'Leap Second' Will Make New Year's Eve Just a Little Bit Longer
A "leap second" will be added to the world's official clocks on Dec. 31, 2016, to accommodate Earth's gradually slowing rotation.
Credit: NASA

For one tiny heartbeat at the stroke of midnight on New Year's Eve, a minute will be 61 seconds long.

World clocks will officially add a "leap second" at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC), which is the time standard set by highly precise atomic clocks. These official clocks, which set the time standard for the world, will shift to 23 hours, 59 minutes and 60 seconds before turning to midnight on Jan. 1.

The extra second of party time is designed to reconcile two ways of keeping time: atomic clocks, and clocks based on the Earth's rotation. [5 of the Most Precise Clocks Ever Made]

"Earth is slowing down over geological time, and that can lead to a problem when you've got a ton of clocks," Demetrios Matsakis, chief scientist at the U.S. Naval Observatory's Time Service Department,  told Live Science last year. "What do you do when the day gets longer?"

Historically, time was hung on the rotation of the Earth in relation to far-flung celestial objects. However, the moon's tug on the Earth slows the planet's spin.

In the past century, however, scientists have shifted to using highly precise atomic clocks to count the ticking of the seconds. These atomic clocks, which are often pegged to the vibration of atoms, are so frighteningly precise that they may not lose a second over the entire age of the universe. The current official atomic clock for the United States bases the second on the vibrational frequency of the cesium atom.

As a result, every day, the rotation-based time loses between 1.5 and 2 milliseconds relative to the atomic clock. That adds up to a full second every 500 to 750 days, Live Science previously reported.

To keep these two types of time in sync, in 1972, the International Earth Rotation and Reference Systems Service (IERS), which keeps time for the world, has snuck 26 leap seconds into atomic clock time. The previous bonus second was added on June 30, 2015.

These leap seconds are always added on either June 30 or Dec. 31, according to the IERS.

Originally published on Live Science.