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.

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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.

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