Doing the Math on Why We Have Leap Day
You may have noticed that there's an extra day on our calendar this year. That's not a typo; it's leap day! Leap day is another name for Feb. 29, a date that typically comes around every four years, during a leap year. Why doesn't Feb. 29 appear on the calendar every year?
Read on to find out how the imperfect match between the length of a calendar year and Earth's orbit results in the need to make small adjustments to our calendar on a regular basis.
The length of a year is based on how long it takes a planet to revolve around the Sun. Earth takes about 365.2422 days to make one revolution around the Sun. That extra 0.2422 days may not seem like much, but that's about six hours longer than the 365 days that we typically include in a calendar year. As a result, every four years we have about 24 extra hours that we add to the calendar at the end of February in the form of leap day. Without leap day, the dates of annual events, such as equinoxes and solstices, would slowly shift to later in the year, changing the dates of each season. After only a century without leap day, summer wouldn’t start until mid-July!
But the peculiar adjustments don't end there. If Earth revolved around the Sun in exactly 365 days and six hours, this system of adding a leap day every four years would need no exceptions. However, Earth takes a little less time than that to orbit the Sun. Rounding up and inserting a 24-hour leap day every four years adds about 45 extra minutes to every four-year leap cycle. That adds up to about three days every 400 years. To correct for that, years that are divisible by 100 don't have leap days unless they’re also divisible by 400. If you do the math, you'll see that the year 2000 was a leap year, but 2100, 2200 and 2300 will not be.
After learning more about leap years with this article from NASA's Space Place, students can do the math for themselves with this leap day problem set. Hey, we've got an extra 24 hours this year, so don't forget to take a little time to relax!