The Rules of a Leap Year
It takes the Earth approximately 365.2425 days to orbit the sun. To keep our calendar aligned with the astronomical seasons, we add an extra day (February 29th) nearly every four years.
The Mathematic Formula:
- If a year is evenly divisible by 4, it is a leap year...
- Unless it is evenly divisible by 100...
- Except if it is also evenly divisible by 400.
Why the Exception?
Because the true solar year isn't precisely 365.25 days (it's slightly less), adding a leap year every four years overcompensates slightly over centuries. Skipping the leap year on century marks (like 1700, 1800, and 1900) fixes this shift. However, we keep it on centuries divisible by 400 (like 2000) for final micro-adjustments.