NASA Reveals June 30 is to Have an Extra Second: Here's Why (VIDEO)

First Posted: Jun 29, 2015 08:10 AM EDT

It turns out that June is getting an extra second. Scientists have announced that Tuesday, June 30, 2015 will receive a "leap" second.

"Earth's rotation in gradually slowing down a bit, so leap seconds are a way to account for that," said Daniel MacMillan of NASA's Goddard Space Flight Center.

A day lasts exactly 86,400 seconds. The duration of one second itself is based on the predictable electromagnetic transitions in atoms of cesium. These transitions are so reliable that the cesium clock is accurate to one second in 1,400,000 years.

The solar day, though, lasts about 86,400.002 seconds. This is because Earth's rotation is gradually slowing down due to a kind of braking force caused by the tug of war occurring between Earth, the moon and the sun. In fact, the mean solar day hasn't been 86,400 seconds long since the year 1820 or so.

The difference between 2 milliseconds, or two thousandths of a second, is hardly noticeable at first. But if this small change were repeated every day for an entire year, it would add up to almost a second.

With that said, the length of a day on Earth is influenced by many factors. However, it's mainly influenced by the atmosphere over periods less than a year. Our seasonal and daily weather variations can affect the length of day by a few milliseconds over a year. Other contributors to this variation include dynamics of the Earth's inner core over long time periods, variations in the atmosphere and oceans, groundwater, ice storage, and oceanic and atmospheric tides.

"In the short term, leap seconds are not as predictable as everyone would like," said Chopo Ma, a geophysicist at Goddard and a member of the directing board of the International Earth and Reference Systems Service, in a news release. "The modeling of the Earth predicts that more and more leap seconds will be called for in the long-term, but we can't say that one will be needed every year."

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