Science & technology | Leap seconds

Their time has come

Are leap seconds about to be abolished?

THE phrase “clockwork universe” is more than a pithy tribute to the exactitude of physics. For thousands of years, the movement of the heavens (or rather, as was eventually realised, the movement of the Earth within the heavens) served as exactly that—a clock. It still does. Even the hyper-accurate atomic clocks now used to record the passage of Coordinated Universal Time (UTC), the globe's official standard, regularly defer to the addition of so-called leap seconds. These are introduced every so often by the time lords of the International Earth Rotation and Reference Systems Service. Their purpose is to match the relentless stream of regular 86,400-second days that pour out of atomic clocks with the slight irregularities that the Earth experiences in its rotation around its axis.

But possibly no longer. Next week, the International Telecommunication Union (ITU) is meeting in Geneva, and one of the items on its agenda is the abolition of the leap second. If the assembled delegates vote in favour, then the next leap second (which will be added one second before midnight on June 30th, causing clocks set to UTC to display 23:59:59 for two seconds instead of one) will be one of the last—and the answer to the question “what time is it?” will have ceased to have anything to do with the revolutions of the heavens.

Worrying about a few stray seconds may remind some readers of medieval debates about the precise number of angels that could be crammed onto the head of a pin. But, say the abolitionists, time—even small amounts of it—does matter. America's Global Positioning System satellites, for instance, do not add leap seconds to their internal clocks, and are therefore out of step with UTC. Receivers on the ground can correct for that discrepancy. But the satellite-navigation systems being launched by China, Europe and Russia use still other definitions of time, so exceptions to UTC are proliferating. That has led to worries that mismatched time signals could cause navigation problems, since even small errors in a time signal would mean positions being off by tens of metres.

Electronic communication is another area where leap seconds are unwelcome. Days in which it is a second before midnight on two occasions can confuse software that relies on accurately timed messages to function. Workarounds exist, but they cause problems of their own. After some of its computers failed to cope with a leap second in 2005, Google, a search-engine firm, has begun running its computer clocks slowly for a short period before one is due, making the change gradual instead of instantaneous. But, notes Peter Whibberley of the National Physical Laboratory (NPL), Britain's official measurements lab, that means that while the correction is being applied Google's watches are out of sync with everybody else's. And because leap seconds are needed irregularly their insertion cannot be automated, which means that fallible humans must insert them by hand.

Opponents of abolition say such problems are overstated. Engineers are used to dealing with the vagaries of leap seconds, according to Markus Kuhn, a computer scientist at the University of Cambridge. He thinks that a lot of those who worry about leap seconds do not actually have much experience of how things like satnav systems really work. And the switch would not be without problems of its own. Astronomers would be particularly cross: they rely on time having some connection with the movement of the heavens, so that their computers can point their telescopes at the correct patch of sky at a given time every night. A few years down the line, says Dr Kuhn, people may find that automated commercial satellite dishes, which also rely on an astronomical definition of time to spot their quarry, start to fail. Fixing them could mean wading through millions of lines of ancient computer code.

In the longer term, more dramatic effects would appear. Over the decades, centuries and millennia, atomic time would begin to diverge blatantly from solar time because the Earth's rotation, besides being irregularly variable, is also gradually slowing down (see chart). In about 2,000 years the two measurements would be roughly four hours out of kilter. Eventually, atomic clocks would say it was midday in the middle of the night.

That may seem like a distant worry, but measurements of time can endure for a long time. It has been customary to divide a day into 24 hours for at least 4,000 years, for instance. The final objection is emotional. “Do we really”, ask the leap seconds' defenders, “want to abandon the sun-based reckoning that humans have relied on for their entire recorded history?”

Sadly for the traditionalists, the odds seem stacked against them. An internal ITU poll, conducted last year, found that, of the 16 countries (out of 192) that bothered to reply, 13 were in favour of abolishing leap seconds, whereas only three—thought to be Britain, Canada and China—wanted to keep them. The clockwork universe, then, has had a good run. But its mainspring may be about to break.

This article appeared in the Science & technology section of the print edition under the headline "Their time has come"

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