The insertion of the leap second is required as Earth does not rotate at a constant speed, whereas atomic clocks, several of which are located at NPL's site in Teddington, are much better at keeping time. Due to the unpredictable nature of Earth's movement, leap seconds are occasionally required to bring atomic time back into alignment with astronomical time. This procedure ensures that on average the Sun remains overhead at noon.

Peter Whibberley, Senior Research Scientist in NPL's Time and Frequency Group, said: "The purpose of leap seconds is to make sure our time scale based on atomic clocks remains in step with the time based on the Earth's rotation. The Earth is a poor timekeeper compared to our clocks, and its rotation changes unpredictably due to changes in its atmosphere and molten core. The leap second correction to our atomic clocks means we get an extra second of summer time."

Historically, time was measured using the passage of the Sun across the sky -- Earth rotation time is still used by astronomers to track stars and spacecraft. Since the start of the 1960s, an atomic time scale, known as Coordinated Universal Time (UTC), has been the world's official time. The stability and global availability of UTC are essential for the smooth operations of satellite navigation and international telecommunications.

Over the last decade there has been considerable debate about the detrimental effects of inserting a leap second on computers and other equipment needing precise time. One minor effect is that some systems fail to implement a leap second at the correct instant and display an inaccurate time, but there is no agreement on the seriousness of this and other problems attributed to leap seconds.

The decision to introduce this year's leap second was taken by the International Earth Rotation and Reference Systems Service (IERS), and timekeeping experts at NPL and other national timing centres will make the necessary changes to the atomic time scales on 00:00 30th June (UTC).

The future of the leap second is one of keen debate among the official international time measurement community. Some countries, including the US, have called for an end to leap seconds, but other countries disagree, and experts at the International Telecommunication Union (ITU) have delayed a decision on the future of the leap second until 2015.

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