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June 30 will have an extra second: NASA

Washington: June 30 this year will officially be a bit longer than usual because an extra second, or 'leap' second, will be added to the day, according to NASA. "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 Centre in Greenbelt.

Strictly speaking, a day lasts 86,400 seconds. That 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 Earth to rotate-is about 86,400.002 seconds long.

That's because Earth's rotation is gradually slowing down a bit, due to a kind of braking force caused by the gravitational tug of war between Earth, the Moon and the Sun.

Scientists estimate that the mean solar day has not been 86,400 seconds long since the year 1820 or so. But if this small discrepancy were repeated every day for an entire year, it would add up to almost a second.

From 1972, when leap seconds were first implemented, through 1999, leap seconds were added at a rate averaging close to one per year. Since then, leap seconds have become less frequent. This June's leap second will be only the fourth to be added since 2000. -PTI

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