasonal corrections of UT2 and the tidal corrections of UT1R. It is the most smoothed form of Universal Time. Its non-uniformities reveal the unpredictable components of Earth rotation, due to atmospheric weather, plate tectonics, and currents in the interior of the Earth.
  • UTC (Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based. It ticks SI seconds, in step with TAI. It usually has 86400 SI seconds per day, but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap seconds. As of 2008 these leaps have always been positive, with a day of 86401 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.
  • UTC-SLS (UTC with Smoothed Leap Seconds) is a proposed modification of UTC that avoids unequal day lengths. It usually ticks the same as UTC, but modifies the length of the second for the last 1000 UTC seconds of a day containing a leap second so that there are always 86400 seconds in the UTC-SLS day.[5]
  • UTS (Smoothed Universal Time) is an obscure form of UT used internally at IERS. The same abbreviation was for a time used to refer to UTC-SLS.[5]

See also

Notes

  1. ^ What is TT?, no date.
  2. ^ Seidelmann, p.52
  3. ^ Earth rotation variations due to zonal tides
  4. ^ United States Naval Observatory, no date.
  5. ^ a b UTC with Smoothed Leap Seconds (UTC-SLS)

References

PD-icon.svg This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

External links

  • Time Lord by Clark Blaise: a biography of Stanford Fleming and the idea of standard time

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

 
 

 

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
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Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/.  Read more
Military Dictionary. US Department of Defense Dictionary of Military and Associated Words, 2003.  Read more
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