Why leap years exist, explained in one simple animation

Mark your calendars: February 29 is on.

Yes, 2024 is a leap year, which is usually momentous, and not just for the people born on February 29 who only get to celebrate their actual birthdate every four years. Leap years also typically coincide with the US presidential election and the Summer Olympic Games. This year, people in the US will also get to see a rare total solar eclipse darken the skies. To top it off, the sun is also nearing the peak of its 11-year magnetic cycle, upping the potential for solar storms. It makes sense that such a jam-packed year should get an extra day.

The simple explanation for why we have leap days is that it takes 365.2422 days for our planet to complete one revolution around the sun. That means each typical 365-day year ends a quarter-day’s worth short of the complete orbit.

The following animation from planetary scientist James O’Donoghue visualizes the discrepancy by showing what would happen without leap years. We can’t add quarter days to each year, and the Earth simply cannot spin any faster to make up for the lag, so a leap day is added every four years, with a few exceptions (more on that below).

O’Donoghue’s visualization makes it clear how, if we abolished leap years right now, sometime in the 2400s, January would drift into summertime for the Northern Hemisphere. And that would just be confusing!

Humans have been accounting for this discrepancy between Earth’s orbit and the length of a day for a while now — as far back as the ancient Egyptians. The current leap day tradition can be traced back to Pope Gregory in the 1500s, who corrected an earlier attempt at a leap year calendar implemented by Julius Caesar.

It’s not the strict case that a leap year happens every four years. You might have noticed that a full orbit around the sun takes 365.2422 days, and not a neat 365.25 days. So if we were to have leap years every four years, then the calendar would still get out of whack over long periods of time.

This happened with Julius Caesar’s calendar, which included a leap year every four years. It appeared to work well for a few centuries, and then the seasons started to be misaligned with the calendar dates.

And so on Pope Gregory’s updated calendar, there are clever, rare exceptions to the leap year rules. As National Geographic explains:

Leap years divisible by 100, like the year 1900, are skipped unless they’re also divisible by 400, like the year 2000, in which case they’re observed. Nobody alive remembers the last lost leap day, but dropping those three leap days every 400 years keeps the calendar on time.

The next skipped leap year will be in 2100. If you’re around to celebrate it, congrats. I hope the world still works for you. (The previous skipped leap year was way back in 1900. The one after 2100 will be in 2200.)

Can we make a calendar without a leap day?

There’s no getting around having to add days to the current calendar. But that doesn’t mean the current calendar is the only possible one.

During the last leap year, two professors at Johns Hopkins University (an economist and a physicist) suggested we switch over to a calendar that doesn’t include a leap day and is more consistent year over year (meaning February 2 will always be a Tuesday, no matter what). To make this work, the professors add a whole leap week every fifth or sixth year. It’s effectively creating a new week-long month that pops up only once a decade.

Is this better? I don’t know. It depends if we can make the leap week a company holiday.

There are merits to our current, chaotic calendar. Over time, you get to have a birthday on every day of the week. In the alternative calendar mentioned above, your birthday will always be on a Tuesday if you're born on a Tuesday. Similarly, isn’t there a joy in learning that Christmas is going to fall on a Monday and that means the holiday has been turned into a three-day weekend?

For now, our calendar remains. And because of it, you have an extra day this year! Make the most of it.