The Leap Second: Interview with Vincent Meens, Head, Frequency Bureau, Centre national d’études spatiales (CNES)
Coordinated Universal Time (UTC) is the primary time standard or international time scale by which the world regulates clocks and time. The international time scale used by distributed services throughout the world, known as Coordinated Universal Time (UTC), is a stepped atomic time scale, as defined by ITU and determined by the International Bureau of Weights and Measures (BIPM) in cooperation with the International Earth reference and Rotation Service (IERS).
The mean solar day – the average length of a day, based on how long it takes Earth to rotate – is about 86,400.002 seconds long. As Earth’s rotation gradually slows down due to gravitational forces and other influences, atomic time (UTC) begins to drift from mean solar time (UT1). It was decided in 1972 to implement a procedure to adjust UTC with mean solar time. Whenever the difference between UTC and UT1, or mean solar time, approaches 0.9 seconds, a second would be inserted, known as the ‘leap second’.
The last adjustment was in June 2012. It was decided to make a further adjustment this year; 30 June 2015 will be 1 second longer than usual as a ‘leap second’ will be inserted at midnight UTC. Normally, clocks move from 23:59:59 to 00:00:00 the next day. On 30 June, UTC will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. In practice, many systems are turned off for one second to compensate.