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Why June 30 Will Be 1 Second Longer

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A new second will be added on June 30, at 23:59:60 GMT. It is called as the leap second. Side note-2015 is not a leap year. On a normal day, the clock moves from 23:59:59 to 00:00:00 the next day. But on June 30, the one minute will have 61 seconds, and the clock will move from 23:59:59 to 23:59:60 before it reaches 00:00:000 on July 1.

Why need a leap second?

Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that. The rotation is slowing down thanks to a kind of braking force caused by the gravitational tug of war among Earth, the sun and the moon.

Due to the slowdown, earth is taking roughly 86,400.002 seconds to rotate around its axis(the average length of a day/ A mean solar day), rather than 86,400 seconds (which is 24 hours * 60 minutes * 60 seconds). Because of this a mean solar day likely hasn’t lasted 86,400 seconds since about 1820.

Other factors:

But, slowdown of rotation is not the only factor responsible for the length of the day.

For instance, seasonal and daily weather changes can influence the length of a day by several milliseconds every year, as can oceanic and atmospheric tides and variations in the atmosphere, oceans, groundwater and ice storage.

Even the cyclic climate phenomenon called El Niño can slow down Earth’s rotation, adding a millisecond onto a day.

These factors can change day length by redistributing mass and momentum within the different parts of Earth. As mass or momentum is redistributed, the rotation rate [of Earth] can change just like the spin of a skater changes when the arms are moved in or out.

By taking all these factors, scientists add a leap second usually on June 30 or Dec. 31 to lessen the gap between our clocks and the more precise atomic clocks.

Scientists added about one leap second every year from 1972 to 1999, but leap seconds have become less frequent since then; this June’s extra second is only the fourth since 2000, the last one was in 2012.


  • In the past, leap seconds have caused problems for computer systems
  • The extra second can create glitches galore for stock traders, computer programmers and airline companies unless their systems are prepared for the change

Recognizing the problems being created by adding a leap second so frequently, alternatives have been proposed to provide accurate time. The International Telecommunication Union, an agency of the United Nations that addresses issues in information and communication technologies, will discuss the proposals and decide the issue.