Today is February 29th, 2016, and by now you probably take for granted that (almost) every four years, February gets an extra day. But do you know why? Did you know that sometimes, we skip a four-year cycle? Read on for all you ever wanted to know about leap years.
Who started this nonsense, anyway?
Believe it or not, Julius Caesar introduced it in 46 BC. The Julian calendar had a number of advantages over the previous Roman calendar, including being a little more in sync with what we know as a solar year - the time it takes Earth to complete a full orbit around the sun.
Why do we do it?
About that solar year thing - we think of a "year" as being 365 days, but it's actually 365.24219 days. Our seasons (spring, summer, autumn, winter) depend on where we are along the orbital path, so a calendar that's out of sync with this path would mean that our months would end up in different seasons - "Winter would feel like summer, and we’d have Christmas in July," as Rachel Wise put it in this Quartz article on leap year babies. So Caesar basically rounded up to 365.25 and tried to make up the difference every four years. The problem is that even though this is a lot closer, it's still off - you end up with a surplus of about 11 minutes each year.
Great, so I'll just leave work 11 minutes early today, since those minutes don't technically exist, right?
Not so fast. In 1582, Pope Gregory fixed this by introducing the Gregorian calendar, which is pretty much what we use today. It added one important change to the "leap year every four years" rule of the Julian calendar: the rule that if it's a century year (1900, 2000, 2100, etc.) then it's only a leap year if it's divisible by 400. So 2000 was a leap year, but 2100 won't be.
Is this the most complicated calendar situation in the world, or what?
Actually, it's one of the simplest. For example, the Chinese calendar has an entire leap month every three years. We should consider ourselves lucky that it's only one day (approximately) every four years!