Although it should not matter much but this 30 June will be one-second longer, says NASA. An extra leap second will be added to account for Earth’s rotation  which is gradually slowing down a bit, said Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Usually, a day consists of 86,400 seconds but due to gravitation pull of the moon and the sun, the earth takes 86,400.002 seconds to complete a day. Thus, every day is 2 milliseconds longer, explains NASA scientist. To compensate it an extra leap second will be added on 30 June. This is the case according to the time standard that people use in their daily lives – Coordinated Universal Time or UTC.

UTC is extremely accurate ‘atomic time’  based on electromagnetic transitions of cesium which can show transitions of even one second in 1,400,000 years accurately.

NASA scientists further explained that solar day hasn’t been 86,400 seconds long since the year 1820. Although the difference of two milliseconds isn’t very huge but it happens everyday and cumulatively it adds up to almost a second in a year. Two milliseconds is even smaller than the time taken by blinking of eyes. If the time is not adjusted then could lead to dysfunction of equipment that are heavily reliant on clock like satellites. According to scientists, even a difference of millisecond could force a satellite to move up to eight kilometers from its location which can disrupt the navigation system and other facilities on the earth.

So, the clock on this 30 June will show 23:59:60 and then it will move to 00:00:00 on July 1. Usually, the clock moves from 23:59:59 to 00:00:00. Since, extra leap second is being added hence the changes.

Leap seconds are generally added either on 30 June or 31 December.

Adding of leap second first started back in 1972. Since then leap seconds were added nearly every year till 1999 after that its frequency got reduced and only four leap seconds have been added since 2000.

NASA scientists further explained that it is not that predictable to add a leap second as everyone would think. Previous leap seconds have created challenges for some computer systems and generated some calls to abandon them altogether. Moreover, computer models suggest that more leap year might be needed in coming future.

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