Children sometimes play leapfrog. We’re often encouraged to take a leap of faith. Technology sometimes takes a quantum leap. And today, the whole world takes a leap — it’s leap day, an extra day added to the calendar every four years.

The calendar is based on Earth’s annual journey around the Sun. But that journey isn’t an even number of days — it’s roughly 365 and a quarter days. So if the calendar was always set to 365 days, it would fall out of sync with the seasons.

To compensate, Julius Caesar ordered that an extra day be added to every fourth February. That made up for the extra few hours it takes Earth to orbit the Sun.

Well, almost. The full year is actually a few minutes less than 365 and a quarter days. Those minutes added up. By the 16th century, the calendar had drifted several days with respect to the seasons.

So Pope Gregory XIII ordered a change to the Leap Year rule. Leap day was dropped from years that are divisible by a hundred, but not by four hundred. So the year 1900 wasn’t a Leap Year, but 2000 was.

With that change, the calendar won’t need another adjustment for thousands of years.

Incidentally, the name “leap” day comes from the fact that a calendar date jumps over a day of the week during Leap Year. Last year, for example, March 1st came on a Friday. In a regular year, the next March 1st would fall on a Saturday. This year, though, it’s on a Sunday — a “leap” forward thanks to Leap Year.

Script by Damond Benningfield