Extra second puts more time on hands

TODAY will be longer than normal – but just by a single second. Nasa’s Goddard Space Flight Centre in the US says because the earth’s rotation is gradually slowing down, an extra second – a “leap” second – has to be added now and then, usually on June 30 or December 31.

Physicist Chopo Ma said over the short term, leap seconds were not as predictable as everyone would like.

“The modelling 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 solar day, based on how long it takes earth to rotate, is about 86 400.002 seconds long.

Earth’s rotation is gradually slowing down because of a kind of braking force caused by the gravitational tug-of-war between Earth, the moon and the sun. Nasa says scientists estimate the mean solar day has not been 84 400 seconds long since about 1820.

“The difference of 2 milliseconds – 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,” Nasa said.

Leap seconds were introduced in 1972 and were added at a rate of about once a year until 1999. Nasa says since then they were less frequent and today will be only the fourth to be added since 2000.

The time we use in our daily lives is called co-ordinated universal time, or UTC. This is “atomic” time and the duration of one second is based on predictable electromagnetic transitions in atoms of caesium. These transitions are so reliable that the caesium clock is accurate to one second in 1 400 000 years.