Origin of the Leap Year

February 29 is a rare occurrence. Every fourth year, which would be divisible by four, becomes a leap year. But where did these rules come from?

Every major culture throughout history has had their own calendar: Chinese, Mayan, Egyptian, Jewish, etc. However, the ancient Roman calendar is the basis of the one we use today. The Romans had a calendar of 355 days with an extra month of 22-23 days every other year. Julius Caesar decided to simplify the calendar, so he put his astronomer Sosigenes in charge of it, and a 365 day calendar was created. However, it wasn't exact. The actual calculation came to 365.242 days (which is slightly less than the 365.25 that we usually think about). To make up for the 1/4 day, every four years, after the 28th of the month of Februarius, an extra day would be added to the calendar. The calendar long surpassed Julius Caesar, and in 1582 Pope Gregory XIII refined this rule further saying that the leap year would occur in years divisible by 4 as described above.

The part about a leap year that most people don't realize is that since the year is actually 365.242 (and not 365.25), there isn't an exact amount of time that is created for every 4th year, meaning that there is .008 of a day that is created that doesn't exist. So, there is another rule about leap years that most people don't know about. It deals with century years (such as 1800, 1900, 2000, etc.). Over a period of 400 years, there is an extra 3 days worth of time created because of this .008, so on certain century years, they may not be leap years. Only 1 out of every 4 century years (you might have guessed it, divisible by 400) are actually leap years. The year 2000 was a leap year because it is divisible by 400, but the years 1900, 1800, and 1700 were not. The year 2100 will not be a leap year. Most people don't know that rule because they don't live long enough to see century leap years (obviously).

Bet you didn't know that!