Today is February 29. Or, as some people call it “leap year day.” Ah yes, leap year. It surfaces every four years so that American presidential hopefuls can have an extra to campaign.
Just kidding. It exists to keep earth in alignment with its astronomical place in our solar system. Most years are 365 days long. But not quite. According to the National Air and Space Museum, the earth completes one orbit of the sun in 365.242190 days, or 365 days 5 hours 48 minutes and 56 seconds. That extra quarter day needs to be accounted for, so every four years we get an extra day and we call it February 29.
Except it’s not quite a quarter day. That’s why some leap years are not leap years. To keep the orbital accounting in balance (as near as possible) three out of every four hundred years there are no leap years at the turns of the centuries. Again, our friends at Air and Space note that there was no February 29 in 1700, 1800 and 1900; but there was one in 2000.
The next time that there won’t be a February 29, in what otherwise would be a leap year, is 2100.