clocks go back change 2015
A leap second helps synchronise atomic clocks with the Earth’s slowing rotationGetty

This month will be one day longer thanks to a leap year. Here is why it is mathematically necessary.

Why does a leap year happen?

Every four years, a leap year with 366 days occurs. As it takes the Earth 365.2422 days to orbit the sun and the Gregorian calendar only has 365 days, an extra day is added to help synchronise the calendar year with the solar year. If this discrepancy was not resolved, the months would gradually become out of sync with the seasons. A year that is not a leap year is called a common year.

There is a problem with the leap year system, however. One additional day every four years is too much as a solar year is not exactly 365.25 days. To solve this, years which are divisible by 100 – such as 1900 – are not normally leap years even though they a divisible by four. For accuracy, years that are divisible by 400 are leap years, such as the year 2000.

Leap years occur in February and were first introduced by Julius Caesar more than 2,000 years ago. Before this, the Roman Empire followed a 355-day calendar with an extra month every four years.

What is a leap second?

A leap second is not directly linked to a leap year, but every now and then a leap second is added to Coordinated Universal Time (UTC) to synchronise atomic clocks with the Earth's slowing rotation. Atomic time is constant but the Earth's rotation is slowing down by around two-thousandths of a second each day.

International Atomic Time, which provides the exact speed for our clocks to tick, is a time scale that combines the output of around 200 extremely precise atomic clocks around the world. Universal Time, also known as Astronomical Time, refers to the Earth's rotation around its axis – which determines the length of a day.

The last leap second was added on 30 June 2015 at 23.59.60 (UTC). The difference between ITC and International Atomic Time is now 36 secs.