Neil Degrasse Tyson Explains the Science of Leap Days

Earlier today, celebrity astrophysicist Neil Degrasse Tyson tweeted, "The Leap Day is misnamed. We’re not leaping anywhere. The calendar is simply, and abruptly, catching up with Earth’s orbit"

As you must know, today is February 29. If you check your planner, you'll notice you didn’t do much on this day last year, or the year before that…or the year before that. Why? Because it never happened.

In case the notion of a leap day is unfamiliar to you, I’ll do a quick break down. A leap day is essentially an extra day that occurs every four years in order to compensate for what is actually an inaccurate way of calculating a year. While we know a year to be 365 days, it actually takes 365¼ days for the earth to orbit the sun. Therefore, we add an extra day to the Gregorian calendar every four years, in order to account for those four quarter days.

“We know how to calculate a year, it’s how long it takes to come back to where you were,” explains Tyson. “But it turns out, if you cut the number of days we live into the year it doesn’t divide evenly."

Unfortunately, by simply adding an extra day every four years, we over compensate. Every 100 years, we have to skip a leap year in order to achieve cosmic balance. 

“Now, by taking out that day you have over-corrected in the opposite direction,” says Tyson. “And now there is a day that you have to put back in — it turns out every 400 years. So every 400 years — in the century year — that would have not had a Leap Day on the century rule, gets a Leap Day put back in on the 400 year rule.”

Now what would happen if we didn't have leap days? Well, today wouldn't be February 29, 2016 but July 15, 2017. But more importantly, we would find that over time, Christmas Eve would be in the middle of summer and the summer solstice in January.