We have leap years to synchronize the calendar year with the solar year, which is about 365.2422 days long. By adding an extra day to the calendar every four years, we keep the calendar in alignment with the Earth’s revolutions around the sun.

Julius Caesar introduced the first leap year around 46 B.C., but his Julian calendar had only one rule: Any year evenly divisible by four would be a leap year. That created too many leap years, but the math wasn’t tweaked until Pope Gregory XIII introduced his Gregorian calendar more than 1,500 years later.

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