Why do we have leap years?

Leaping Through History

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 the 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., Julius Caesar created the Julian calendar to include leap years. His version estimated one year to be 365.25 days [sources: U.S. Navy; ROG Learning Team]. 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 [source: Doggett]. At the time he made the changes, the typical dates of new moons were several days off target. 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.

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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) http://astro.nmsu.edu/~lhuber/leaphist.html
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  • 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
  • ROG Learning Team. "Leap Years and Leap Seconds." Royal Museum Greenwich. Dec. 30, 2002. (Jan. 31, 2012) http://www.rmg.co.uk/explore/astronomy-and-time/time-facts/leap-years
  • U.S. Navy. "Leap Years." Naval Oceanography Portal. (Feb. 2, 2012) http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/leap-year