Tuesday Will Have a 'Leap Second,' Says Nasa

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.

(Also see: The Second That Almost Brought Down the Internet)

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.

This difference of 2 milliseconds, or two thousandths of a second - far less than the blink of an eye - hardly seems noticeable at first.

But if this small discrepancy were repeated every day for an entire year, it would add up to almost a second.

Although Earth's rotation is slowing down on average, the length of each individual day varies in an unpredictable way.

The length of day is influenced by many factors, mainly the atmosphere over periods less than a year.

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. In practice, many systems are instead turned off for one second.

Previous leap seconds have created challenges for some computer systems and generated some calls to abandon them altogether. One reason is that the need to add a leap second cannot be anticipated far in advance.

From 1972, when leap seconds were first implemented, through 1999, leap seconds were added at a rate averaging close to one per year.