In the 1970s the calculation of atomic time replaced the calculation of time based on the irregular rotation of the Earth. Studies of the Earth's dynamics show that the velocity of the Earth's rotation is decreasing, and in consequence a rotational day is shorter than a day of 86 400 atomic seconds.
When atomic time was adopted, some communities of users in particular those using celestial navigation requested that atomic time be synchronized with the rotation of the Earth. To compensate for the Earth's irregular velocity of rotation, the International Telecommunication Union (ITU) defined in 1972 a procedure for adding (or suppressing) a second as necessary, to ensure that the difference between the international time reference and rotational time remained less than 0.9 s. The resulting time scale is Coordinated Universal Time (UTC), the atomic time scale maintained at the BIPM with the contribution of 69 national institutes that operate about 400 atomic clocks.
The International Earth Rotation and Reference Systems Service (IERS) is responsible for monitoring the Earth's rotation and announces the dates of application of any leap seconds required, usually timed for the end of 30 June or 31 December.
|The IERS has announced that no leap second will be added to UTC on 31 December 2013.|
The difference between Coordinated Universal Time (UTC) and International Atomic Time (TAI) will remain unchanged at 35 s until further notice.