Why leap years exist, explained in one simple animation (2024)

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 coincidewith 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.

Why leap years exist, explained in one simple animation (2024)


Why do leap years exist? ›

It takes Earth 365 days, 5 hours, 48 minutes and 46 seconds to orbit the sun, according to NASA — and while that is rounded down to the 365 days we recognize as a typical year, those nearly six extra hours don't disappear. Instead, leap years are added to account for the difference.

How do you explain leap year? ›

A leap year (also known as an intercalary year or bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) compared to a common year. The 366th day (or 13th month) is added to keep the calendar year synchronised with the astronomical year or seasonal year.

What is the logic behind leap years? ›

It takes approximately 365.25 days for our planet Earth to orbit the Sun — that is a solar year. We usually round the days in a calendar year to 365, that is 365 days in a year. To make up for the missing partial day, we add one day to our calendar approximately every four years and that is known as a leap year.

How to explain leap year to kids? ›

A leap year is a year that has one day more than the normal 365 days. The extra day is February 29. Leap years generally occur once every four years. However, century years are only given an extra day if they are exactly divisible by 400, without a fraction.

Why do we have a leap year very short answer? ›

Leap years are necessary because the Earth's orbit around the Sun is not exactly 365 days. We add an extra day every four years to correct this discrepancy. The rule for determining a leap year is that it must be divisible by 4, unless it is also divisible by 100 but not by 400.

What is the mathematical reason we must have a leap year? ›

Earth takes about 365.2422 days to make one revolution around the Sun. That's about six hours longer than the 365 days that we typically include in a calendar year. As a result, every four years, we have about 24 extra hours that we add to the calendar at the end of February in the form of leap day.

What would happen if we didn't have a leap year? ›

Eventually, nothing good in terms of when major events fall, when farmers plant and how seasons align with the sun and the moon. “Without the leap years, after a few hundred years we will have summer in November,” said Younas Khan, a physics instructor at the University of Alabama at Birmingham.

What happens if you are born on February 29 legally? ›

When it comes to legal documents, such as obtaining a passport or driver's license, the date February 29th is recognized as the official birthday for leaplings in most countries. However, some states in the U.S. allow the leapling to celebrate their birthday on either February 28th or March 1st on non-leap years.

What is the scientific explanation for leap year? ›

It's not random, it's astronomy. Leap days happen due to a mismatch between the number of calendar days and the actual time it takes for the Earth to orbit the sun. One calendar year is 365 days, but one solar year is technically 365 days plus 5 hours, 48 minutes and several dozen seconds.

What is the secret behind leap year? ›

It's because the Earth takes approximately 365.25 days to go around the sun once. So, without leap years, our calendar would slowly get out of sync with the seasons. To fix this, we add an extra day to the calendar almost every four years. That way, our calendar stays in line with the seasons.

What is the simple logic for leap year? ›

The rules for determining if a year is a leap year are as follows:
  • If a year is evenly divisible by 4, 100, and 400, then it is a leap year.
  • If a year is divisible by 4 but not by 100 and not divisible by 400, then it is also a leap year.
  • If a year is not divisible by 4, then it is not a leap year.
May 23, 2023

What are the three rules for leap years? ›

To determine whether a year is a leap year, follow these steps:
  • If the year is evenly divisible by 4, go to step 2. ...
  • If the year is evenly divisible by 100, go to step 3. ...
  • If the year is evenly divisible by 400, go to step 4. ...
  • The year is a leap year (it has 366 days).
  • The year is not a leap year (it has 365 days).
Jun 6, 2024

What is leap year in simple words? ›

In an ordinary year, if you were to count all the days in a calendar from January to December, you'd count 365 days. But approximately every four years, February has 29 days instead of 28. So, there are 366 days in the year. This is called a leap year.

Why do we have leap years and how did they come about? ›

This was known even to ancient peoples, and when Julius Caesar decided to change the basis of the Roman calendar from using the moon to the sun, he also decreed that every fourth year an extra day would be added to keep everything in sync. Congratulations! Happy leap day!

What are 5 facts about leap year? ›

10 Wildly Strange Leap Year Facts That Are Absolutely True
  • We Didn't Have a Leap Year Until Julius Caesar Decided We Needed One. ...
  • The Julian Calendar's Fix Wasn't Quite Perfect. ...
  • The 29th Marks a Dark Day in Salem. ...
  • There's a Name for Leap Day Babies: Leaplings. ...
  • February 29th Is Believed to Be Unlucky for Love.
Feb 29, 2024

What would happen if there were no leap years? ›

"If we didn't account for this extra time, the seasons would begin to drift," the article goes on to say. "This would be annoying if not devastating, because over a period of about 700 years our summers, which we've come to expect in June in the northern hemisphere, would begin to occur in December."

Why do we have a leap year every 4 years and not 3 to 5 years? ›

It's because the Earth takes approximately 365.25 days to go around the sun once. So, without leap years, our calendar would slowly get out of sync with the seasons. To fix this, we add an extra day to the calendar almost every four years. That way, our calendar stays in line with the seasons.

Why was February chosen for leap year? ›

This means that when the Roman calendar added an extra day in February, they were in fact adding a day at the end of their year. So the simple answer is that we put the leap day at the end of February because the Romans did.

Why is leap year skipped every 100 years? ›

Enter Pope Gregory XIII, who reformed the calendar again in 1582. He decreed that every 100th year (to make it simple, years ending in 00) would not be a leap year, so no leap day would be added.

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