Blue Sky Science: What’s the science of leap year?

Oskar Westin


What’s the science of leap year?

2016 is a leap year, meaning we insert an extra day, February 29, into the calendar. We do this once every four years in order to keep our calendar aligned with the Earth’s seasons.

Every time the earth spins around once, that’s one day. And as the earth orbits around the sun, it will rotate approximately 365 times. Since it’s not exactly 365 rotations, we have to take action to correct the discrepancy.

If we go one complete orbit of the earth, the earth actually rotates about 365 and a quarter days. The next year, it’s again 365 and a quarter days and now the earth is a half turn ahead. Another year and the earth will be three quarters of a turn ahead. In the fourth year, the earth will have done one full extra revolution that we didn’t count because we were ignoring those quarter days.

If we didn’t insert that extra day into the calendar in the fourth year, in the leap year, then the earth wouldn’t be in the same place in its orbit and the calendar would be out of sync with the seasons.

We like to have a certain number of whole days in our calendar. This is why we avoid fractional days and instead alternate between normal years of 365 days and leap years of 366 days.

The leap year rule does have exceptions that fine tune the calendar even further.

We skip a leap year in a year that’s divisible by 100, unless it’s also divisible by 400.

For example, 1900 is a century year. It’s divisible by four so it would have been a leap year like 1904 or 1908. But since it’s divisible by 100, we had a normal year with 365 days instead of a leap year.

In the year 2000, that’s another century year as well, because it’s divisible by 400, we skipped the skipping. So we actually had a leap year in 2000 and that exception only happens every 400 years.