Universal Time

The universal time ( YOU , or in English UT ) is a measurement of the Temps based on the Rotation of the Ground. It is about the modern prolongation of the average Temps of Greenwich (GMT), which was the average solar time with the Méridien of Greenwich. GMT was replaced in the facts by UTC and UT1, two incarnations of the universal time, since 1972.

Measure

The universal Time (TU) is a measurement of time based on the Rotation of the Earth. Such a measurement can be carried out by observing each day the passage of celestial objects to the Méridien of the place of observation. The astronomers privileged the observation of star S compared to that of the Sun, because these observations are more precise. Currently, the universal time is determined by the observations by interferometry at very long base of Quasar S remote, a method which has a precision of about a microsecond.

The rotation of the Earth is not regular, because of effects of Marée of the the Moon and the Sun, as well as earthquakes. To get rid of these effects, the astronomers introduced the terrestrial Temps (TT), determined from now on starting from the international atomic Time (TAI), measured starting from a whole of atomic clocks distributed on Earth. YOU being synchronous with terrestrial rotation, it deviate gradually from TAI; a regular correction is thus added to TAI, in the form of second guides, in order to obtain a civil time which is a compromise which follows TAI but change sometimes in order to avoid to him deviating too much from the average solar time.

Terrestrial rotation and YOU are managed by the International Service of terrestrial Rotation and Frames of reference (IERS). The International Astronomical Union is also implied in the development of the standards, but the final referee of the standards of diffusion is the International union of telecommunications.

Versions

There exist several versions of the universal Time:

• UT0 : Universal time measured in an observatory by observing the diurnal movement of star S or radiosources extragalactic and multiple observations of the the artificial satellite Moon and of terrestrial. It is not corrected a movement of the geographical poles terrestrial compared to the axis of rotation of planet; this polar Mouvement induces a variation of several meters in the geographical position of an unspecified point of the Earth and different observatories measure values different of UT0 at the same time. This value is thus not universal strictly speaking.

• UT1 : obtained by correcting UT0 of the effects of the polar movement to the longitude of the observatory. UT1 is the same one in any point of the Earth and defines a true swing angle of planet compared to a fixed reference frame. The rotation of the Earth not being uniform, UT1 has an uncertainty of ± 3ms per day.

• UT 1R: a smoothed value of UT1, on which the short-term variations (up to 35 days) are filtered.

• UT2 : seldom used and of a primarily historical interest. It is about a smoothed version of UT1. The seasonal variations of UT1 can be removed by using the following conventional correction:

$UT2\; =\; UT1\; +\; 0.0220\; \backslash \; cdot\; \backslash \; sin\; (2\; \backslash \; pi\; T)\; -\; 0.0120\; \backslash \; cdot\; \backslash \; cos\; (2\; \backslash \; pi\; T)\; -\; 0.0060\; \backslash \; cdot\; \backslash \; sin\; (4\; \backslash \; pi\; T)\; +\; 0.0070\; \backslash \; cdot\; \backslash \; cos\; (4\; \backslash \; pi\; T)\; \backslash ;\; \backslash \; mbox\; \{seconds\}$

where T is time, like fraction of the Année besselienne.

• UTC: universal Time coordinated, international standard on which civil time is based. It is based on the international atomic Time (TAI), even based to him on the definition of the second and measured using a whole of atomic clocks distributed on Earth. UTC and TAI differ from an integer of seconds, but UTC is maintained with less than 0,9 S of UT1 by introducing a intercalated Seconde with each time necessary. When the desired precision is not larger than one second, UTC can be used as approximation of UT1. The difference between UT1 and UTC is called DUT1.

See too

Internal bonds

• Greenwich Mean Time
• International Earth Rotation and Refers Systems Service
• Sandford Fleming
• Second guide
• international atomic Time
• barycentric dynamic Temps
• sidereal Temps
• solar Temps
• universal Time coordinated

External bonds

• Scales of Time, giving the current values of the time universal, terrestrial, sidereal, etc
• 'The Birthplace off Standard Time '', commemorative plaque in Ontario, in Canada

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