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Universal Time
Time
(UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time
Greenwich Mean Time
(GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, London, UK. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time
Coordinated Universal Time
(UTC) and UT1 (see below).[1] All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC
UTC
is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.

Contents

1 Universal Time
Time
and standard time 2 Measurement 3 Versions 4 Adoption in various countries 5 See also 6 Notes 7 References 8 External links

Universal Time
Time
and standard time Prior to the introduction of standard time, each municipality throughout the civilized world set its official clock, if it had one, according to the local position of the Sun
Sun
(see solar time). This served adequately until the introduction of rail travel in Britain, which made it possible to travel fast enough over long distances to require continuous re-setting of timepieces as a train progressed in its daily run through several towns. Greenwich Mean Time, where all clocks in Britain were set to the same time, was established to solve this problem. Chronometers or telegraphy were used to synchronize these clocks.[2]

Standard time
Standard time
zones of the world since 2016. The number at the bottom of each timezone specifies the number of hours to add to UTC
UTC
to convert it to the local time.

Standard time, as originally proposed by Scottish-Canadian Sir Sandford Fleming
Sandford Fleming
in 1879, divided the world into twenty-four time zones, each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Greenwich Observatory
Royal Greenwich Observatory
in Greenwich, England, was adopted as standard on 22 October 1884 at the end of the International Meridian Conference,[3][4] leading to the widespread use of Greenwich Mean Time to set local clocks. This location was chosen because by 1884 two-thirds of all nautical charts and maps already used it as their prime meridian.[5] The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, which was to choose a basis for universal time (as well as a prime meridian). During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.[6] In 1935, the term Universal Time
Time
was recommended by the International Astronomical Union as a more precise term than Greenwich Mean Time, because GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight.[7] The term Greenwich Mean Time persists, however, in common usage to this day in reference to civil timekeeping. Measurement Based on the rotation of the Earth, time can be measured by observing celestial bodies crossing the meridian every day. Astronomers found that it was more accurate to establish time by observing stars as they crossed a meridian rather than by observing the position of the Sun
Sun
in the sky. Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry
Very Long Baseline Interferometry
(VLBI) observations of distant quasars, a method which can determine UT1 to within 4 milliseconds.[8][9]

An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time

The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service (IERS). The International Astronomical Union also is involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union or ITU.[10] The rotation of the Earth is somewhat irregular, and is very gradually slowing due to tidal acceleration. Furthermore, the length of the second was determined from observations of the Moon
Moon
between 1750 and 1890. All of these factors cause the mean solar day, on the average, to be slightly longer than the nominal 86,400 SI seconds, the traditional number of seconds per day. As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT). Because Universal Time
Time
is synchronous with night and day, and more precise atomic-frequency standards drift away from this, however, UT is still used to produce a correction (called a leap second) to atomic time, in order to obtain a broadcast form of civil time that carries atomic frequency. Thus, civil broadcast standards for time and frequency usually follow International Atomic Time closely, but occasionally step (or "leap") in order to prevent them from drifting too far from mean solar time. 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.[11] First, 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. Next, the time scales based on Earth's rotation are not uniform and therefore, are not suitable for predicting the motion of bodies in our solar system. Versions There are several versions of Universal Time:

UT0 is Universal Time
Time
determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon
Moon
and artificial Earth satellites. The location of the observatory is considered to have fixed coordinates in a terrestrial reference frame (such as the International Terrestrial Reference Frame) but the position of the rotational axis of the Earth wanders over the surface of the Earth; this is known as polar motion. UT0 does not contain any correction for polar motion. The difference between UT0 and UT1 is on the order of a few tens of milliseconds. The designation UT0 is no longer in common use.[12] UT1 is the principal form of Universal Time. While conceptually it is mean solar time at 0° longitude, precise measurements of the Sun
Sun
are difficult. Hence, it is computed from observations of distant quasars using long baseline interferometry, laser ranging of the Moon
Moon
and artificial satellites, as well as the determination of GPS satellite orbits. UT1 is the same everywhere on Earth, and is proportional to the rotation angle of the Earth with respect to distant quasars, specifically, the International Celestial Reference Frame (ICRF), neglecting some small adjustments. The observations allow the determination of a measure of the Earth's angle with respect to the ICRF, called the Earth Rotation Angle (ERA, which serves as a modern replacement for Greenwich Mean Sidereal Time). UT1 is required to follow the relationship

