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So you’d like a little more free time to kick back during this holiday season? Enjoy New Year’s Eve’s leap second.

On Wednesday, at 11:59:59 pm Universal Time (6:59:59 p.m. Eastern time), atomic clocks around the world will add one second to the day.

The idea of adding time at the end of the year dates back to the ancient Egyptians, who added a day every four years. But the leap second is a newcomer. This year, the world’s timekeepers will add its 24th leap second since 1972. Wednesday’s extra second will bring the world’s high-tech atomic clocks back into sync with time as defined by Earth’s rotation.

Why does the world need leap seconds? Chalk it up to the moon’s braking action on Earth’s rotation and to modern timekeeping that has become so precise it can make your head spin. Indeed, as timekeeping becomes more accurate and portable, it could improve everything from GPS navigation to cellphone reception.

Scientists developed the first atomic clock in 1949. As the clocks improved, researchers found themselves using timepieces of a precision beyond exquisite. Today’s versions might gain or lose one second after 60 million years.

These clocks became the standard for global timekeeping. No swinging pendulums or vibrating wristwatch-scale quartz crystals here. One second is formally defined as 9,192,631,770 cycles, or “ticks and tocks,” of microwave energy emitted or absorbed by cesium-133 atoms under carefully controlled conditions.

Meanwhile, researchers were also taking the measure of Earth’s rotation with greater precision, using a global network of radio telescopes aimed at quasars – the active cores of galaxies in the distant early universe. Quasars are so far away that they constitute the best frame of reference for measuring Earth’s rotation rate.

The moon is the single largest influence on Earth’s spin, slowing it by an average of 2 milliseconds per century. Since Earth’s rotation rate varies, so would the value of one second when it’s defined as a fraction of the time it takes for one spin of the Earth on its axis.

So where a leap year periodically makes up for the difference between a year on the calendar (365 days) and a year’s trip around the sun (365 days plus 6 hours), the leap second makes up the difference between an atomic clock’s second and one second as defined by astronomical time keeping.

At the US Naval Observatory in Washington, its newest atomic clock is getting a climate-controlled room of its own for the first time. The hardware forming the core of the new clock tips the scale at some 800 pounds, says Geoff Chester, a spokesman for the observatory. One component looks like a six-foot water heater.

That works for a 24/7/365 service whose job is to act as the nation’s timekeeper. But for a range of applications – from better GPS systems to more jam-resistant navigation gear – smaller may be better.

The current generation of portable atomic clocks is roughly the size of a pack of playing cards. But the devices need a car battery to run them. John Kitching and colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colo., are designing atomic clocks that range in size from a sugar cube to a grain of rice. Their goal: mini atomic clocks that can operate on AA batteries.

With mini atomic clocks, hand-held or dashboard GPS navigation devices would pick up GPS satellite signals faster. They would require fewer satellites in view to get a good reading on a position – a feature handy for driving amid tall buildings. And for the military, tiny clocks in navigation gear would make jamming weak GPS satellite signals more difficult.

Such devices could also improve the reliability of cell phones, whose signals must be carefully timed as they travel from phone to tower. Protoypes are accurate to within 10 microseconds per day, or roughly 1 second per 274 years.

As for larger atomic clocks, current models measure the interaction of microwaves with electrically charged atoms, or ions, to measure one second. NIST researchers are working on a new generation of clocks that replace microwaves with light. A prototype “optical” clock  is so stable it runs neither a second too fast or too slow for at least 400 million years.

It could result in more secure transfer of data over the Internet, as well as increased capacity of existing network pathways to handle more Web traffic.

And there are the unanticipated uses. NIST physicist James Bergquist notes that the idea of using a constellation of satellites for navigation came up only after atomic clocks were developed. He says something similar could well happen with the quantum leap in accuracy that optical clocks promise.

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