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A clock at Greenwich Observatory, Greenwich, England.

The Shepherd 24-hour gate clock at the Royal Observatory in Greenwich, England.

Photograph by Eric Nathan, Alamy

Brian Handwerk

for National Geographic News

Updated June 29, 2012

Blink and you'll miss it, but this weekend's going to be a little longer than most. Just after 7:59:59 p.m. ET—23:59:59 coordinated universal time—a "leap second" will be tacked on to the world's atomic timepieces.

The chronological change spotlights some of the quirks of an increasingly critical task—keeping the world's clocks perfectly synced.

(Related: "Leap Year: How the World Makes Up for Lost Time".)

Coordinated universal time (UTC) is an atomic time scale derived from a variation of the metallic element cesium's atom. This truly atomic "clock" ticks with microwave light about nine billion times each second, allowing us to slice and dice time with extreme precision.

The official UTC time is set by the Paris-based International Bureau of Weights and Measures, which gathers contributions from labs in some 50 nations and computes an internationally agreed-on average.

Why a Leap Second?

This year's leap second—the 26th to be added to UTC since 1972—exists because time was traditionally based on a full rotation of the Earth and was related to heavenly bodies, which defined the length of the day.

This rotational time, called UT1, divides the day into 86,400 seconds.

But the atomic era demanded more exact timekeeping, and the world began doing business by UTC in 1972.

The two time scales, though, aren't quite in sync, because Earth spins a bit slower each year due to tides and internal processes that create a gap between the two scales.

The International Earth Rotation and Reference Systems Service monitors this difference and periodically inserts a leap second to keep the two in tandem.

Leap Second Pains

The difference between atomic and rotational time is tiny—only an hour or so every thousand years.

But the leap second causes a host of timekeeping issues, because no clock can accommodate an extra second. Instead, clocks are traditionally stopped at 23:59:59 for one second—but life goes on, and gets in the way.

"It might seem stupid to say that you have a difference of only one second," Elisa Felicitas Arias of the International Bureau of Weights and Measures told National Geographic News in 2008.

"But for the stock exchange, one second is important. For an airport, one second is important. For global navigation satellite systems, the difference of a second is unacceptable."

Navigation systems work by measuring the time it takes a signal to travel between a known satellite location and a receiver. Such systems require extreme precision on the level of nanoseconds, or billionths of a second.

"In one billionth of a second, light travels about one foot [30 centimeters]," said Dennis McCarthy of the U.S. Naval Observatory in Washington, D.C.

"So for navigational accuracy, [even a] billionth of a second can be important."

Mobile phone networks have blacked out in past years when their timekeeping got out of sync because of failure to observe the leap second, said Judah Levine of the National Institute of Standards and Technology in Boulder, Colorado.

The electric power grid is also vulnerable.

"Companies share power and it's very important that all generating stations are running at the same frequency," said Levine, sometimes called the nation's timekeeper. "That's a very serious issue."

Goodbye, Leap Second?

Arias of the International Bureau of Weights and Measures is part of a working group that's arguing to abolish the leap second.

"The leap second was created in 1972 because there was a need to have a time scale somehow linked to rotational time," she said.

"At that time celestial navigation was common, and people needed to have time linked to the rotation of the Earth."

"Today we don't need to the leap second for navigation, because the GPS system exists for finding directions in the sea, or anywhere on Earth."

(Related: "An Extra Day for Everyone—Lobbying for Leap Year Status.")

But if the leap second is abolished, it could cause problems for astronomers. Complex adjustments may be needed for research that has long relied on a strong connection between clocks and the sun.

For now, the leap second ticks on, despite the fact that its implications can rattle even the nation's timekeeper.

"It can become super-duper confusing," Levine said.

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