Leap years. We all know about them. We learned all about them in school; It takes the Earth just a little over 365 days to travel around the sun. We’re taught it’s 365 and a quarter days each year. That quarter is about 6 hours. So every four years (6 * 4 = 24 hours) we add an extra day. That extra day is added to the end of February to give us February 29th once every four years. We know when it’s a leap year because if we can divide the year by 4 and end up with a whole number, it’s a leap year. 2016 (the next leap year) divided by 4 = 504. Simple maths. Or at least that’s what I have believed for the last 25 years. Turns out, that’s not entirely accurate.

It does take the Earth a little over 365 days to orbit the sun, but it’s not 365 ¼ days exactly. It’s actually 365 days, 5 hours 49 minutes and 16 seconds. That’s 10 minutes and 44 second under 6 hours. Adding an extra day every four years, means we are actually adding a bit extra each time, which over the course of thousands of years would eventually throw our seasons out of sync. So, there’s an additional rule we need to know about leap years. If the year is divisible by 4 we add a leap year, unless it is the start of a new century i.e. a year divisible by 100. For example 1900 / 4 = 475 (a whole number), but because 1900 is the start of a new century, it’s NOT a leap year.

OK. Got it. Simple. A year divisible by 4, unless it is also divisible by 100 is a leap year. Actually… There’s still a little problem. By not having a leap year every 100 years when there should be one (following the 4-year rule), we’ve lost an extra day again. You see, if we always added a day every four years (even on the years divisible by 100) then the extra days added over 400 years is just about 3. So, the rule needs to be adjusted again; A leap year occurs every 4 years. **It’s a leap year if the year is divisible by 4, unless the year is divisible by 100 when we don’t have a leap year, unless the year is divisible by 400 when we do actually have a leap year.** In other words, the years 1700, 1800 and 1900 were not leap years, even though they are divisible by 4, but the year 2000 was a leap year (I remember it!) because you can divide it by 400. Perhaps that’s why I’d never heard of this extra rule—I’ve never experienced it. Neither have you (unless you are 115 years old). And the thing is, I will almost certainly never experience missing a leap year on a new century because I’m probably not going to be around in the year 2100.

So because the number of rotations the Earth does in the time it takes it to travel once around the sun don’t match up exactly, we need to have these little corrections. You’d have thought that was enough, and it generally is. However, all these corrections are not quite perfect, and it means that in around 1000 years, there’s still going to need to be another correction, but our scientists today are happy to leave it up to the people of the future to sort out.

In case you were wondering, the insertion of a leap day (or sometimes week/month in other calendars) into a calendar is known as an *intercalation *and a year that contains a leap day is also known as an *intercalary. *A year that doesn’t contain a leap day, is just known as a *common year*.

So there you go. Something that I thought was so simple growing up, turns out to be somewhat more complicated. And that is what I learned today.

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I wonder why this came to your attention today. Are you a leap year birthday? I was quite glad when 2000 was a leap year – not because I like longer years (although 366 is a prettier number than 365) but because it seemed unfair that a leap year baby who had been waiting for a birthday for four whole years would suddenly find out they’d be waiting even longer. Only mildly annoying when you’re 20, sure, but imagine if you were 12 and birthdays were important.

I enjoy teaching this to my students when it comes up because they are always so amazed to think that there are people their age who have only had a quarter of the birthdays.

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I’m not sure why it came to my attention suddenly. I was just trawling through random facts and came across it. I was so surprised to find out about it, just because I’d been under the assumption that the 4-year rule was absolute. I’m not a leap year baby, so doesn’t really affect me.

One interesting (kind of) thing that I also read yesterday, but forgot to put into the post, was that different countries treat it differently when it comes to the legal day of your birthday, In the UK, for example, you legally age a year on March 1st, whereas NZ, it’s Feb 28th.

And, I must say that I have to disagree with you when you say 366 is prettier. I think 365 is so much better. 366 is so lopsided :p

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Lopsided indeed! But being a leap year baby myself, I’m afraid I’m somewhat biased. I never thought to wonder, though, when I legally age a year. It seems the jury is still out in the US. But my DL expires on 2.28.

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Were you born on Feb 29th?!?

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In 1980. That makes me soon to be 8 and three quarters. (my friends always joked that my birthday explains so much and they were glad i was not dropped on my head as a child) 😛

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Haha, I think you might be the first person I’ve actually ever met who was born on Feb 29th! Big celebration next year then! When do you celebrate your birdthay, Feb 28 or Mar 1?

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Haha. As a kid we always celebrated it on the last Sunday of February. Now I only celebrate it when it comes around. But I think of it as the breath between 2.28 and 3.1.

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