[Mp3] Leap years and why we need them

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[Reading level: B2 – Upper Intermediate]

2020 is a leap year. That means there’s an extra day in the calendar, the 29th of February. But why do we have leap years and how do we decide when to have them? The answer is a little more complicated than you may think. So here’s how it works.

 

We measure a day as how long it takes the earth to spin once on its own axis. That’s 24 hours. And we measure a calendar year as how long it takes the earth to orbit the Sun, 365 days, except the time it actually takes for the earth to circle the Sun is 365.24 days. So, that’s roughly a quarter of a day longer, which adds up to a full day every four years.

 

To keep everything in sync, this full day is added to the fourth year’s shortest month, February. And that’s what we call a leap year.

 

But we don’t actually have a leap year every four years. And here’s why. Remember how we rounded up that 0.24 to a quarter, well that difference does eventually add up, pushing the whole system out of sync again, by three days out of every four hundred years to be precise. In order to redress the slight imbalance, we have to skip a leap year every now and then, so not add that extra day.

 

But how do we decide when to have a leap year, and when to skip?

 

The first rule is that the year to add an extra day must be divisible by four.

 

The second rule is that a leap year cannot fall on a year that’s divisible by 100. If it does, no leap day is added to that calendar year. But to make things even more complicated, there’s an exception to this second rule. If a leap year can be divided by 400, the leap day is added after.

 

All that’s why a leap day happened in the year 1600 and 2000 but not in 1700 1800 or 1900. We have Pope Gregory XIII to thank for creating this century rule some 400 years ago. His changes marked the beginning of the Gregorian calendar, which is still in use across the world today.

 

So, leap years exist to help us stay in sync with the real astronomical year. And if you want to get real nerdy, there’s also the more recent introduction of leap seconds, but maybe we’ll explain that another year.

 

Source: BBC News

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