Is time running out for the 'leap second'?

Unable to reach a consensus, world leaders have delayed a vote on nixing the leap second, keeping atomic clocks tied to the solar day until at least 2015.

Tue, Jan 17 2012 at 12:36 PM

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UN, Environmental Research, Science

Photo: NASA Earth Observatory

UPDATED, JAN. 19: The decision to keep or abandon the "leap second" has been pushed back for another three years, the BBC reports. Delegates at the U.N.'s International Telecommunication Union reportedly couldn't reach a consensus on the issue, so they have punted to a future meeting in 2015. The next leap second is currently scheduled to take place June 30.

Unlike leap years — which deal with Earth's 365.24-day orbit by adding an extra day to the calendar every four years — leap seconds exist because Earth's rotation periodically changes speeds, which makes it an inherently inaccurate way to keep track of the time.

Leap seconds were introduced 40 years ago in response to atomic clocks, which are the most accurate timepieces known. Since atomic clocks rely on atomic radiation rather than sunlight to tell time, they inevitably outpace the gradually lengthening solar day. A leap second, therefore, simply "pauses" all atomic clocks whenever they start getting too far ahead.

The U.S. and several other U.N. countries support scrapping the leap second, since it handicaps our most precise clocks to accomodate our relatively imprecise planet. "This would be an important decision because the problem of introducing the leap second would disappear, and we would have a more steady time than we have today," ITU official Vincent Meens tells the Associated Press.

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According to Judah Levine, a physicist at the U.S. National Institute of Standards and Technology, "most of the people who operate time services favor discontinuing leap seconds," including operators of cellphone networks, financial markets, airports and spaceports. "The main problem is that the leap second is usually implemented by stopping the clock for one second," he tells the AP. "However, the world doesn't stop."

Not everyone wants to nix leap seconds, though, which became clear during this week's ITU meeting in Geneva. China warns the move could make it hard for astronomers to compare long-term data, while the U.K. says it could render Greenwich Mean Time (GMT) meaningless. Canada also has reservations, and even some U.S. officials seem hesitant. "Leap seconds are an inconvenience to the telecommunications people, but there are many other users of time who should be considered," says Ken Seidelmann of the U.S. Naval Observatory.

Without the leap second, atomic clocks would outpace the solar day by about 90 seconds per century, or roughly one hour every 4,000 years. And after a few dozen millennia, that means atomic clocks would say it's afternoon before the sun has risen.

"We have to have some means of making a correction, but at the moment no one knows how that's going to be done" if leap seconds are eventually abolished, Peter Whibberley of the U.K.'s National Physical Laboratory tells CNN.

But, speaking to the BBC on Thursday, Whibberley expressed relief the decision is now delayed. "This result achieves the U.K.'s aims of securing much broader debate and understanding of the consequences of ending the link between UTC [Coordinated Universal Time] and solar time before a final decision is taken," he says. Delegates still plan to discuss the issue at another ITU meeting next week, the AP reports, but no formal decision is expected until 2015.

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Comments: 3

Steve Jan 20 2012 at 7:45 PM

There are perfectly adequate solutions for this, that keep time consistent with the Earth's rotation, provide a steady tick for timing things (no, you don't "stop the clock for a second"), and allow accurately comparing times across time zones and leap seconds. Basically, there already IS a time standard that doesn't have leap seconds, doesn't stay synchronized to the Earth's rotation, and is perfectly usable (and is used). See International Atomic Time (TAI). GPS uses this, for example, along

.... More

with an offset from that value to UTC (which is then converted to a local time for display). Computer systems that use TAI as the underlying clock, with conversions to local time, UTC and any other time standard done with a standard library, work just fine. Unfortunately, the "standard" (POSIX, Unix) time is based on UTC and thus must be adjusted when there's a leap second. Simply changing operating systems to use TAI as the underlying base, with a conversion to POSIX time (based on UTC) for older programs, and using the TZ database for conversions, would make all the problems people are complaining about go away. You can already configure systems to do this, it just needs to be made the standard way of doing it. Fixing NTP to use TAI, along with broadcasting current UTC offset, and a formal method of updating the local leap-second database would basically fix everything while keeping time consistent with the Earth's rotation. Software is going to need to be flexible in the future to deal with time delays (speed of light), different planetary time systems, and even relativity when calculating time, a needless "simplification" now is just going to complicate things more later on. The standard for displaying times when there's a leap second is to either go to a 60th second (when adding a second) or skipping 59 (when deleting a second). This would be a lot messier if the jump was anything more than a second, which is why it needs to be done as often as it is.

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pat Jan 20 2012 at 10:31 AM

Just wait 20 or 40 years and then make a single adjustment to the clocks, ie change it but not as often.

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SixDegrees Jan 20 2012 at 6:41 PM

Sporadic discontinuities are what they're hoping to avoid. They play havoc with software that expects a continuously increasing timescale without such jumps. We've actually been fairly lucky so far that all of the recent leap seconds added have been positive; negative changes are very bad for software and difficult to work around in any reasonable way. Given that the long-term trend is for the earth's rotation to slow, such negative leap seconds will be the rule if things continue as they have.