A Leap Year Does Not Occur Every Four Years Without Exception

The familiar rule of a leap year occurring every four years has ancient roots, dating back to the Roman Julian calendar. This system was a vast improvement over previous methods, but it was built on a slight mathematical overestimation. The Julian calendar assumed a year was precisely 365.25 days long. In reality, the Earth's journey around the sun, known as a tropical year, is a little bit shorter, lasting approximately 365.2422 days. This tiny discrepancy of about 11 minutes per year might seem insignificant, but over centuries, it caused the calendar to drift noticeably out of sync with the seasons.

By the 16th century, this accumulated error meant the calendar was off by about 10 days, affecting the timing of astronomical events like the spring equinox. To correct this drift, Pope Gregory XIII introduced the Gregorian calendar in 1582. Its brilliant solution was a more precise rule: a century year is skipped as a leap year unless it is divisible by 400. This clever exception removes three leap days every 400 years, bringing the average calendar year remarkably close to the true solar year. This is why 1700, 1800, and 1900 were not leap years, but 2000 was, ensuring our calendar remains accurate for millennia.