Leap Year: Every Four Years?

Everybody thinks that years divisible by four are always leap years. Right?

Wrong!

In the Gregorian calendar, the one we use here in the United States, most years evenly divisible by 4 are leap years. Adding an extra day to the calendar every four years helps correct for the fact that it takes 365.242374 days to go once around the sun (a "solar year"). If the calendar year were always 365 days long, things would get screwed up pretty rapidly. Spring would come about a day later with every four years that passed.

Enter the leap year.

But adding a leap year every four years would correct the problem perfectly only if the solar year were exactly 365.25 days long. But, as we said, it's only 365.242374 days long.

So what actually happens is that a leap year is omitted in three out of every four centuries.

The rule is that years divisible by 100 are not leap years, unless they are also divisible by 400.

So 1600 and 2000 were leap years, but 1700, 1800 and 1900 weren't.

Unfortunately, only the very youngest (and longest-lived) of us will ever get to use this rule. The next omitted leap year isn't until 2100.