The Sciences

'Leap Seconds' Slow Clocks to Match Calculations

Every year or so we figure out that Earth hasn't been rotating quite as fast as we thought, so we add a second to the clock. And that messes up everything.

By Karen WrightMar 28, 2004 6:00 AM


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Time is fundamental to so many things that we do, people take it for granted,” says physicist Ronald Beard of the U.S. Naval Research Laboratory in Washington, D.C. “But it’s a generated thing, not a naturally occurring phenomenon we’re just monitoring. We actually make time.”

Nobody knows that better than Beard, who is trying to corral a rancorous constituency of experts into reaching an agreement on what time it is. The experts are arguing about the future of the leap second, an extra second added to the world’s clocks every year or so to keep the artificial constructs of hours and minutes in lockstep with the actual length of the day.

Leap seconds are to a clock what leap days are to the calendar. An extra day is added every fourth February because the planet takes 365 and a quarter days to circle the sun. So an ordinary calendar year is a few hours shy of the actual length of a year, and leap days keep the calendar in sync with Earth’s orbit. Similarly, the standard 24-hour day is slightly shorter than day length as determined by the rate at which Earth turns on its axis. That’s because the moon’s gravitational pull is constantly slowing Earth’s rotation. It takes a network of ground-based radio telescopes, triangulating from the most distant objects in the universe, to determine Earth’s rotation day by day, hour by hour, minute by minute. Leap seconds “slow” clocks to match these calculations.

Leap seconds are also like leap days, says Judah Levine, a physicist in charge of time services at the National Institute of Standards and Technology in Boulder, Colorado, in that “once they’re inserted, they’re forgotten.”

But computer software designers haven’t adapted very well to the occasional added second, so experts in air traffic control, satellite communications, and electronic fund transfers have been lobbying to abolish the tinkering. A leap second may have caused the Russian satellite navigation system to crash for hours, and critics claim the added instants could cause commercial airliners to crash as well. “A one-second jump can cause significant problems for systems that require continuous, uninterrupted time,” Beard says.

If it seems strange to contemplate changing time to suit a computer, that’s because most people don’t realize how time is made. At the standards institute, Levine presides over more than a dozen atomic clocks—the most accurate, uniform timekeeping devices ever invented. Yet the clocks rarely agree. So Levine calculates an average time using a computer program that accounts for each clock’s offsets. Then he transmits the average in a series of digital ticks. People running communications networks, satellite navigation systems, and multimillion-dollar telescopes take note.

Levine and his counterparts at 50 timing labs in other countries also send their signals to the International Bureau of Weights and Measures in Paris, the agency that determines the world’s official atomic time. The bureau takes the weighted average of about 200 clocks and publishes it in Circular T, a list of official times for every fifth day of the preceding month. Clocks in timing labs across the globe can then be synchronized, albeit in retrospect, to a billionth of a second.

“It’s like, ‘Remember last Thursday? Well, last Thursday you were five nanoseconds off,’ ” says Levine.

And every so often, another international agency tells Levine and his peers to add an entire second to the time given by their atomic clocks. When the International Earth Rotation Service calls for it, timing labs in all time zones add a leap second just before midnight in Greenwich, England. Leap seconds create the civil time used by TV stations, hospitals, factories, schools, airports, and wristwatches. All told, 22 seconds have been added since the practice was formally adopted more than 30 years ago. “In 1972 the leap second was considered a step forward,” says astronomer Dennis McCarthy, who runs the Washington-based time directorate for the International Earth Rotation Service.

But today many computer-based systems, including Global Positioning System satellites, use uncorrected atomic time. Foes of the leap second fear that growing discrepancies between atomic time and civil time could sabotage military operations, disrupt financial markets, disable cellular phones, and even cause midair collisions. Commercial jets, for example, use atomic time–based GPS satellites to set their flight paths, but air traffic controllers typically use civil time. “You have to be very careful that everybody understands that the timing by which we navigate and the timing by which we live are very different,” adds astronomer William Klepczynski, a senior analyst in the State Department’s Office of Space and Advanced Technology. “It makes for an operational headache.”

But most astronomers like leap seconds. They use time as a proxy for Earth’s position in space. If time is divorced from Earth’s rotation, they say, they won’t know when to aim their telescopes where. “It may take hundreds of years for it to matter to civil time,” says Steve Allen of the Lick Observatory in Santa Cruz, California. “But we [astronomers] would have to rewrite the software that points many of our telescopes within five years of discontinuing leap seconds.”

