The French legal time

The French legal time

The French legal time was defined on August 9, 1978 by law : "the legal time is got by adding or substracting an integer number of hours to the Coordinated Universal Time", this integer number being defined by law. The legal French time is made and broadcast by the LTE in Paris Observatory.
The phone clock of Paris Observatory is accessible through phone call (on 3699).
The Coordinated Universal Time (UTC) is the world standard time issued from the International Atomic Time (TAI) made by BIPM (Bureau International des Poids et Mesures) and from the time scale UT1provided by the IERS (Service International Earth Rotation Service).
TAI is made from several hundreds of atomic clocks spread all around the world.
UT1 is a time scale directly related to the Earth rotation determined by the IERS essentially from the observation of extragalactic quasars extra galactiques using VLBI (Very Long Baseline Interferometry). UT1 is not uniform since the rotation of the Earth around its axis is slowing down because of the gravitational effects of the Moon and the Sun. More, the rotation of the Earth is perturbated by its internal structure and by atmosphere and oceans which modify its rotation.

An universal time scale

Since 1972, the Coordinated Universal Time (UTC) is linked to the Universal Time UT1 thanks to the adding of leap second in the time scale UTC. These leap seconds are added if necessary either on June 30 or on December 31 at 23h 59m 60s in order to have a difference between UT1 and UTC always smaller than one second of time. It is the International Earth Rotation service (IERS) in the LTE laboratory which is in charge of the calculation of this leap second and to predict when it will be added in UTC. This decision is then proposed to the national and international authorities in charge of the diffusion of the time. The dates where a leap second will be introduced is not predictable with accuracy. At the present time, it occurs every 3 or 4 years. On June 30, 2012, the 25th introduction of leap second occurs, all positive making the difference between TAI and UTC to 35 seconds.