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians where Tu = (Julian UT1 date - 2451545.0)[13]

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.[14] 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

U T 2 = U T 1 + 0.022 ⋅ sin ⁡ ( 2 π t ) − 0.012 ⋅ cos ⁡ ( 2 π t ) − 0.006 ⋅ sin ⁡ ( 4 π t ) + 0.007 ⋅ cos ⁡ ( 4 π t )

seconds

displaystyle UT2=UT1+0.022cdot sin(2pi t)-0.012cdot cos(2pi t)-0.006cdot sin(4pi t)+0.007cdot cos(4pi t); mbox seconds

where t is the time as fraction of the Besselian year.[15]

UTC
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 86,400 SI seconds per day but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap seconds. As of 2016[update], these leaps have always been positive (the days which contained a leap second were 86,401 seconds long). Whenever a level of accuracy better than one second is not required, UTC
UTC
can be used as an approximation of UT1. The difference between UT1 and UTC
UTC
is known as DUT1.[16]

Adoption in various countries

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The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.

Year Countries [17]

1848 Great Britain [18]

1880 Ireland (entire island)

1883 Canada, USA [19]

1884 Serbia

1888 Japan

1892 Belgium, the Netherlands,[20] S. Africa[21]

1893 Italy, Germany, Austria-Hungary (railways)

1894 Bulgaria, Denmark, Norway, Switzerland, Romania, Turkey (railways)

1895 Australia, New Zealand, Natal

1896 Formosa (Taiwan)

1899 Puerto Rico, Philippines

1900 Sweden, Egypt, Alaska

1901 Spain

1902 Mozambique, Rhodesia

1903 Ts'intao, Tientsin

1904 China Coast, Korea, Manchuria, N. Borneo

1905 Chile

1906 India,[22] Ceylon(Sri Lanka), Seychelles

1907 Mauritius, Chagos

1908 Faroe Is., Iceland

1911 France, Algeria, Tunis,[23] British West Indies

1912 Portugal,[24] other French possessions, Samoa, Hawaii, Midway and Guam, Timor, Bismarck Arch., Jamaica, Bahamas Is.

1913 British Honduras, Dahomey

1914 Albania, Brazil, Colombia

1916 Greece, Poland, Turkey

Year Countries

1917 Iraq, Palestine

1918 Guatemala, Panama, Gambia, Gold Coast

1919 Latvia, Nigeria

1920 Argentine, Uruguay, Burma, Siam

1921 Finland, Estonia, Costa Rica

1922 Mexico

1924 Java, USSR

1925 Cuba

1928 China Inland

1930 Bermuda

1931 Paraguay

1932 Barbados, Bolivia, Dutch East Indies

1934 Nicaragua, E. Niger

By 1936 Labrador, Norfolk I.

By 1937 Cayman Is., Curaçao, Ecuador, Newfoundland

By 1939 Fernando Po, Persia

By 1940 Lord Howe I.

1940 The Netherlands

By 1948 Aden, Ascension I., Bahrein, British Somaliland, Calcutta, Dutch Guiana, Kenya, Federated Malay States, Oman, Straits Settlements, St. Helena, Uganda, Zanzibar

By 1953 Raratonga, South Georgia

By 1954 Cook Is.

By 1959 Maldive I. Republic

By 1961 Friendly Is., Tonga Is.

By 1962 Saudi Arabia

By 1964 Niue Is.

