An extra second in 2008

In the scale of international time " UTC ", this additional second, will take place on December 31, 2008, just before midnight. The scientists thus tend to consider that it belongs to 2008. But in France, because of the time shift compared to UTC, of one hour in winter (+1 H), it will arrive actually at 1 a.m. on January 1... 2009.

It is at Paris Observatory that is played this second. Indeed, the department " Reference Frames for Time and Space" -SYRTE, by its activities in the measurement of the Earth rotation and the metrology of time, plays a key role in this event.

The rotation of the Earth on itself which determines the passage of days and nights, slows down on the long term, because mainly of the effects of luni-solar attraction. Moreover, our planet is disturbed by its internal components (core, mantle) and external (atmosphere, oceans).

However, time is measured today by means insensitive to Earth moods, thanks to 250 atomic clocks belonging to several countries of the globe, including 25 in France. Together, they allow to calculate the Coordinated Universal Time - UTC(1).

And the UTC is so regular that it appears quickly a shift between it and the time of days and nights determined by the rotation of the Earth on itself.

This shift which can be awkward for certain applications, an international agreement signed in 1972 stipulates that the difference between the two should never exceed a second (2). This is the reason why additional seconds were created : when the shift between UTC and Earth rotation time approaches one second, the insertion of a "leap" second in UTC allows to readjust the two scales.

Paris Observatory ensures scientific services entrusted by national and international organizations. It is for this reason that a component of the "International Earth Rotation Service" - IERS, located at SYRTE, is responsible for the prediction and the advertisement of these leap seconds. This decision is then implemented by the international and national authorities responsible for the diffusion of time.

For France, the LNE-SYRTE(3) is in charge of this mission. This other component of the SYRTE makes the Coordinated Universal Time of the Observatory of Paris - UTC(OP). This reference of high degree of accuracy is used by the speaking clock of France Telecom, lodged at the Observatory of Paris, to diffuse French legal time ; one can listen to it by composing the 36 99. The legal time based on UTC(OP) is also diffused in a transparent way by encoding on the radio carrier wave of France Inter, so as to be used continuously by laboratories, industrials, communities, etc, everywhere in Metropolitan France(4).

International discussions in progress since several years could lead to a modification of this system. UTC would become dissociated from the Earth rotation and we would not have to add leap seconds any more.

• (2) Until 1960, the unit of second was defined as the 864002th fraction of one solar day, averaged over the year 1900. Today it is defined as 9 192 631 770 periods of radiation corresponding to the transition between two hyperfine levels from the fundamental state of cesium 133.
• (3) A contract between the National Laboratory of Metrology and Tests - LNE, the Observatory of Paris and CNRS created, within the SYRTE, the LNE-SYRTE, laboratory charged to carry out and broadcast the national references for metrology of time and frequencies.
• (4) A partnership between the French Office of the Clock Industry and Microtechniques - CFHM and the LNE manages this means of diffusion of legal time.

Dernière modification le 21 décembre 2021