Leap second to be added in June 2015
The International Earth Rotation and Reference Systems Service (IERS), based at the Paris Observatory, has announced that a leap second will be added to Coordinated Universal Time (UTC) at the end of June this year. The extra second will be added immediately before midnight on 30 June, at 23:59:60 UTC (or 00:59:60 BST).
For many centuries, timekeeping was based on the rotation of the Earth by observing mean solar time at astronomical observatories, with Greenwich Mean Time (GMT) becoming adopted globally. However, the rotation speed of the Earth fluctuates unpredictably, and in the long term is slowing down due to friction caused by the ocean tides. This means that the length of the mean solar day is not consistent enough for use in precise timekeeping.
Today, the atomic clocks housed at the National Physical Laboratory (NPL) and other timing centres worldwide are many millions of times better at keeping time than the rotation of the Earth. More than 400 clocks in around 70 institutes are combined to form the international time scale, called Coordinated Universal Time or UTC. The variations in the Earth's rotation cause GMT to diverge gradually from the extremely stable UTC time scale. To prevent this difference building up to more than 0.9 seconds, UTC is adjusted occasionally with a one-second step known as a 'leap second'.
On average, in recent years the Earth's rotation has been a little slower than atomic time. Since the leap second procedure was introduced on 1 January 1972, a total of 25 leap seconds have been needed. The leap second at the end of June 2015 will be the 26th. Coordinated Universal Time is therefore a compromise between mean solar time (GMT) and atomic clock time, and provides a combination of the stability and accuracy of atomic clocks with a measure of the Earth's rotation.
There is an ongoing debate over whether to abolish leap seconds and allow UTC to gradually drift away from mean solar time.
Read Peter Whibberley's comments on the addition of the leap second in The Guardian
Find out more about leap seconds
For more information, contact Peter Whibberley
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