My goal to learn something new every day
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.