Tomorrow is February 29 instead of March 1 because this year is a leap year. But why do we have a leap year every four years?

Years and days are measured differently, and they don’t quite add up.

• A year is the time for the earth to complete its annual orbit around the sun.
• A day is the time required for the earth to make one complete revolution on its axis (24 hours).

Going by those measures, there are almost 365 and a quarter days in a year. That’s an extra quarter of a day a year. And every four years, we have an extra day on our hands.

To make up for those extra four quarters, every year exactly divisible by 4 is a leap year: a year with 366 days instead of the usual 365. That extra day is always February 29, and 2012 is just such a year.

However… We had said there are almost 365 and a quarter days in a year. There are actually 365.2422 days in a solar year, which equates to 365 days, 5 hours, 48 minutes and 46 seconds.

That’s only 11 minutes and 14 seconds less than 365 and a quarter days, but after 125 years, it would mean we’ve added a whole extra day too many.

Thanks to that gap of 11 minutes, years that end in ‘00’ that are not exactly divisible by 400 are not leap years. Therefore, 2000 was a leap year, but 1900, 1800, and 1700 weren’t. 2100 won’t be a leap year either. So, if you have anything due March 1, 2100, don’t procrastinate thinking that you’ll have February 29th, 2100 to take care of it, because you won’t.

Information courtesy of the Newton Ask A Scientist! Program of the Argonne National Laboratory.