The world's clocks will get an extra second during a the last minute of the day on Tuesday, June 30. File photo by UPI/John Angelillo | License Photo

WASHINGTON, June 29 (UPI) -- The Earth's rotation is slowing ever so slightly. To keep the world's clock in synch with Earth's circadian rhythm, time-keepers are forced to squeeze in an extra second now and again.

On Tuesday, June 30, the last minute of the day will feature an extra second. A 61-second minute will mark the transition from Tuesday to Wednesday, the world over. The last leap second was added in 2012.


"Earth's rotation is gradually slowing down a bit, so leap seconds are a way to account for that," NASA's Daniel MacMillan explained in a statement.

Of course, analog clocks will have to be adjusted manually. But for digital clocks synched to Coordinated Universal Time (UTC), the additional second will be added automatically.

UTC time -- the system that governs most civil times systems around the world -- is regularly synched with the Earth's rotation. But it's governed by an atomic clock which measures the length of a second based on changes in the amount of radiation emitted by caesium atoms.

But because the Earth's rotation is slowing every so slightly, while atomic decay remains constant, the system has to be periodically recalibrate.

There is always some worry that manipulation of the UTC system will cause problems for digital and Internet-based programs that rely on regular time to function. But on the many previous occasions when extra seconds have been added-in, anomalies have been minor in nature and fixed relatively quickly.

Still, the adjusted time does require some computer server systems to adapt. Google has developed a program to slowly work in slivers of the extra second into their computer systems over the course of the day.

But writing the code to enable said adaptation requires some preplanning -- a warning -- and scientists can't always predict when the next leap second will be necessary.

"The leap second is a hiccup in the time scale that's not predictable," John Lowe, an expert in time-keeping at the National Institute of Standards and Technology, told Slate. "If you're writing code right now you know when every leap day is going to occur all the way into the future. But leap seconds can't be predicted. There's five or six months of advanced notice, but that can be a problem for long-term programs that are already written."