Why Do We Have Leap Years?

Feb. 29 calendar
You only see February 29 on the calendar once every four years. bgblue/Getty Images

If you were born on February 29, it might feel like the ultimate gyp to only celebrate your birthday once every four years. So why is this fab day so scarce?

Leap years, or those years with the extra day of Feb. 29, compensate for our underestimating of Earth's orbit. It takes just a little more than 365 days for Earth to go around the sun once.


Humans prefer round, neat units – like seconds, minutes, hours and days. Over time, however, our ability to measure the Earth's rotation and orbit became more refined and precise, forcing us to adjust how we tracked time over the centuries, whether it's with paper-thin calendars or high-tech smartphones.

Here's how it works: Earth orbits the sun in a little less than 365.25 days. One common way to predict leap years is to see if the year can be evenly divided by four. It makes sense if you think about it: Those six extra hours each year add up to 24 hours (one whole day) over the course of four years. It's like sucking down that free coffee after getting your frequent buyers card stamped four times.

But, there's an exception to the "divisible by 4" rule (you knew there would be). For a while we've known of a more precise estimate of Earth's orbit. That number is about 365.2422 days, or 365 days, 5 hours, 48 minutes and 46 seconds – a tad bit under the 365.25 days we just talked about. By comparing the numbers, you'll see we're now overestimating Earth's orbit, even if it's by a fraction. To make up for this, a rule states there can only be 97 leap years over the span of 400 years, not 100 as you might think. One way to remember the rule is that years occurring at the turn of centuries – 1900 and 2000, for example – must be evenly divisible by 400 [source: Royal Museums Greenwich]. This is why 1900 wasn't a leap year but 2000 was.


History of Leap Years

Tacking that extra day onto the end of February holds more meaning than you might think. Before modern conveniences such as alarm clocks, computers and digital calendars, people relied on their own measurements to stay on track. Communities that accurately predicted the arrival of the seasons were more likely to be prepared. Without leap years to balance the extra few hours of Earth's orbit, people's expectations for seasons gradually fell out of sync with when the seasons actually occurred.

Thankfully, some bright minds and world leaders learned more about the Earth's elliptical journey around the sun.


In 46 B.C.E., Julius Caesar created the Julian calendar to include leap years. His version included three years of 365 days followed by one year with 366 days [source: Royal Museums Greenwich]. Sosigenes, an astronomer and math expert to Caesar, is often credited as the brains behind leap years. Since March 1 was commonly used as the beginning of the New Year under the Romans, the end of February seemed like a natural place to add on an extra day.

But we can thank a pope, Pope Gregory XIII in the 16th century, for revising the calendar again to reflect a more precise interpretation of the Earth's orbit. Remember a year is slightly less than 365.25 days. It's actually 365.2422 days. With time, that tiny overestimation adds up – even if it's roughly three days every 10,000 years.

In his spare time, the pope decided to fix what he believed was the problem by creating the Gregorian calendar, the standard for most of the world today and the one that incorporated the rule about centurial years needing to be divisible by 400 to qualify as leap years. At the time he made the changes, the typical dates of new moons were several days off target [source: Doggett]. Without accurate calendar records, religious holidays such as Easter would also be days off.

Still, some issues, like leap seconds, can't be resolved with calendar fixes and popes.

Leap seconds are added to standard atomic clocks to compensate for inconsistencies in the Earth's rotation on its axis. This isn't the same as an orbit. Instead, think of rotation as the spin responsible for night and day.

As long as the planet still makes its mathematically messy journey around the sun, we'll have time to welcome Feb. 29 and calculate the next one.


Lots More Information

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  • Answers.USA.gov. "Standards: Official Time." (Jan. 31, 2012) http://answers.usa.gov/system/selfservice.controller?CONFIGURATION=1000&PARTITION_ID=1&CMD=VIEW_ARTICLE&USERTYPE=1&LANGUAGE=en&COUNTRY=US&ARTICLE_ID=9704
  • Doggett, L.E. "Calendars." Explanatory Supplement to the Astronomical Almanac. 2005. Sausalito, Calif. University Science Books. (Feb. 2, 2012) https://eclipse.gsfc.nasa.gov/SEhelp/calendars.html
  • Encyclopaedia Britannica. "Sosigenes of Alexandria." Encyclopaedia Britannica Online. (Feb. 2, 2012). http://www.britannica.com/EBchecked/topic/555018/Sosigenes-of-Alexandria
  • The Honor Society of Leap Year Day Babies. "The Honor Society of Leap Year Day Babies." (Feb. 2, 2012) http://leapyearday.com/feb29/home
  • The National Institute of Standards and Technology. "What leap seconds are, how they are implemented and their future." NIST.gov. Jan. 18, 2011. (Jan. 31, 2012) http://www.nist.gov/pml/div688/leapseconds.cfm
  • Royal Museums Greenwich. "Which Years Are Leap Years?" (Feb. 25, 2020) https://www.rmg.co.uk/discover/explore/which-years-are-leap-years-and-can-you-have-leap-seconds