What is the science behind a leap year?

A back-to-school lesson in leap years

a yearly planner with a bronze pen
Leap Year A leap year happens every four years, or if the centenary year is divisible by 400

It’s February 29 – so what? So we only get to see this magical day every four years. But do you actually know why?

Here’s everything you need to know – from the science, to the history, to the maths – about why we sometimes have 366 days in a year.

Why is an extra day added?

An extra day is added so that the Gregorian calendar year (which we most commonly use) stays in sync with the seasonal year.

Because the earth’s orbit around the sun actually takes about 365.2422 days (so 365 days and a quarter, or six hours) – but the Gregorian calendar uses 365 days – the two would begin to drift out of line if we always had a common 365-day year.

And, in case you didn’t realise, we need to keep up with the solar system because we completely rely on the seasons it gives us – from our farming habits to our ability to actually see time pass us by.

Therefore by inserting an extra day into the year, this drift is corrected as best as possible.

Why add a day every four years?

Because after this amount of time, the six hours that we lose after each common year would equate to the loss of one whole day (6 hours x 4 = 24 hours, y’see?). So add a day, and it makes up for the slip.

What if we didn’t add the extra day?

Well we’d live in a crazy, crazy world. Eventually the loss of one day every four years would get us to the point where the summer solstice would be in December and winter would be in July. That means Christmas would be sunny in the UK and cold in Australia, and there’d sometimes be snow at Glastonbury Festival – WHAT A THOUGHT.

So yeah, we should probably keep adding the extra day.

Why is the extra day in February?

That’s all down to the Roman Emperor Caesar Augustus and his ego.

Under Julius Caesar’s reign, his month of July had 31 days, while February had 30 days and August had just 29 days.

Augustus was clearly pretty jel so when he became Emperor he pinched two days from February to add to his month of August. Poor February.

What are your chances of being born on a leap day?

It’s about one in 1,461 – four years equates to 1,460 days, plus one for the leap year. So pretty steep odds but it happens – and those people can feel special (or cheated, depending on their outlook) for the rest of their lives.

So is every fourth year a leap year then?

Not quite. Pope Gregory XIII decided in 1582 that if a year is divisible by 100, but not by 400, then it’s not a leap year. For example, the year 2000 was a leap year but 1700, 1800 and 1900 were not.

But there’s a good reason for it – remember how the earth’s orbit actually takes 365.2422 days? We round the point-2422 of a day up to six hours to make it easier – but technically it’s still too much of a correction to the drift. So this maths sorts that out, and has stuck around ever since.

So there you have it. Lesson over, class dismissed.