Why We Have Leap Year

It's leap day, and scientist Chris Turner uses his extra time to muse in the New York Times on its origins. Julius Caesar came up with a plan—pinning the calendar to the Earth's circling of the sun—in 46 BC as a way to synchronize months with the seasons. But one circuit takes 365 days, 5 hours, 49 minutes, creating the need for an extra day every 4 years.

By 45 BC, having suffered just one confusing year of temporary corrective months, Caesar had done the trick. Almost. His Julian calendar proved 11 minutes per year too long; by the 16th century, it was was running 10 days ahead. Enter Pope Gregory XIII. In 1582, he got the discrepancy down to 30 seconds a year, creating the calendar we still follow. (More leap year stories.)