If 2015 seems a tiny bit longer, there’s a reason

If you were hoping to find a little more time for yourself or your loved ones in the new year, you are in luck: 2015 is going to be exactly one second longer than 2014.

The “leap second” was decreed last week by astronomers at the International Earth Rotation and Reference Systems Service in Paris who measure Earth’s rotation and compare it to the time kept by atomic clocks. It’s the 26th time this has happened since atomic clocks started governing our time.

“It’s not like the leap day, which everyone knows about years in advance,” said John Lowe, who works at the National Institute of Standards and Technology in Boulder, Colo. “It’s not predictable. Nobody knows when the next one will be.”

The extra second will be tacked on to the final minute of June 30. On that day, the official atomic clocks that keep Universal Coordinated Time will mark the time as 23h 59m 59s, followed by the leap second 23h 59m 60s. July 1 will continue as usual.

The international timekeeping community has two ways of measuring the passing of our days. The first, known as astronomical time, is based on how long it takes Earth to make one complete spin on its axis.

Atomic time, on the other hand, defines a second as exactly 9,192,631,770 oscillations of a cesium-133 atom. This is what determines the time that displays on your computer or cellphone.

When the 13th General Conference on Weights and Measures came up with atomic time back in 1967, it was designed to be in sync with astronomical time. But it hasn’t always worked out that way.