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Leap-second decision delayed by eight years

Some want to scrap adjustment that keeps atomic time in sync with Earth's rotation.

Countries can't agree on whether the benefits of a leap second outweight its drawbacks. Credit: F.G.I CO., LTD./Alamy

A leap second is gone in the blink of an eye. But a long-awaited decision on whether to ditch these occasional time insertions — which ensure that official time is synced with Earth’s rotation — has been delayed for at least eight years.

After representatives who gathered this month at the World Radiocommunication Conference in Geneva, Switzerland failed to agree on whether the costs of the leap second outweigh its benefits, the International Telecommunication Union (ITU) announced that it would defer a decision until 2023, when it will have more information on the impacts of getting rid of the leap second.

Hyper-precise atomic clocks face off to redefine time

The union did, however, make a decision that could shift the responsibility of defining the official Coordinated Universal Time (utc) — and in turn the leap second — to the body that is already responsible for defining the second, along with the other SI units.

Leap seconds are necessary because Earth’s rotation is slowing in an unpredictable way. Without them, the time of day when the Sun is at the highest point in the sky would drift by about one minute over about 100 years. However, these extra seconds have to be programmed into electronic systems manually and can upset systems that depend on accurate timings.

Scrap the second

Most countries, including China, the United States and many in Europe, favour scrapping the leap second and basing utc on the continuous tick of atomic clocks.

Official time would slowly move out of sync with Earth’s rotation, but — given that it would take thousands of years to accumulate a difference that is greater than the kinds of shifts already caused by changing the clocks backwards and forwards for daylight savings time — many argue that this would cause few problems.

Kilogram conflict resolved at last

“If we have an offset from solar time, it is not extremely important,” says Elisa Felicitas Arias, director of the Time Department at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, who wants to scrap the leap second. “We are already shifted by one hour in summer compared to winter time. Are we affected because of that?"

Once the drift is appreciable, the argument goes, a correction could be added much further down the line, perhaps by adding a leap minute or hour.

Cultural argument

A small number of countries however, including Russia and the United Kingdom, want to keep the leap second. Russia is concerned about how GLONASS, its Global Navigation System — the only one to incorporate leap seconds — would cope, says Vincent Meens, from France’s National Centre for Space Studies, and the chair of the ITU subgroup tasked with debating the topic.

Britain’s argument is largely based on the desire to keep a link between official time and Earth’s rotation, says Peter Whibberley, a metrologist at the National Physical Laboratory in Teddington, UK.

Astronomers are among those who would be affected if the leap second were scrapped. Their software would need to cope with Earth's rotational time — which defines when stars and galaxies are seen in the sky — being offset by more than a second from universal time, says Meens.

Language tweak

Historically, the ITU has borne responsibility for the definition of utc, through an international treaty that also governs how nations share radiowaves. But at the Geneva conference, the ITU announced that it would modify the treaty. Rather than having a stand-alone definition of utc, the treaty will only cite an SI definition — and mention of the leap second will be moved from the utc definition-proper to a mere ‘description’ in a subsidiary resolution, which expires in 2023.

Special relativity aces time trial

Whibberley says that that the biggest effect of these seemingly subtle changes will be to remove responsibility for defining UTC, and therefore the leap second from the ITU. The General Conference on Weights and Measures (CGPM), which already has ultimate responsibility for defining SI units, including the second, is most likely to become the authority in the future, he adds.

BIPM, a subsidiary of the CGPM, is responsible for generating International Atomic Time, on which utc is based, from the results of 500 clocks distributed around the world. “In effect, therefore, the BIPM will ‘own’ the definition of utc," says Whibberley, "even if there is no formal process to transfer responsibility.”

The CGPM's involvement is unlikely to mean a decision on whether to scrap the leap second will come sooner than 2023, however: the organization's next chance to even propose a change would not come until 2018.

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Gibney, E. Leap-second decision delayed by eight years. Nature 527, 421–422 (2015). https://doi.org/10.1038/nature.2015.18855

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