Waiting for that New Year’s Eve midnight kiss? Be careful to time it right – as it’s coming a second later this year.

When New Year revellers are expecting 2009 to start, the world’s ultra-precise atomic clock will actually show the time as 23 hours, 59 minutes and 60 seconds.

The extra second will make 2008 – already a leap year – the longest in the last 16 years.

So-called “leap seconds” are added to compensate for the Earth's slowing rotation. It’s caused by the Moon’s gravity and other factors influencing the speed of rotation – from inside the Earth and the atmosphere.

Time is counted according to the Earth’s rotation around the sun and to Coordinated Universal Time – a time standard based on International Atomic Time. Solar mean time is calculated at the Royal Observatory in Greenwich, London.

Since 1972, leap seconds have been added 23 times – to keep both time standards less than 0.9 seconds apart.

The last time a leap second was added was on December 31, 2005.

Psychologists say that adding a second will not affect people in any way.

Differences in the duration of days were first recognised in the 19th century.

But it was only with the invention of the atomic clock in 1949 that scientists could measure precisely the difference between terrestrial and astronomical time.