National Physical Laboratory

Leap second to make 30 June longest day of the year

Immediately before midnight on 30 June 2015, the National Physical Laboratory (NPL) introduced the 26th leap second into UTC(NPL), the UK's representation of the international time scale Coordinated Universal Time (UTC).

One of NPL's atomic clocks shows a leap second making a 61-second minute in 1994
One of NPL's atomic clocks shows a leap
second making a 61-second minute in 1994

Following the directive issued earlier this year by the International Earth Rotation and Reference Systems Service (IERS) based at the Paris Observatory, the additional second will help keep the international reference time based on atomic clocks in sync with solar time based on the Earth's rotation.

Atomic clocks are more than a million times better at keeping time than the rotation of the Earth, which fluctuates unpredictably and in the long term is slowing down. Leap seconds are therefore essential to ensuring civil time does not drift away from time based on the Earth's rotation. If not corrected, such a drift would eventually result in clocks showing the middle of the day occurring at night. While it would take hundreds of years for the difference to become obvious to most people, astronomers and celestial navigators rely on time being consistent with the conventional positions of the Sun, Moon and stars to within a fraction of a second.

Ever since their introduction in 1972, the international community has been split as to the need for leap seconds. The UK Government believes that the consequences of breaking the link between civil time and the Earth's rotation are not fully understood, and that the problems leap seconds currently cause can be dealt with by technical improvements. The fate of the leap second is expected to be decided at the World Radiocommunication Conference in November 2015.

Peter Whibberley, Senior Research Scientist in the Time & Frequency Group at NPL, said: "There are consequences of tinkering with time. Because leap seconds are only introduced sporadically it is difficult to implement them in computers and mistakes can cause systems to fail temporarily. However, we have always taken the Earth's rotation as the ultimate reference for timekeeping, and astronomers and navigators still make use of it. We shouldn't break the link without carefully weighing the consequences."

As the UK's home of precise timing, NPL is developing ways to measure time ever more accurately and continuing to improve the performance of its atomic clocks. The next generation of atomic clocks - optical clocks - should achieve accuracies equivalent to losing or gaining no more than one second in the age of the universe.

NPL's Leon Lobo was interviewed for an article in The Independent on the leap second, and was featured in an episode of the BBC News Channel's Click programme, which was filmed at NPL.

NPL's Peter Whibberley was quoted in an article on the BBC website on the leap second, and was interviewed on The Naked Scientists radio show.

Coverage also appeared in The Guardian, The Economist, The Telegraph, and The Times, and BBC World Service's Click

Find out more about NPL's work on Time & Frequency

Find out more about Leap Seconds

For further information, contact Peter Whibberley

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Last Updated: 23 Sep 2015
Created: 30 Jun 2015


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