Science explained: What is a leap second?

A decision on whether to abolish the leap second - the occasional, extra second added to the world's time - has been deferred.

Experts at the International Telecommunication Union were unable to reach a consensus, so moved the matter to a meeting in 2015.

But what is a leap second? Why do some people want to get rid of it? And what would happen if we do? Science reporter Rebecca Morelle heads to the Royal Observatory, Greenwich, to explain.