Today is leap day, Feb. 29, a date observed only once roughly every four years in the international standard Gregorian calendar. While the extra day serves a technical function, its uniqueness prompts celebrations around the world—especially by the estimated 5 million Leaplings born on the rare day.
Leap years are necessary because the Earth takes slightly longer than 365 days to revolve around the sun—roughly five hours and 48 minutes more (see explainer). For millennia, agricultural societies using a solar calendar to schedule plantings and harvests saw the seasons drift over time and instituted the extra day to compensate. The Romans made the current leap year approach official under Julius Caesar, but that calendar was overhauled 1,500 years later by Pope Gregory XIII because the seasons had drifted off by roughly 10 days. Learn how this prompted the disappearance of 10 days in 1582 here.
In contrast, China's cultural calendar utilizes a leap month every three years. Learn how other non-Gregorian cultures approach leap years here.
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