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 a leap second will be added to UTC on 30 June 2012 at 23 h 59 m 59 s UTC.|
This will be the 26th occasion since 1972 that a leap second adjustment has been made. The previous leap second was introduced on 31 December
The application of the leap second will be made at the same time (UTC hour) on all clocks maintaining local representations of UTC in the world.