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Alien Planets Would Likely Have Leap Years, Too

Today is a leap day, a calendar oddity that helps align our timekeeping with the orbit of the Earth around the sun. But leap day may not be restricted to the Earth — it could occur on planets around other stars as well, as long as there are beings living there to mark the days, scientists say.

The extra day of Feb. 29 is added to the second month of the year every four years, making that year a leap year, according to our Gregorian calendar. This extra day is necessary to keep our calendar in line with the seasons, which depend on Earth’s revolution around the sun.

That’s because the year — the time it takes Earth to make one circuit around the sun — can’t be evenly divided into days — the time it takes Earth to make one full rotation.

“We have a leap year because the spin rate of the Earth, which is 23 hours and 56 minutes, doesn’t divide completely evenly into the length of the year, which is 365.242374 days,” said astronomer Greg Laughlin of the University of California, Santa Cruz. “You can’t fit an exact number of spins into one trip around the sun without having some left over.”