Allen and other proponents of the status quo also raise the larger issue of whether humankind is ready for time and sunlight to go their separate ways. “Time to most everything on the planet is Earth turning around and the sun going up and down,” he says. “Atomic time is a bunch of cesium atoms vibrating. It doesn’t know about day or night, months or years. It’s forcing the question on humanity: How much do you care about when the sun comes up?”

The most ancient clock, the sundial, couldn’t help but measure the actual length of a day. The Egyptians divided each day into 12 hours of light and 12 hours of darkness, but the intervals represented by those hours changed with the seasons. A summer daylight hour, for example, lasted longer than a winter one. “It was not until the fourteenth century that an hour of uniform length became customary due to the invention of mechanical clocks,” Beard writes with several coauthors in a recent review of the leap-second debate. “These clocks were significant, not only because they were masterpieces of mechanical ingenuity, but also because they altered the public’s perception of time.”

From that moment on, the public perceived seconds, minutes, and hours as fixed intervals. But fixed relative to what? For most of history, there were only two possible reference points. One is Earth’s rotation, or day length, which can be divided into seconds, minutes, and hours. The other is the length of Earth’s orbit around the sun, or year length, which can then be broken into smaller units. In the past century, technological advances both provided and necessitated more precise measurements of time. The requirements of specialized machines drove a proliferation of timescales customized to each user: Universal time, sidereal time, ephemeris time, barycentric time, and terrestrial time are just a few examples. Every timescale, no matter how sophisticated, was based either on the length of the day or the length of a year. The goal in every case was to define a uniform, unchanging second.

Atomic clocks, introduced in the 1950s, were able to provide a consistent measure of time intervals independent of Earth’s motion in space. Atoms in these clocks resonate at uniform and predictable frequencies as they flip-flop between energy states. But the frequencies still have to be calibrated, either to Earth’s rotation or its orbit. Because the orbit provides a more uniform timescale, it was used when the atomic second was defined a half century ago. A second, members of international standards conferences agreed, represents both 1/31,556,925.9747 of a year and 9,192,631,770 transition periods in cesium atoms. For physicists and engineers, this formulation passed for progress.

But because a second defined in that way fails to account for the slowing of Earth’s rotation, it wreaked havoc with celestial navigation, which in the 1960s was still guiding ships across the globe. “It was so uniform that it didn’t conform to the nonuniform length of the day,” Beard says. Specifically, the second based on year length and atomic resonance is shorter than the second based on day length. So, beginning in 1972, another international body agreed to add leap seconds to atomic time to create a civil timescale that was both uniform and consonant with day length.

Since then, satellites have usurped stars in navigation, and most watches are based on computer chips. If leap seconds were as predictable as leap days, they wouldn’t be so troublesome to computer programmers. “People ask us why we can’t just tell them when the leap seconds are going to be,” says McCarthy. “But it’s not that simple.” Earth’s rotation is erratic. Although it has lost three hours in 2,000 years, within that slowing there are random fits and starts. The moon’s gravity brakes the spinning planet in weekly and monthly waves, and Earth’s core shifts in irregular cycles that can hasten or retard rotation. Even ocean currents cause variations.

McCarthy says the discrepancy between Earth-based and atom-based timescales will become “dangerously annoying” as the two continue to diverge. “In about 50 years we will be putting in leap seconds at the rate of a couple a year,” he says. “Do you want to be doing that? Probably not.”

Yet developing new standards of time will not be easy. The simplest options seem to have the most extreme consequences. Abandoning leap seconds, for example, would create chaos for world governments, because most national legal codes and international treaties are based on civil time. Redefining the atomic second to conform to current day length “would alter the value of every physical measurement and render obsolete every instrument related to time,” write Beard and his coauthors in their review.

Last May Beard led a group convened in Turin, Italy, by the International Telecommunication Union to consider other ways of redefining time. Some recognize that 21st-century civilization may no longer need a daily accounting of Earth’s rotation. Leap seconds could be inserted every four years along with the February leap day, for example, or leap minutes could be added every half century or so. If such solutions seem awkward and unnatural, it may be because we tend to think of time as the governor of our lives, when in fact it is we who govern time.

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