WHAT'S A LEAP YEAR?
A leap year is a year when an extra day is added at the end of the month of February. This happens approximately (but NOT every) four years. This applies to anyone who uses the Gregorian calendar - which is pretty much the whole world.
WHY DO WE HAVE LEAP YEARS?
We have a leap year because a standard year is not actually exactly 365 days long - it's 365.2422 days long, so slightly longer than we count in a standard year calendar.
That long number - 365.2422 - is the number of days it takes the planet Earth, on average, to make a full rotation around the sun.
So, an extra day is added onto our shortest month, about once every four years, in order to keep the calendar months in light with their assigned seasons. If we didn't add the day, the wintry month of February would occur during summer.
The concept of the leap year dates back to Julius Caesar and the ancient Roman Empire, when he commissioned scientists to create the Julian calendar in 46 BC.
SO IT HAPPENS EVERY FOUR YEARS?
Well, this may surprise you, but not exactly.
In general, leap years occur every year that is divisible by four. But the first year of a new century is generally NOT a leap year - EXCEPT if that year is a multiple of 400.
For example, 1900 was not a leap year, but 2000 was.
Why? Pope Gregory XIII made up these rules in 1582. Adding one day to the calendar every four years still puts us just a smidge too far ahead of the season, so we use the calendar system he created to this day.
IS THIS REALLY HOW THE CALENDAR HAS TO WORK?
Not necessarily. Scientists have long tried to get rid of leap years and simplify the calendar.
Most recently, two professors at Johns Hopkins University, Richard Conn Henry and Steve Hanke, proposed a new system that would keep each date on the same day of the week year after the year (imagine never having to buy a new calendar!).
Under the Henry-Hanke model, most years would be 364 days long, and an extra week would be tacked on to the end of December about every six years.
