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.
