Why do we have leap years?

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.

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