Happy Leap Day! 5 simple things to know about a (surprisingly complicated) moment in time

Once every four years, today happens: Feb. 29. But why do we have leap days, and leap years? With the help of Keely Finkelstein, a University of Texas astronomer, we have five things you should know about leap days:

  1. The Western calendar, which uses leap days, is based on the Earth’s rotation around the sun. It takes the Earth 365 days to complete a circuit around the sun. Thus 365 days in a year.
  2. Well, it takes roughly 365 days. The actual number is 365.2422 days. Back in 46 BC, when Julius Caesar ordered the creation of a new calendar, his astronomers knew this. The discrepancy meant that every four years, a day was missing, which could mess with endeavors such as agriculture.
  3. There was a solution, though. Every four years, that missing day should be added back. Thus, today: February 29.
  4. But wait – Caesar’s solution doesn’t quite work. Leap day actually adds too much time to the calendar. Pope Gregory’s people noticed this, too. To account for that discrepancy, the pope decided in the 1580s that every so often we should skip a leap year. So as part of the Gregorian calendar, which we use today, we skipped leap year at the start of every century: in 1700, in 1800, and again in 1900.
  5. But wait … wasn’t 2000 a leap year? Yes, it was, actually. That’s because the pope’s astronomers also figured out that skipping leap year every 100 years would take too much time off the calendar. So to account for that discrepancy, some leap days were added back. We observed Feb. 29 in 2000, and we will again in 2100, 2200 and 2300. But to make the math work, there will be no Feb. 29 in 2400 – at least for those people still living on Earth that point.

leap year

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