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

This is the case according to the time standard that people use in their daily lives - Coordinated Universal Time or UTC. UTC is 'atomic time' -- the duration of one second is based on extremely 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. However, the mean solar day -- the average length of a day, based on how long it takes the Earth to rotate -- is about 86,400.002 seconds long.

"That is because the Earth's rotation is gradually slowing down a bit owing to a kind of braking force caused by the gravitational tug of war between the Earth, the Moon and the Sun," the US space agency said in a statement.

Typically, a leap second is inserted either on June 30 or December 31.Normally, the clock would move from 23:59:59 to 00:00:00 the next day. But with the leap second on June 30, UTC will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1.



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