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Happy bissextus! Inside the weird science of leap year

The great leap somewhere

Need to Know

It’s Leap Day, that curious quirk when the clock strikes 13, the calendar reads February 29, and you’re spared paying the rent for just one more day.

The technical name for it is bissextile, from the Latin bissextus dies, which the OED helpfully explains is “the name given to the intercalary day inserted by the Julian calendar every fourth year after the sixth day before the calends of March.” Got that?

The calendar, as you might have gathered, is a far more complicated and strange technology than our first grade teacher ever let on.

The solar year is not evenly divisible by 365 – the actual time it takes the Earth to make a complete revolution around the Sun is 365 days, 5 hours, 48 minutes, and 46 seconds. If we didn’t account for that ugly remainder, we would soon be celebrating July 4th in the middle of winter.

The leap year was one of Julius Caesar’s innovations in 45 BCE, although they initially occurred every three years. Roman priests caught the error soon enough, and the quadrennial leap year was instituted in 8 BCE.

Unfortunately the math still doesn’t quite add up – over a period of 400 years, it results in three extra days added to the calendar. Enter the 1582 introduction of the Gregorian calendar, which determined that only one of every four century years (1600, e.g.) is considered a leap year.

Quartz has a great essay from seven-year-old Rachel Wise, whose birthday falls today (she’s actually 28). 25-year-old Daisy Belle Ward is the nation’s oldest leap baby (she’s 100).

In Ireland and the U.K., Leap Day is traditionally the day when women can ask men to marry them; according to legend Saint Patrick is responsible for the idea.

And finally, if you’re paid an annual salary, you’re pretty much working for free today.

Now go forth (and get marching).