Why Do Leap Years Have 366 Days?

You might think that Earth takes exactly 365 days to complete its orbit around the Sun, but the reality is a bit more nuanced. The journey actually lasts about 365 and a quarter days. To keep our 12-month calendar in sync with Earth’s movement, we have leap years. After four years, those extra hours accumulate to form a whole day. During a leap year, we add this extra day to February, extending it to 29 days instead of the usual 28.

The concept of leap years dates back to ancient Rome, where their calendar had only 355 days, based on lunar cycles. They noticed that their calendar was drifting out of sync with the seasons. To catch up, they introduced an extra month called Mercedonius every two years. Later, in 45 B.C.E., Roman emperor Julius Caesar introduced the Julian calendar, which added an extra day to February every four years. However, this correction was slightly more than needed.

In 1582, Pope Gregory XIII refined the system. The Gregorian calendar retained leap years every four years, except for “century” years (divisible by 100) unless they were also divisible by 400. This adjustment made the calendar even more accurate. Without leap years, our calendar would gradually lose alignment with the seasons, leading to confusion for farmers and others relying on seasonal cues.

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