We’ve all heard the term leap day and leap year. But do you know exactly why it exists?

About three-fourths of the time, today would be the last day of February. But not this year—tomorrow is Leap Day, February 29. As part of our solemn duty to make you smarter than your coworkers, we’re going to quickly walk you through why Leap Day exists in the first place. 

The short answer

The Earth takes 365.242 days to rotate around the sun, but we round down to 365. That quarter-day puts us approximately one day behind schedule every four years, when we tack on February 29.

The longer answer

In 45 BCE, Julius Caesar adopted the Egyptians’ 365-day solar calendar, and added Leap Days to keep the seasons on track. So Caesar is the reason why leap day exists?

  • But Caesar’s formulation still made the year about 11 minutes too long…which adds up over time. So, in the 16th century, Pope Gregory XIII made a new rule: We’d skip Leap Day on centennial years (1700, 1900) unless they’re divisible by 400 (so there was a Leap Day in 2000). Then, he deleted 10 days from October of 1582 and ruined a bunch of Libras’ birthdays.

Fast forward to 1752: The U.S. and Britain adopted the Gregorian calendar, and we haven’t changed a thing since. However, some scientists think it’s time we do.

And since we’re already talking about time and planets, why don’t you take a look at our post about the relative rotation and axial tilts of mapped planets?