What’s a Leap Year, Anyway?
Why February 29 is so rare
Why do we get an extra day every four years? (Hint: It’s not so we can make the most of Boston’s beautiful February weather.)
Here’s the easy part: our modern calendar is made up of a single year with 365 days. Each time the Earth spins all the way around on its axis, it’s a 24-hour day. Every time the Earth completes an entire orbit of the sun, it’s a year.
The hard part: a year doesn’t actually add up to an whole number of 24-hour periods. It takes the Earth about 365.2425 days to orbit the sun, leaving an extra quarter of a day unaccounted for each year. Doesn’t sound like a lot, right? But think about it this way: if those days were just left to build up, then the seasons would begin to get out of sync, and pretty soon we’d be having summery green Christmases and snow-white Fourth of Julys.
The original leap year was part of the Julian calendar, created by Julius Caesar in 46 BC. In Caesar’s version, the leap day came between February 23 and 24, creating a 48-hour day instead of an extra one. The calendar we use in the United States today is called the Gregorian calendar, which is a slightly more precise version of the Julian calendar instituted by Pope Gregory in 1582.
Devin Hahn can be reached at dhahn@bu.edu.
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