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Those who can't wait to see 2008 come to a close will have to endure an extra second — literally. On Dec. 31, just before 7 p.m. Eastern time, a leap second will be added to atomic clocks around the world to realign Coordinated Universal Time (UTC), the international standard for atomic clocks, with Earth's rotational period. The reason for the intermittent mismatch between these two measurements lies not with the clocks but with the movement of the planet, which is decelerating at an average rate of two milliseconds a day.
Space dust, magnetic storms, solar winds and the Earth's atmosphere all create drag, which slows down the planet. Even the amount of snow covering the polar ice caps adds to the rotational lag. But one of the main obstacles is tidal friction. Because the gravitational pull between the moon and the Earth is not uniform, the tidal force stretches the Earth — core, mantle, crust, oceans and all — producing bulges. The Earth's rotation pushes the tidal bulge slightly ahead of the Earth-moon alignment; the moon's gravity, however, yanks the bulges back to keep them in line. This tug-of-war — essentially a transfer of energy between the Earth and the moon — boosts the moon's orbital momentum, pushing it away from the Earth at a rate of 1.6 inches a year (the moon's distance from us will increase by the length of a football field over the course of 2,250 years), while reducing the Earth's rotational speed and lengthening the day. (See pictures of Earth from space.)
No one could truly appreciate the global slowdown until the invention of the atomic clock, which uses the oscillation frequencies of atoms such as cesium, hydrogen or rubidium to mark the passage of time. According to Andrew Novick, an engineer with the time and frequency division of the National Institute of Standards and Technology (NIST), there exist three types of atomic clocks: primary standard clocks, which are state-of-the-art instruments owned by only a handful of nations, such as Germany, Britain and the U.S. (there's one at NIST); smaller, rack-mounted commercially available versions that can cost as much as $40,000; and widely available radio-controlled clocks, whose time is set by a daily radio signal received from a primary standard clock — usually the one at NIST's headquarters in Boulder, Colo.
Without the unerring measurement provided by atomic clocks, we couldn't have landed a rover on Mars, the Internet wouldn't be able to process data superfast and GPS navigation would be a fantasy. These clocks are so precise that they literally redefined time: Once tied to the mean solar day, the official measure of a second was changed in 1967 to refer to the duration of more than nine billion periods of radiation between two levels of the cesium 133 atom.
If scientists did not periodically correct the difference between the atomic clock and Earth's rotation, within a few hundred years the position of the sun in the sky would noticeably differ with the time on your kitchen clock, an aspect of earthly timekeeping that has caused much consternation historically, vexing everyone from Julius Caesar to Pope Gregory XIII. So, in 1972, an international agreement decreed that instead of continually revising the definition of a second, atomic clocks would be adjusted by adding a leap second each time an appreciable discrepancy was detected by observations made at the International Earth Rotation Service in Paris. Since then, 24 leap seconds have been added, either on June 30 or Dec. 31; the last one was tacked on in 2005. At that rate, "it will be 5,040 years before we gain an extra hour," Novick says.
For now, you needn't worry about resynchronizing the clocks on your electronic devices; they'll adjust themselves. Cell phones, for instance, will receive a signal from a cell-phone base station, many of which often rely on commercially available rubidium atomic clocks. But if you would like to witness the leap second pass with your own eyes, log on to NIST's Web clock shortly before midnight Greenwich Mean Time, and watch as 23:59:59 changes to 23:59:60, a feat that only NIST's clock can achieve.
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- Danielle Dowling