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. Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... Time zones of Europe: Light colours indicate countries not observing summer time Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. ... Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon. ... On the earth, a meridian is a north-south line between the North Pole and the South Pole. ... Greenwich (pronounced grenn-itch or by some grinn-itch ) is a town, now part of the south eastern urban sprawl of London, on the south bank of the River Thames in the London Borough of Greenwich. ... Motto (French) God and my right Anthem God Save the King (Queen) England() – on the European continent() – in the United Kingdom() Capital (and largest city) London (de facto) Official languages English (de facto) Government Constitutional monarchy  -  Queen Queen Elizabeth II  -  Prime Minister Tony Blair MP Unification  -  by Athelstan 967  Area... Synonyms can be nouns, adverbs or adjectives, as long as both members of the pair are the same part of speech. ... ...

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. 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. // The term steam engine may also refer to an entire railroad steam locomotive. ... This is the top-level page of WikiProject trains Rail tracks Rail transport refers to the land transport of passengers and goods along railways or railroads. ... A wrist watch A clock (from the Latin cloca, bell) is an instrument for measuring time. ... An SP freight train west of Chicago in 1992. ... A marine chronometer is a timekeeper precise enough to be used as a portable time standard, used to determine longitude by means of celestial navigation. ... Optical Telegraf of Claude Chappe on the Litermont near Nalbach, Germany Telegraph and telegram redirect here. ...

Standard time originally divided the world into twenty-four "time zones", each one covering exactly 15 degrees of longitude. However, political considerations have increased the number of standard time zones to 39. 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. The conference rejected Sir Sandford 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 time zones. A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time. ... Royal Observatory, Greenwich The original site of the Royal Greenwich Observatory (RGO), which was built as a workplace for the Astronomer Royal, was on a hill in Greenwich Park in Greenwich, London, overlooking the River Thames. ... The Prime Meridian, Greenwich The Prime Meridian is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England; it is the meridian at which longitude is 0 degrees. ... Location of the Prime Meridian Prime Meridian in Greenwich The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England — it is the meridian at which longitude is 0 degrees. ... Sir Sandford Fleming Sir Sandford Fleming (January 7, 1827 – July 22, 1915) was a prolific Canadian engineer and inventor, known for the introduction of Universal Standard Time, Canadas first postage stamp, a huge body of surveying and map making, engineering much of the Intercolonial Railway and the Canadian Pacific...

In the United States and Canada, standard time zones were introduced on November 18 1883, by American and Canadian railroads. Newspapers referred to that day as "the Day of Two Noons." There was no legislative enactment or ruling: the railroads simply adopted a five zone system encompassing North America from Nova Scotia to California, and assumed the public would follow. The American Railway Association, an organization of railroad managers, had noticed growing scientific interest in standardizing time. The ARA devised their own system, which had irregular zone boundaries which followed then-existing boundaries of different lines, partly in order to head off government action which might have been inconvenient to their operations. Most people simply accepted the new time, but a number of cities and counties refused to accept "railroad time", which, after all, had not been made law. In, for example, the expiration of a contract--what does "midnight" mean? In one Iowa Supreme Court case, the owner of a saloon argued that he operated by local (sun) time, not "railroad time," and so he had not violated laws about closing time. Standard time remained a local matter until 1918, when it was made law as part of the introduction of daylight saving. November 18 is the 322nd day of the year (323rd in leap years) in the Gregorian calendar. ... See also: 1882 in Canada, 1884 in Canada and the Timeline of Canadian history. ... This is the top-level page of WikiProject trains Rail tracks Rail transport refers to the land transport of passengers and goods along railways or railroads. ... Daylight saving time (also called DST, or Summer Time) is the local time a region is designated for a portion of the year, usually an hour forward from its standard official 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. November 2 is the 306th day of the year (307th in leap years) in the Gregorian calendar, with 59 days remaining. ... 1868 (MDCCCLXVIII) was a leap year starting on Wednesday (see link for calendar) of the Gregorian calendar or a leap year starting on Friday of the 12-day-slower Julian calendar. ...

