By Edward Cardenas

SOUTHFIELD (CBS Detroit) – Summer will have one extra second Tuesday as an extra second, or “leap” second, will be added to the day.

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The added time is needed because the Earth’s rotation is slowing down, and according to NASA, the extra second is needed.

“Earth’s rotation is 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 in Greenbelt, Maryland, in a blog post.

A day lasts 86,400 seconds according to Coordinated Universal Time, or UTC – which bases the duration of one second on the “predictable electromagnetic transitions in atoms of cesium” – NASA stated.

But the average length of a day based on how long it takes Earth to rotate is about 86,400.002 seconds long. NASA states the slowing of the Earth’s rotation is due to a “braking force caused by the gravitational tug of war between Earth, the moon and the sun.”

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The length of day is also influenced by seasonal and daily weather variations, dynamics of the Earth’s inner core, variations in the atmosphere and oceans, groundwater, ice storage and atmospheric variations due to El Niño.

Over the course of the year, the of 2 milliseconds, or two thousandths of a second add up nearly one second over a year.

Scientists are able to monitor how long it actually takes Earth to complete a full rotation using a technique called Very Long Baseline Interferometry (VLBI). These measurements are conducted by a worldwide network of stations, NASA stated.

Universal Time 1, or UT1, time standard is based on VLBI measurements of Earth’s rotation. But since UT1 isn’t as uniform as the cesium clock, the two time measurement tools drift apart and leap seconds are added when needed. That decision is made by a unit within the International Earth Rotation and Reference Systems Service.

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“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 Rotation and Reference Systems Service, in the post. “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.”