National Physical Laboratory

The end of the leap second?

In Geneva, Switzerland, representatives from countries all over the world are gathered at the International Telecommunications Union's Radiocommunication Assembly, which runs from 16-20 January 2012. They are discussing a change to the world time system (Co-ordinated Universal Time or UTC) that is based upon a global network of extremely accurate atomic clocks such as those kept at NPL.

Clock (Image courtesy of iStockphoto)
(Image courtesy of iStockphoto)

The proposed change concerns a feature of UTC known as 'leap seconds'. These are extra seconds that are added to, or taken away from, the timescale every few years to ensure that atomic clocks match up to the rotation of the Earth. This is done because the rotation of the Earth varies unpredictably, and is gradually slowing down over time due to the action of ocean tides and changes within the Earth's core. The time signal received from atomic clocks is more constant, and therefore a more accurate source of time, than the rotation of the Earth.

Many people consider a day to be the period of time it takes for the Earth to complete one rotation, and refer to Greenwich Mean Time (GMT), which is based on the sun passing over the meridian line at exactly 12 noon. The more practical definition of the day used by the world time system is of 86,400 SI seconds, as defined by atomic clocks. Although GMT has not been used as the practical definition of time for over 40 years, by adding leap seconds to the time system, we ensure that the realisation of time is synchronised with the Earth's rotation and is always within one second of GMT.

Some countries have suggested the abolition of leap seconds because technologies such as satellite navigation systems and communications networks require an extremely stable continuous timescale to operate correctly but leap seconds have to be added manually, as it cannot be accurately predicted when they are needed. This is inconvenient and introduces the potential for error.

However, the removal of leap seconds will mean that the atomic timescale will gradually drift apart from the solar timescale. This will take a long time, it will be about 50 years before the difference reaches a minute and several hundred years before it reaches an hour, but eventually some adjustment will be needed to ensure that the two timescales do not diverge too far.

After considering all of the evidence, for and against the change, the UK government position is that UTC with leap seconds provides a viable basis for civil timekeeping worldwide and there is no urgent need to change its definition. It is therefore against the proposal to remove leap seconds from UTC and would welcome further discussion among all users of precise time, with the aim of securing greater consensus before any change is made.

David Willetts, Minister of State for Universities and Science, said: "The UK position is that we should stick to the current system used throughout the world. Without leap seconds we will eventually lose the link between time and people's everyday experience of day and night."

NPL's Peter Whibberley is representing the UK in Geneva. An announcement is expected before the end of the Assembly as to whether or not we will keep the current leap second system, or whether further discussions are needed before any action is taken.

The debate over the abolition of leap seconds has been covered in The Economist, Financial Times, The Observer and The Telegraph.

Coverage has also appeared on the BBC website and on Channel 4 News.

More on the leap second debate.

More information about the world time system and atomic clocks.

More on NPL's work on Time and Frequency.

Last Updated: 5 Mar 2012
Created: 17 Jan 2012