Leap Year 2024

February 29th only comes once every four years – but what exactly is a leap year and why was an extra calendar day needed?

Accounting for Earth’s True Revolution

  • A solar year equals one full revolution of Earth around the sun. This takes 365 days plus 5 hours, 48 minutes and 46 seconds.
  • Early calendars just approximated 365 days. But the extra hours aggregated over centuries. To compensate, an extra day is added to February every four years.
  • This realigns the calendar with Earth’s rotation and prevents seasons drifting. The day is in February as it’s the shortest month.

When Was the Concept Introduced?

  • The leap year was conceived in 46 BC under Julius Caesar’s scholars. The Julian calendar added a 366th day to every fourth year.
  • But 6 hours used for calculations still differed from the actual 5 hours 48 minutes extra time. Small errors crept in over the ages.
  • By 1500s, dates had shifted 10 days ahead of Earth’s revolution. So 10 days were cut in 1582 to reset the correlation.

The Gregorian Fix

  • Pope Gregory XIII also devised a long term solution – dropping leap years for most years ending in double zeroes.
  • This prevented excess day buildup from too many quadrennial additions. However, exceptions were 400-divisible years like 2000.
  • The refinement finally aligned the Gregorian calendar with Earth’s tropical orbit to within 26 seconds.



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