Why do we have leap years?

Those leap year babies look pretty excited about their birthday cake. Could that be because they only eat cake once every four years? The year they were born is printed on their jerseys. See pictures of time measurement.
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On the eve of the 21st century, some partied like it was 1999, while others doggedly worried about the world's computers. The Y2K mess-that-wasn't spurred its share of anxiety and bottled-water buying.

But there was another issue: The year 2000 was also a leap year. Would computer systems jettison their calendars, thinking the added last day in February was March 1, too?

We now know that things turned out OK on both fronts, but leap years are trickier than you might think. Why was 2000 a leap year, while 1900 wasn't? It all depends on how long it takes the planet we call home to complete one orbit around the sun.

It turns out the time it takes for the Earth to orbit the sun is a little more than 365 days. Leap years, or those with the extra day of Feb. 29, compensate for our underestimating of the Earth's orbit.

But how did we come to realize that leap years are necessary?

The short answer is that humans prefer round, neat units -- like seconds, minutes, hours and days. Over time, however, our ability to measure the Earth's rotation and orbit has become more refined and precise, forcing us to adjust how we track time over the centuries, whether it's with paper-thin Dilbert calendars or glossy, hi-tech smartphones.

Here's how it works: The 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.

Alas, 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 the 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, 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 [source: U.S. Navy]. 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. This is why 1900 wasn't a leap year but 2000 was.

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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  • U.S. Navy. "Leap Years." Naval Oceanography Portal. (Feb. 2, 2012) http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/leap-year