Leap year still leaves our calendars off

This month's extra day will go a long way in bringing our clocks and calendars back in line, says a University of Georgia mathematics education researcher, but the result is still not perfect.

That's because each year is just a little bit longer than 365 days-365.2421, to be more exact, says Kevin C. Moore, associate professor of mathematics education in the UGA College of Education. And even when you add in a leap day every four years, the timing still hasn't quite balanced.

"What we want our calendar to do is perfectly align with how long it takes to make one rotation around the sun. That's the ideal situation," said Moore, who specializes in the teaching of algebra and calculus at the high school level. "But 365.2421 days is how long it takes for us to make one complete orbit, not 365 or 366 days."

Which means that when we add in a day for leap year every four years, our calendars are still off. It's a small margin, but over time it adds up. "When you do a leap year, you're actually 45 minutes in surplus over four years," he added.

As a result, after a while you need to skip a leap year in order to make up for this surplus of time. This is why leap years take place every four years-except in years evenly divisible by 100 but not 400. The year 1900, for example, was one not a leap year.

"This system works well, but over 10,000 years, even the system we have is just a little bit off. So there will be a difference in the future," said Moore. "But for now, this keeps us pretty close."