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 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. Look up day in Wiktionary, the free dictionary. ... STAR is an acronym for: Organizations Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... The Sun is the star at the center of the Solar System. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... Very Long Baseline Interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy, in which the data received at each antenna in the array is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. ... Artists impression of quasar GB1508 A quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including radio waves and visible light. ...

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 International Earth Rotation and Reference Systems Service is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... The International Telecommunication Union (ITU; French: Union internationale des télécommunications, Spanish: Unión Internacional de Telecomunicaciones) is an international organization established to standardize and regulate international radio and telecommunications. ...

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. It has been suggested that Tidal friction be merged into this article or section. ... Look up second in Wiktionary, the free dictionary. ... Apparent magnitude: up to -12. ... Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... Look up second in Wiktionary, the free dictionary. ... Ephemeris Time (ET) is a now obsolete time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. ... Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... Civil time is another name for mean solar time reckoned from midnight. ... Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon. ...

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. Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960. ... The eight planets and three dwarf planets of the Solar System. ... This long range radar antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll. ... Newtons First and Second laws, in Latin, from the original 1687 edition of the Principia Mathematica. ... General relativity (GR) [also called the general theory of relativity (GTR) and general relativity theory (GRT)] is the geometrical theory of gravitation published by Albert Einstein in 1915/16. ...

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. Time zones of Europe: Light colours indicate countries not observing summer time Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. ... Civil time is another name for mean solar time reckoned from midnight. ...

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).^[1]

• 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.^[2]

• 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:

where t is the time as fraction of the Besselian year.

• 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 2007 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 modified form 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.^[3]

• 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.^[3]

This article needs to be wikified. ... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ... The Julian day or Julian day number (JDN) is the (integer) number of days that have elapsed since Monday, January 1, 4713 BC in the proleptic Julian calendar [1]. That day is counted as Julian day zero. ... In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. ... A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... Coordinated Universal Time (UTC) is a high-precison atomic time standard. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... 2007 is a common year starting on Monday of the Gregorian calendar. ... In the fields of science, engineering, industry and statistics, accuracy is the degree of conformity of a measured or calculated quantity to its actual (true) value. ... The time correction DUT1 is the difference between the Universal Time scale UT1 (which corresponds to the Earths rotation) and the Coordinated Universal Time (UTC) (which corresponds, except for a constant offset, to International Atomic Time (TAI)). DUT1 = UT1 - UTC DUT1 is maintained in the range -0. ... The International Earth Rotation and Reference Systems Service is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups. ...

Notes

1. ^ Seidelmann, p.52
2. ^ Earth rotation variations due to zonal tides
3. ^ ^{a b} UTC with Smoothed Leap Seconds (UTC-SLS)

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 0-393-02001-0. Discusses the history of time standardization.
• O'Malley, Michael. Keeping watch: A history of American time. Washington: Smithsonian, 1996. ISBN 1-56098-672-7
• Seidelmann, P. Kenneth, ed. Explanatory supplement to the Astronomical Almanac. Mill Valley, California: University Science Books, 1992. ISBN 0-935702-68-7.
• Dennis D. McCarthy. Astronomical Time. Proceedings of the IEEE, Vol. 79, No. 7, July 1991, pp. 915-920.

Federal Standard 1037C, entitled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. ... The Department of Defense Dictionary of Military and Associated Terms is a compedium of terminology used by the United States Department of Defense (DOD). ... A time scale specifies divisions (scale) of time. ... Peter Galison is a professor of physics and the history of science at Harvard University. ...

See also

• Coordinated Universal Time (UTC)
• Greenwich Mean Time (GMT)
• Sir Sandford Fleming
• Sidereal Time
• Time scale

Coordinated Universal Time (UTC) is a high-precison atomic time standard. ... Time zones of Europe: Light colours indicate countries not observing summer time Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. ... Sir Sandford Fleming Sir Sandford Fleming (January 7, 1827 – July 22, 1915) was a prolific Canadian engineer and inventor, known for the introduction of Universal Standard Time, Canadas first postage stamp, a huge body of surveying and map making, engineering much of the Intercolonial Railway and the Canadian Pacific... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ... A time scale specifies divisions (scale) of time. ...

External links

• Ontario Plaques - The Birthplace of Standard Time