UTC is a time scale between:

• the International Atomic Time (TAI), stable but disconnected from the Earth's rotation;
• the Universal Time (UT), directly linked to the rotation of the Earth and therefore slowly varying.

The term "coordinated" indicates that the Coordinated Universal Time is actually identical to the International Atomic Time (he has the stability and accuracy) to a whole number of seconds around, allowing it to stick to Universal Time 0, 9 s close.

Coordinated Universal Time was short in UTC instead of CUT corresponding to the acronym in English or TUC corresponding to the acronym in French. Indeed, if the experts of the ITU agreed to define a common abbreviation in all languages, they were divided on the choice of language. Finally, the UTC compromise, requiring an effort of both parties, was chosen. This notation is used by the standard ISO 8601.

UTC and International Atomic Time

UTC Europe

The problem of the Universal Time UT is that it defines the day as the average length of Earth's rotation around its axis. However, this rotation is not constant, it slows down slowly under the action of tides and, moreover, has irregularities unpredictable duration of the days UT increases very slowly on average. But in the 1960s and until recent years, several activities (celestial navigation, tracking of space probes, etc.) always needed in UT, that is to say, always referred to the Earth's rotation, while requiring a time scale as stable as possible.

Initially, before the establishment of TAI, issued by the atomic time clocks atomic frequency was changed to monitor the Earth's rotation and make the difference UTC-UT remains in a fixed limit. This system soon became too cumbersome and complicated to implement. To address these problems in 1972 we instituted a International Atomic Time (TAI) and is tied intangible UTC to TAI.

UTC has the same course and same frequency as TAI, but differs by an integral number of seconds. To do so, UTC is occasionally incremented or decremented by a second atomic whole, to ensure that the difference between UTC and Universal Time UT is less than 0.9 s, while maintaining a gap of an integer second atomic with respect to atomic time.

The International Atomic Time is set by the International Bureau of Weights and Measures (BIPM) ( Pavillon de Breteuil in Svres in France ) from some 349 (December 2008) atomic clocks in cesium distributed worldwide.

Details and Implementation

To keep the Coordinated Universal Time synchronized with the rotation of the Earth , a leap second is occasionally added or subtracted at the end of the month of June or December (in English "leap second"). So far these leap seconds have been added, never subtracted: UTC time delay so the TAI time. These insertions are not systematic, they are determined by the International Earth Rotation and Reference Systems Service (IERS), based in particular at the Observatoire de Paris, in the light of developments in the Earth's rotation.

To add or subtract a second, counting the time displayed by the atomic clocks is simply modified:

• In normal times, the transition from midnight UTC, atomic clock would indicate: 23 h 59 min 59 sec, then: 00 h 00 min 00 s.

• To add a second you see him: 23 h 59 min 59 sec, then: 23 h 59 min 60 s, and finally: 00 h 00 min 00 s. A second atomic clock on which we would not be reached then display 00 h 00 min 01 s: the first has taken a second behind.

• To remove a second, he would show: 23 h 59 min 58 sec, then: 00 h 00 min 00 s. Again, a second atomic clock on which it would display would not be reached then 23 h 59 min 59 sec: the first has taken a second.

Changing the display automatically. Simply set the date on which the leap second to be held.

History of the difference between UTC and TAI

• ¹January 2006 : Adding a second, the gap increased to 33 seconds after standing at 32 s for 7 years (since 1999 ).
• ¹January 2009 : Adding a second.

The current mismatch between UTC and TAI, since 1 January 2009 , is 34 seconds UTC = TAI - 34 seconds.

UTC and GMT

Using the old standard naming time using Greenwich Mean Time (GMT, English Greenwich Mean Time ) is now discouraged because its definition is ambiguous, instead of UTC, which is to be preferred.

This acronym was imposed by the preponderance of the Marine Corps during the ^nineteenth century and was later renamed Universal Time (UT, English Universal Time).

See also

Related articles

• Time Zone
• Leap second
• International Bureau of Weights and Measures
• International System of Units
• International Telecommunication Union
• WWV (radio station)

External Links

• (En) World Time
• (In) real-time UTC
• (In) IERS

• Science portal

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