Leap year: What are they and why do we have leap days?

Normally our calendar years are made up of 365 days, but every four years we add a day to make what is known as a ‘leap year’.

Here's why:

The Gregorian calendar, the most widely used across the globe, measures a year to be 365 days following the Earth's orbit around the Sun.

However, the Earth's orbit takes 365.24 days to complete its solar orbit and an extra day every four years is added to keep calendar seasons synchronised with solar seasons.

If we didn’t do this the seasons would shift around the calendar, and in 750 years’ time June would find itself in the middle of winter.

The origins of the modern calendar as we know it date back to the Roman times.

Julius Caesar introduced the Julian calendar in 46 BC, which consists of 365 days with 366 in every fourth year.

However, in his calendar February had 30 days and August 29.

When Caesar Augustus became emperor he changed this so "his" month had the same amount as Caesar’s. He reduced February to 28 days to accommodate the amendment.

In 1582 the UK switched to the Gregorian calendar and worked out the Julian Calendar didn’t actually reflect the actual time the Earth takes to orbit the sun, or tropical year.

The Gregorian calendar follows three rules to determine which years are leap years: if the year can be divided by four it is a leap year, but if it can be evenly divided by 100 or by 400 then it is not.

Despite this complicated process the calendar is still not perfect, every 3,236 years it will be out by a day.

Comparably, the Julian calendar is currently lagging behind the Gregorian calendar by 13 days.

Statistics expert Courtney Taylor, writing on website About.com, says around 0.07 per cent of the world’s population was born on a leap day, or 4.8 million people

Those born on a leap year are invited to join the Honour Society for Leap Year Babies, which currently has 11,000 members.

There are surprisingly few Leap Year traditions, but the custom of women proposing to men is the most famous.

The legend goes that in 5th Century Ireland, St Bridget complained to St Patrick about women having to wait too long for men to propose, to which St Patrick allowed women to propose on 29 February.

Although the more observant may note this could still result in a wait of nearly four years, the tradition has nonetheless gained popularity since the 19th Century.