2020 is a leap year, and as we all know leap years happen every four years, right? Wrong. That extra day in February is designed to keep the Earth in sync with the astrological or seasonal¹ year. But, what you might not realise is that the maths isn't that simple and occasionally we have to wait eight years.

 

The method behind calculating when leap years occur might be a bit more complicated than you think. Way back in 1582 the calendar we now use was first implemented². The Gregorian calendar was named after the pope at the time, Gregory X1V. The people who created it realised it wasn't perfect to just add an extra day every four years. 

 

Here's where the maths comes in. A day, of course, is measured by the amount of time it takes for the Earth to complete one full rotation³ on its axis⁴. A year is the number of days it takes the Earth to orbit the Sun, which most people assume is 365. However, the Earth actually takes just under 365 and a quarter days to complete one full revolution around the Sun.

 

So, every four years we get an extra day, which falls on the 29th February. However, as mentioned earlier, it is in fact just under a quarter of a day. So, by rounding it up, we put the calendar out of sync again – which means that after a while we have too many days.

 

That imbalance has to be readdressed, otherwise after 400 years you'd end up with three extra days. So back in 1582, they realised that every turn of a century should skip their extra day to subtract those three extra days. This is why the years 1700, 1800, and 1900 weren't leap years.

 

But wait, there's more. The year 2000, the millennium⁵, was a leap year. That's because over a period of four hundred years we only need to remove three days. So, every 400 years the turn of the century is a leap year. This means the next time you hear someone saying 'leap years happen every four years' you can tell them why they're wrong.

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