# One Extra Second On Tuesday, June 30 2015!

Tomorrow (Tuesday, 30.06.15) will be a longer day: one extra second longer. According to NASA scientists, one more second, called the leap second, will be added to the normal 86,400 seconds.

A day lasts 86,400 seconds, as per the scientific standardisation of time called “Coordinated Universal Time”, or UTC, and also known as “atomic time”. The latter dictates that the duration of one second is determined by electromagnetic transitions in atoms of cesium; the cesium clock is actually accurate to one second in 1,400,000 years.

On the other hand, the mean length of a day governed by how long the Earth rotates – the mean solar day – is 86,400.002 seconds long. This is so because our planet’s is slowing down in terms of rotation as a result of the gravitational pull that exists between Earth, the moon and the sun that acts like a “braking force”.

“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.

But, who will actually feel the difference of the 2 milliseconds? The effect would be seen if it were to happen every single day: by the end of one year, the difference would have reached one second. Though, in reality, the Earth’s rotation decelerates in unpredictable ways daily.

Now, how to decide when to add the leap second? Scientists monitor the time it takes for our planet to do a full rotation. An extremely precise methodology is used to do so: the Very Long Baseline Interferometry (VLBI). Another time standard known as UT1 is based on VLBI measurements.

UT1 and UTC are not normally the same. Therefore, leap seconds are added as required to keep the difference between two time standards within 0.9 seconds. This decision is made by a team from the International Earth Rotation and Reference Systems Service.

As such, a leap second is added on June 30 or December 31. As a consequence, if the leap second is inserted in June, UTC will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. To prevent this, some systems are switched off for one second; in the past, adding leap seconds had complicated things for computer systems.

“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. “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.”

The leap second to be added tomorrow will be the 4th one inserted since 2000.