A French decision of international importance
The decision to add a new intercalory second to the international time scale "UTC", is made by the «Earth rotation section - Service de la Rotation de la Terre » of the Service international de la rotation de la Terre et des systèmes de référence (International Earth Rotation Service) – IERS. Housed at the Paris Observatory as part of the SYRTE department (Observatoire de Paris/CNRS/UPMC/LNE), this unit measures the changes in the orientation of the Earth , and is responsible for the prediction and the announcement of these intercalory seconds. The previous insertion was made on July 1st 2015.
Each country is responsible for realizing the change
In France, in the context of a partnership with the "Laboratoire national de métrologie et d’essais", the LNE-SYRTE laboratory of the Paris Observatory which creates the national time standards will thus be responsible for the application of the intercalory second.
International atomic time
To-day, time is measured by methods which do not depend on the moods of the Earth, using almost 400 atomic clocks in many countries, 25 of which are in France. Overall, they enable the BIPM [2] to compute the Universal coordinated time (le Temps Universel Coordonné – UTC). Atomic time is currently determined to a relative frequency accuracy of a few times 10-16. This level of accuracy corresponds to drift of one second in 300 million years.
Irregularities in the terrestrial rotation and the leap second
The rotation of the Earth, responsible for the day and night, slows down regularly due to the tidal effects of the Moon on our planet.
Consequently, the astronomical second – the 86400th fraction of the mean solar day – has lengthened on the average by 3 thousandth of a second since the second half of the XIXth century. Since the length of the atomic second was taken as the length of the astronomical second of that epoch, somewhat shorter than the present astronomical second, terrestrial rotation time passes more slowly than atomic time, on which civil time is based.
Since the gradually increasing difference between UTC and terrestrial rotation time would cause difficulties in certain cases, an international agreement signed in 1972 decrees that the difference between the two time must never exceed one second. That is the purpose of the leap second: when Earth rotation time falls behind UTC by close to one second, the two scales are brought into conformity by adding one second to UTC, i.e. by slowing down UTC by one second.
However, on scales of from 1 to 30 years, the secular slowing down often transforms into an acceleration due to winds or movements in the Earth’s liquid core. That is why the leap second is not introduced regularly, but depends on the observed irregularities in the rotation of the Earth.
Since the signature of the international agreement in 1972, the difference between UTC International Atomic Time (Temps Atomique International - TAI) has constantly increased and will be 37 seconds after Januay 1st 2017.
[1] Until 1960, the second was defined as the 86400th fraction of the mean solar day. Since 1967, it is defined in the Système international (SI) as « 9 192 631 770 vibration periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom».
[2] The Bureau International des Poids et Mesures - BIPM, an international organization in Sèvres, is responsible for the computation at the end of each month of the UTC for the past month. Every country which needs to have an precise time measurement must set up its own approximate version of UTC in real time: the UTC(k). The differences between UTC and UTC(k), given each month by the BIPM, are known de 5 to 6 weeks later. In the case of France, this standard UTC(k) is the Temps Universel Coordonné of the Paris Observatory - UTC(OP) determined by the LNE-SYRTE.