Any good gradeschooler knows how many days are in a year: 365. But that’s not quite the case. It takes Earth just a little bit more than 365 days to complete our orbit around the sun–about 365-and-a-quarter days. But 365.2422 doesn’t fly off the tongue so easy does it? So we stick with the old 365-days story, saving up that extra quarter-day each year until the fourth year when we’ve saved up enough quarter-days to add a day back to the calendar. And that’s leap day, folks.
But it turns out, it’s not that simple. Here’s a little bonus leap day knowledge bomb, as presented by our favorite astrophysicist and StarTalk’s own Neil deGrasse Tyson.
Over time, the Gregorian calendar’s leap year system actually over-corrects the leap day in the opposite direction. Eventually, that .2422 fraction of a day adds up to the point where every 100 years, we’ve over-corrected by a full day–and need to put another leap day back into the calendar under what Tyson refers to as the “400-year rule.” This happened most recently in the year 2000, a century-year that normally would not get a leap day–but since it’s divisible by 400, we put the leap day back in. It sounds a bit complicated, but it all has profound implications in ensuring our months stay in sync with the seasons.
Watch Tyson explain the logic behind the leap year, along with a brief history of calendars: