What's the deal with "leap year," anyway?

Tomorrow's "leap day." This means we get an extra day this year: February 29th. Why?

It turns out that a year isn't exactly 365 days long. A year is more precisely 365 days and about six hours (or, about a quarter of a day). Every four years, the "solar year" is out of sync with the calendar year, so an extra day is added to the calendar year to catch up and resync the calendars.

It's not QUITE as simple as that, of course. Because it's not EXACTLY 365 days and six hours, over the course of a hundred years or so, things can get out of sync again. So years that end in "00"and are divisible by 400 (like the year 2000) are also leap years, but other years that end in "00" and aren't divisible by 400 (1900, for example) are not leap years.

Enjoy your extra day!