1972 Liberia

Apart from the Nepal Time Zone (UTC+05:45), the Chatham Standard Time Zone (UTC+12:45) used in New Zealand's Chatham Islands[25] and the officially unsanctioned Central Western Time
Time
Zone (UTC+8:45) used in Eucla, Western Australia
Eucla, Western Australia
and surrounding areas, all timezones in use are defined by an offset from UTC
UTC
that is a multiple of half an hour, and in most cases a multiple of an hour. See also

List of international common standards Coordinated Mars Time
Time
(MTC) Unix time UTC

Notes

^ Guinot 2011, p. S181. ^ Howse 1997, ch. 4. ^ voting took place on 13 October ^ Howse 1997, pp. 12, 137. ^ Howse 1997, ch. 5. ^ Howse 1997, ch. 6. ^ McCarthy & Seidelmann 2009, p. 14. ^ McCarthy & Seidelmann 2009, pp. 68–9. ^ Urban & Seidelmann 2013, p. 175. ^ McCarthy & Seidelmann 2009, Ch. 18. ^ Urban & Seidelmann 2013, p. 7. Strictly speaking, a major producer of ephemerides, the Jet Propulsion Laboratory, uses a time scale they derive, Teph, which is functionally equivalent to TDB. ^ Urban & Seidelmann 2013, p. 81. ^ McCarthy & Seidelmann 2009, pp. 15–17, 62–64, 68–69, 76. ^ IERS n.d. ^ Date and Time
Time
Definitions n.d. ^ McCarthy & Seidelmann 2009, Ch. 14. ^ Howse 1980, pp. 154–5. Names have not been updated. ^ legal in 1880 ^ legal in 1918 (Standard Time
Time
Act) ^ Legal time reverted to Amsterdam time 1909; to Central European Time 1940, ^ except Natal ^ except Calcutta ^ and many French overseas possessions, ^ and overseas possessions, ^ HM Nautical Almanac Office 2015.

References

"Date and Time
Time
Definitions". United States Naval Observatory. Retrieved 3 March 2013.  "Earth Rotation Variations Due to Zonal Tides". Paris: Earth Orientation Center. Retrieved 2 October 2011.  Galison, Peter (2003). Einstein's clocks, Poincaré's maps: Empires of time. New York: W.W. Norton & Co. ISBN 0-393-02001-0.  Discusses the history of time standardization. Guinot, Bernard (July 2011). "Solar time, legal time, time in use". Metrologia. 48 (4): S181–S185. Bibcode:2011Metro..48S.181G. doi:10.1088/0026-1394/48/4/S08.  HM Nautical Almanac Office (April 2015). "World Time
Time
Zone Map".  Howse, Derek (1980). Greenwich Time
Time
and the discovery of the longitude. Oxford Univ Press. pp. 154–5. . Names have not been updated. Howse, Derek (1997). Greenwich Time
Time
and the Longitude. Phillip Wilson. ISBN 0-85667-468-0.  McCarthy, Dennis D. (July 1991). "Astronomical Time" (PDF). Proceedings of the IEEE. 79 (7): 915–920. doi:10.1109/5.84967.  McCarthy, Dennis; Seidelmann, P. Kenneth (2009). TIME—From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA. ISBN 978-3-527-40780-4.  O'Malley, Michael (1996). Keeping watch: A history of American time. Washington DC: Smithsonian. ISBN 1-56098-672-7.  Seidelmann, P. Kenneth (1992). Explanatory supplement to the Astronomical Almanac. Mill Valley, California: University Science Books. ISBN 0-935702-68-7.  Urban, Sean; Seidelmann, P. Kenneth, eds. (2013). Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, California: University Science Books.  "UT1R". International Earth Rotation and Reference System Service. Retrieved 6 March 2013.  "What is TT?". Naval Oceanography Portal. United States Naval Observatory. Retrieved 3 March 2013. 

 This article incorporates public domain material from the General Services Administration
General Services Administration
document "Federal Standard 1037C". External links

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