Standard of time

A Standard of time is the specification is rate/rhythm of the time which master key either of fixed times, or of both. For example the standard of civil time specifies both at the same time: temporal intervals and the hour of time.

Historically, the standards of time were based over the terrestrial period of rotation. However the rotation of the Earth is not uniform. Initially the reference to terrestrial rotation was replaced by the interval corresponding to the period of revolution of the Earth around the Sun. Because of complexity of its use and measurements that Ci was replaced, following the progress made in the measurement of time, by the time intervals based on the properties of the atom, intervals counted to the atomic clocks.

The time interval of reference recognized internationally is the second S.I. The other current time intervals, (minute S, hour S, Day S, and Year S) are based over the second.

Some standards of time

  • the solar Temps is based over the solar day, which is the period of time between two passages of the sun to the meridian local. One day solar lasts 24 hours on average. However, the effects of the characteristics of the revolution of the Earth around the Sun and the rotation of the Earth involve variations which can reach up to 15 minutes compared to the intermediate duration of solar time, it is the equation of time. The solar Temps corrected equation of time gives the average Temps .

  • the sidereal Temps is the time located compared to stars. It is used in astronomy of position. One day sidereal is time that the Earth puts to carry out a complete revolution compared to stars: in fact the time separates two successive passages from the same star to the meridian line of a terrestrial place. One day sidereal lasts 23 hours roughly 56 minutes 4 seconds.

  • the universal Time Universal Time ( UT ) is a scale of time based over the solar day, defined in a way as uniform as possible, in spite of the variations of terrestrial rotation.

  • UT0 results from rough measurement of the period of rotation of the Earth in a place of observation. It is measured according to the diurnal movement of stars or according to radio sources extra terrestrial.

  • UT1 is deduced from UT0 by correction of the effect of the movement of the pole on longitude of the site of observation. It is not uniform because of the irregularities of terrestrial rotation.

The international atomic Time ( TAI ) is the standard of international time, on the basis of which all the other standards of time, as a paticulier the UTC, are established. The TAI is placed under the responsibility of BIPM (International office of the Weights and measures), and is calculated according to the comparison of the steps of approximately 300 atomic clocks in the world, each one being corrected effects of its environment and relativity.

  • the Universal Time Coordinated ( UTC ) differs from the TAI by an integer of seconds. The UTC is maintained in an interval of 0,9 second of the UT1 by the possible introduction of second guides. It is in practice the base of time of use to the international scales.

  • the legal Time or civil time is the hour in force in a given country or an area. She is deduced from the UTC by addition or subtraction of a certain number of hours (generally of the full hours), so that the new day begins roughly in the middle of the local Nuit. This jet lag can be modified, in general of an hour, twice a year: it is the system of Summer-time/Heure of winter.

  • Greenwich Mean Time ( GMT ) is the time of the Méridien basic of Greenwich, close to London. GMT was time international standard: it is in force any more, neither technically nor legally. It was replaced by the Universal Time (Universal Time. Greenwich Mean Time was also the international standard for civil time. Although not being more in force, GMT is still very often used like Synonyme with UTC, which is the new international standard; it is then translated by Greenwich Meridian Time .

To also consult

External bonds

  • BIPM

  • GMT
  • UTC

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