Curiosities: Why do we need leap days?

Leap days appear every four years or so, including this year, and they are needed because one orbit around the sun does not occur in an exact number of days, says Jim Lattis, director of UW Space Place, in the UW–Madison astronomy department.

“It would be so easy if Earth simply rotated 365 times every time it orbits the sun once, but it doesn’t. If it was exactly 365 days, we could add one day every four years, and that would compensate for the irregularity and stay exactly in synch. The spring equinox (when day is the same length as the night) would fall on the same day of the year every year.”

It doesn’t work out that way, however, because the orbital period is a bit less than 365 days, and “the fraction is extremely inconvenient,” Lattis says.

In 1582, Pope Gregory instituted a calendar reform (which is why we use the “Gregorian Calendar”), which compensates as follows: We add a leap day every four years, except in years divisible by 100 (like 1900), but those can also be leap years if they are divisible by 400.

“So we would have skipped a leap day in 2000,” says Lattis, “but it’s also divisible by 400, and that’s the exception to the exception. It all starts from the fact that there is not an even number of days in the orbital period.”