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Save the leap second! Astronomers need to mobilize to keep our clocks in sync with Earth's slowing rotation.

EARTH SPINS on its axis once a day, and each day lasts 24 hours, right? Almost. Our planet doesn't complete one turn in exactly 86,400 seconds; it's gradually, measurably slowing down. The rate of this slowdown isn't very regular and can't be accurately predicted. And now the mechanism that has been used to adjust the world's clocks to keep them in step with the planet's rotation is being threatened with extinction.

In order to keep Coordinated Universal Time (UTC), which evolved from Greenwich Mean Time, in sync with Earth's spin, the scientific community agreed that beginning in 1972 a "leap second" could be inserted at the end of a month when needed. Such leap seconds are announced typically five months before they take place.

Since 1972 a total of 22 leap seconds have been added. The last was on December 31, 1998, and the next will be on December 31, 2005. Despite this seven-year gap without a leap second, nothing guarantees that the next interstice will be as long. We do know that leap seconds will be needed more frequently in the future. A half century from now, two leap seconds will be required in some years. And 1,500 years hence, a leap second will be needed every month.

Are these stitches in time really necessary? They are, if we want to keep the Sun crossing the local meridian approximately at noon. Without them, by about AD 2900 midday will occur at 1 p.m. local time.

The decision ultimately rests with the International Telecommunication Union, Radio Communication Sector (ITU-R), but the International Astronomical Union (IAU) and the International Union of Radio Science are also involved. Yet it took even many ITU members by surprise when, in late 2004, US Working Party 7A of the ITU-R recommended redefining the UTC standard and abandoning leap seconds by the end of 2007. The move has upset many astronomers as well as some government officials.

As part of the ongoing debate, the IAU Working Group on the Redefinition of UTC is studying the issue and will report its findings to the IAU General Assembly in August 2006. The chair of this group is Dennis McCarthy (, and if Sky & Telescope readers wish to make their feelings known, they should do so very soon.

Apparently there are people in the communications field who don't appreciate the age-old link between time and astronomy. But those involved with satellite positioning systems (GPS, GLONASS, Galileo) aren't asking for the change; they've adjusted for leap seconds in the past and have no problem doing so in the future. Your hand-held GPS receiver won't show an incorrect place or speed when the leap second is inserted. If the time displayed is ever wrong due to bad reception or any other cause, it's corrected when the next clear signal is received. Radio time services such as WWV in the US or DCF77 in Europe have no problem with leap seconds either; you simply hear 61 beeps, rather than 60, in the final minute before midnight UTC.

Still, computer glitches were reported on the occasions of the 1995, 1997, and 1998 leap seconds, and today even more computer activities rely on a timing accuracy of better than 1 second. The seven-year lull means that many newer systems will experience their first leap second at the end of this month. How prepared is the information-technology (IT) sector? Even less so, apparently, than it was for the Y2K situation, which cost a lot to analyze and correct where needed. But no planes fell out of the sky on January 1, 2000, and a doomsday scenario is not appropriate for January 1, 2006, either. If critical navigation and other systems aren't already hardened against loss of synchronization, we have much more than the leap second to worry about!

Whether or not the leap second is abandoned after 2007, there will be one at the end of this month and perhaps one or two more in the meantime. The methods that have been (and are being) devised by responsible IT people can easily handle leap-second insertions for at least another 50 years. Then--not now--would be the time to take up the profound question: Do we want our clocks to continue counting Earth's rotations, or should we let them drift off on their own?

Electrical engineer Chris Steyaert is a prime mover in Vereniging voor Sterrenkunde, Belgium's foremost astronomical society.
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Author:Steyaert, Christian
Publication:Sky & Telescope
Geographic Code:1USA
Date:Dec 1, 2005
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