Time may be running out for leap second

The world’s timekeepers will decide on Thursday whether to break the age-old link between their official clocks and astronomical time based on Earth’s rotation.

If so, the many activities that depend on precise timekeeping, from computing and telecommunications to navigation, will no longer suffer the occasional disruption of adjusting their clocks by adding “leap seconds”. These are added occasionally to synchronise ultra-accurate atomic clocks with the real length of the day, which varies slightly because of irregularities in Earth’s rotation around its own axis.

The International Telecommunication Union’s Radiocommunication Assembly in Geneva will debate a controversial proposal to abandon these leap seconds.

The ITU, the united agency responsible for global timekeeping, declares a leap second every two to three years on average – the next one will be added at midnight on June 30. But unfortunately the need for one cannot be predicted well ahead and programmed into computers, unlike leap years which keep the annual calendar in step with Earth’s orbit around the sun.

Timekeeping has been transformed since leap seconds were introduced in 1972, with atomic clocks now so accurate that they gain or lose no more than a second in millions of years. Over the same period the activities that depend on atomic clocks, from telecommunications and satellite navigation to high frequency financial trading, have grown enormously.

The argument in favour of dropping leap seconds is that millions of high-precision computer systems, including many that are safety-critical, have to be adjusted manually when an extra second is introduced. This is not only time-consuming but also potentially dangerous.

Defenders of the status quo, such as Peter Whibberley of the National Physical Laboratory who will represent the UK at the ITU meeting, respond that there have been no proven examples of trouble after 24 leap second introductions over 40 years.

But their main point, says Mr Whibberley, is “not technical but more of a social or civil question. Do we want to keep the traditional link with astronomical time? I think we do”.

At first no one would notice the gradual divergence of atomic and astronomical time, averaging about a second every 500 days, but after a century they would be several minutes apart.

The ITU says Thursday’s decision is unpredictable. Out of 192 member states, only 16 have disclosed their views in advance. Three, including the UK, oppose the change while 13 countries, including the US, are in favour.

A possible compromise would be to drop leap seconds but maintain the notional link with astronomical time by introducing instead a leap minute every few decades or even a leap hour every few centuries. Or leap seconds might be preserved but announced several years ahead to give more preparation time, which would mean tolerating a wider divergence between atomic clocks and astronomical time than the ITU permits now.