Friday, June 29, 2012

Summer Get Longer As Scientists Add A Leap Second To Atomic Clocks, Earth Doesn't Rotate At Constant Speed

Scientists at the National Physical Laboratory (NPL) will be adding a leap second at 00:59 BST on 1st July to its atomic clocks, to ensure UK time remains synchronised with international time.

A leap second will be introduced on 30 June 2012 following a decision made by the International Earth Rotation and Reference Systems Service (IERS) earlier this year. This could potentially be one of the last ever leap seconds added, as a decision may be made in the next few years to abolish the practice.

The insertion of the leap second is required as the 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 the 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.

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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.
 
Leap seconds are added to Co-ordinated Universal Time (UTC) to keep the time scale from atomic clocks within one second of that determined by the rotation of the Earth. The time scale produced by atomic clocks is much more stable and reliable than that based on the Earth's rotation, and without leap second adjustments the two would diverge by ever increasing amounts.

There is ongoing debate over whether or not to abolish leap seconds and allow atomic time to gradually drift away from solar time. For now, a decision has been deferred until 2015, but if agreement is reached then to abolish the leap second, the second added on 30 June 2012 could be one of the last.

Some countries have proposed that leap seconds should be abolished because of the difficulties they cause for systems reliant on precise timing, and the time and effort needed to programme them manually into equipment, with the resulting risk of human error. They also argue that the need for predictable timekeeping outweighs that for a link between civil timekeeping and the Earth's rotation.

However, other countries argue that the current leap second system works adequately for the majority of users and the international community needs to be absolutely sure about the long term consequences before making any potentially irreversible changes to it.

Find out more about the World Time System

Contacts and sources:
Natasha Warren
National Physical Laboratory

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