As the calendar turns from 2008 to 2009, clocks around the world will stutter. Their timekeepers have been instructed by an international committee to add what is called a “leap second” to official time. Timekeepers in Greenwich, England, the location of a world time standard since the reign of Queen Victoria, will tuck the second in just ahead of midnight. The leap second will show up on U.S. East Coast clocks five hours before the big ball drops in Times Square. In Tokyo, revelers will be sleeping off their celebration when the extra second arrives just before the stroke of 8 a.m. on January 1. The leap-second insertion may be the only human event that occurs simultaneously worldwide.
Why do we need it?
Earth’s rotation is slowing. We like to think our planet takes a dependable 24 hours for a single spin, but ocean tides force Earth to give up a bit of its rotational energy. Since 1972, Earth’s loss of oomph has prompted international groups in charge of global reference systems—currently, the International Earth Rotation Service—to ask timekeepers on 23 occasions to add leap seconds onto a broadcast global time standard with the French acronym UTC, for Universal Coordinated Time. (No, 1972 isn’t the first year the Earth became sluggish; our home planet has been slowing down for eons. Earth went around so much faster in the Paleozoic era that a day then was about two hours shorter than a day in 2008.) The seconds have been added to keep Universal Coordinated Time synchronized with the apparent movement of the sun in the sky.
Universal Coordinated Time is a relatively modern system, based on a scale far more stable and predictable than even that offered by planetary motion. (Earth’s spin is an imperfect timekeeper, but it’s no slouch. Over the past 200 years, the length of the day has increased by only 0.0025 second.) UTC is counted by cycles in the frequency of microwaves emitted by an isotope of cesium. Its basic unit, the second—defined in 1967 and one of seven standard units presided over by the International Bureau of Weights and Measures in Sevres, France—is the duration of 9,192,631,770 cycles, or hertz, of that specific energy. The atomic clocks that measure the frequency are the most accurate scientific instruments in existence. They will neither lose nor gain a second over hundreds of millions of years.
According to Peter Whibberley, Senior Research Scientist for Time and Frequency at the British National Physical Laboratory, the clocks were created not because their inventors felt the need to mark time more precisely, but “to keep track of the variation in the Earth’s rotation.” Astronomers have long known that they couldn’t really set their watches by Earth. From timing the lunar eclipses of certain stars, they learned Earth’s rate of rotation was changeable.
The forces that may speed or slow Earth include seasonal effects on oceans and winds, the swirlings of molten metal deep in the core, a tightening of the middle latitudes that is making the planet slightly rounder than before, and thinning of glaciers caused by global warming. It comes down to conservation of angular momentum. Think of a skater doing a pirouette with her arms flung out: whenever she pulls her arms in, her spin rate must go up, to conserve the angular momentum. If Earth’s densest molten rock settles closer to the core, all of us Earth-riders speed up—a little. This may sometimes counter the tidal action slowing us down.
For reasons not yet clear, reports Richard Gross, a geophysicist at NASA’s Jet Propulsion Laboratory in California, Earth ran unusually slow for a few months in 1912, making for the longest days in the 20th century. By contrast, on July 13, 2003, the Earth was speedy enough that it beat the clock by one millisecond, going around in 86,399.999 seconds flat. Still, on average the days of our lives must get longer.
But no expert or computer can predict when forces will combine to slow Earth so much that we need another second again. Although leap seconds have generally been added every year or two, Earth had something unusual going on in its core during a six-year stretch after the New Year’s of 1999. During that period, timekeepers required only one leap second to keep the accounts straight.
How does anybody know that a given day in 2003 took a millisecond less than the standard day? The answer comes from radio antennas spaced on continents around the world, portrayed lovingly but erroneously in the Jodie Foster movie Contact. Together they make up the Very Large Baseline Interferometry, or VLBI, network. Magic with signal processing and precision timing turn the global network into one giant antenna, thousands of miles in diameter. That size gives it very sharp vision in the radio spectrum.
The VLBI network was set up to plumb the depths of the distant universe, the farthest objects of which are quasars, giant galactic cores that blast radio waves and X-rays across billions of light years. Because they are so far away, quasars appear to receivers on Earth almost stationary, so astronomers use them as a fixed frame of reference. Using radio antennas to pick up signals from quasars, scientists can monitor the rotation of Earth with great precision.
Here’s how they do it: Using atomic clocks, geodetic researchers measure the slight time differences between the arrival of a quasar’s signals at several widely separated radio telescopes. The delays in arrival times change as the Earth rotates. Knowing the fixed positions of the telescopes and the changes in the time differences makes it possible to calculate the rate of the Earth’s rotation.
The Jet Propulsion Laboratory needs to keep a close eye on Earth’s rotation because it uses tracking measurements taken by telescopes located on the rotating Earth to help spacecraft navigate around the solar system. That’s why the lab has a geophysicist—Richard Gross—among its astrophysicists.
Because Earth’s movements are so unpredictable, atomic clockmakers can’t helpfully pre-program leap seconds into new clocks. Nobody knows about the next one more than six months in advance.
What timekeepers do know is that the number and frequency of leap seconds required can only increase. “My guess is that we may need more than one in five or 10 years,” says Bill Klepczynski, the former director of time at the U.S. Naval Observatory, “but nobody knows for sure.” The growing complexity of electrical transmission, broadcast, Internet, and telephone systems, all of which rely on precise synchronization, makes frequent insertions risky. The 2005 leap second revealed a programming problem at the Swiss time-broadcasting station HBG, and some “network time protocol” servers on the Internet suffered computer hiccups. Such dangers have prompted several scientific organizations, including the U.S. Naval Observatory, to recommend that leap seconds be discontinued.
Others believe that leap seconds should no longer be inserted in the broadcast time scale, to which society’s sensitive machines are tuned, but should continue to be inserted in global civil time. Observatories, which rely on UTC when steering automated telescopes, have joined to fight off a proposal from anti-leap-seconders to drop the leap second and make a change only every 600 to 900 years, by inserting a full hour instead. “Civil time that tracks the sun means that we keep a conventional meaning of time that is consistent with all of human history,” argues researcher Steve Allen of the University of California’s Lick Observatory. In any case international discussions about changing time take lots of time, so leap seconds are in our future through 2019 and probably longer.