New Year's Eve revelers will have a bit of extra time to enjoy tonight, as officials add a "leap second" to the super-accurate atomic clocks that serve as the world's timekeeping standard.
At 6:59:60 p.m. EST, the 16th leap second since 1972 will be added to keep those "master" clocks in synchronization with humankind's oldest clock, the rotating Earth, whose spin has been erratically slowing for millions of years.
"We don't break out a bottle of champagne or anything," said astronomer Dennis McCarthy, chief of Earth orientation parameters at the U.S. Naval Observatory in Washington. "But if we didn't put leap seconds in, it wouldn't be long before things were way out of whack."
Leap seconds are analogous to leap years, when an extra day, Feb. 29, is inserted in the calendar to compensate for the fact that Earth takes 5 hours, 48 minutes and 45.51 seconds longer than 365 days to revolve around the sun, adding up to about 24 hours in four years.
Without leap years, the seasons would gradually shift away from their calendar guideposts; without leap seconds, it would eventually be "dark at noon," Dr. McCarthy said.
The Naval Observatory, keeper of the United States' master clock, belongs to the International Earth Rotation Service in Paris, which orders leap seconds when the gap between atomic time and rotational time approaches 0.9 of a second.
The Earth's 24-hour turn is being slowed by friction from ocean tides on the surface, as well as global winds and the movement of molten material in the planet's interior, Dr. McCarthy said.
Those factors increase the length of a day by an average 1/1000 to 3/1000 of a second daily, and the cumulative effect is dramatic. Scientists estimate a day was 22 hours long during the time of the dinosaurs, some 100 million years ago.
But the slowdown hasn't been regular, with erratic changes in rate illustrated by the history of leap seconds since they were begun in 1972: Some years had none, some had two, with the intervals ranging from six months to two years.
Before 1972, timekeepers used an error-prone method of compensating for the irregular decrease in spin by adjusting their clocks -- initially pendulum clocks, then quartz crystal clocks -- more frequently.
For the past 34 years, the Naval Observatory has used as its standard timekeeping device the cesium clock. It now has 25 of the incredibly precise atomic clocks, accurate to one-billionth of a second.
Their readings are averaged by computer to obtain the master atomic time, and that value is continuously compared to rotational time to determine when a leap second is needed.
Dr. McCarthy said the rotation rate is primarily measured with radio telescopes thousands of miles apart, listening for signals from quasars deep in the universe and comparing the minute differences in arrival time of the radiation at widely separated points on Earth.
Why do we care about such precision in keeping time? "It mostly applies to navigation and communication," he said. "If you're flying a plane by instruments and you're off by one second, you're going to miss the runway by nearly one-fifth of a mile."
It's of critical importance to the Defense Department's satellite network, the Global Positioning System, to commercial and military aviation and shipping, to the journeys of spacecraft to distant points in the solar system.
Although Dr. McCarthy and his colleagues will be making the official addition in early evening, at midnight Universal Time, the rest of us can pretend along with the crowd at Times Square that the leap second arrives at midnight EST.
The famous ball that has marked the arrival of the new year for 83 years will pause in its descent for one second, for observers "to live and think a positive thought," said Tama Starr, president of the company that presents the annual ceremony.