Tonight's countdown to 2017 will be 11 seconds long

Tonight's traditional New Year's Eve countdown to 2017 will take 11 seconds instead of 10.

The National Physical Laboratory, the body responsible for measuring time in the UK, will add a leap second to the last few moments of 2016.

Clocks will strike 23:59:60 for a second before 2017 dawns to account for the slowing rotation of the earth.

"The atomic clocks housed at NPL are nearly a million times better at keeping time than the rotation of the Earth," the laboratory said.

"Leap seconds are used to provide a link between the extremely stable time scale based on atomic clocks and the more variable time scale of the solar day," the NPL said.

This will be the 27th second added since 1972, with the last one occurring on June 30 last year and the one before that in 2012. The length of a day today is very slightly longer than the length of the same day last year. In the 1800s, a day was defined as 86,400 seconds, while now it is 86,400.002 seconds.

Atomic clocks are accurate to around a billionth of a second a day, and so are sensitive to variations in the planet's daily rotation.

Here's a chart of changes in the earth's rotation that shows just how volatile it is: