Leap Day!

February 29th is Leap Day for us using the Gregorian Calendar, a very common solar calendar.

The reason we have Leap Years and Days is because the actual time it takes the Earth to rotate around the sun is not 365 days, but 365.242374, and increasing. Since this number is almost 365-and-a-quarter, every four years we've gained another day.

Well, almost. Since .242374 is just less than .25, if we make every four years leap years, we'll be ahead of ourselves after some time. To correct for this, it was long ago decreed that with the Gregorian Calendar years divisible by 100 will not be Leap Years, unless also divisible by 400; 1700, 1800, 1900 not Leap Years while 1600 and 2000 were.

Does anybody want to guess what event is so important that it must always fall in the same time each year?

If you guessed either the summer or winter solstice, you're wrong. It's the vernal equinox, desired to be on or very near March 21st.

This keeps Easter in relatively the same time of year on the calendar, March/April, or, if you're using an Eastern Orthodoxy calendar, April/May.

But, given a long enough time frame, even the self-correction of skipping leap years will yield some error. That span? 8000 years. Over the course of 8000 years, the Gregorian Calendar will be off by one day, but the time it takes for vernal equinoxes (365.242374 days) will have changed--but not predictably, so they'll probably just go with Herschel's suggestion and make 4000 a non-leap year.

Wily old Herschel...