Every four years, our calendar expands by one day. February, usually the shortest month of the year with 28 days, receives an extra day: February 29th. This year is known as a "leap year." But why do we have this calendar quirk, and what would happen if we decided to skip it?
The Solar Orbit Inaccuracy
Our standard calendar year is exactly 365 days long. However, the time it takes for planet Earth to complete one full revolution around the Sun (a tropical or solar year) is not a perfect whole number. It actually takes approximately 365.2422 days (roughly 365 days, 5 hours, 48 minutes, and 46 seconds).
If we ignored this fractional decimal, our calendar would slowly drift out of alignment with the astronomical seasons. Adding up the extra 5 hours and change over a century would shift the calendar by approximately 24 days. Eventually, July would occur during winter in the Northern Hemisphere.
The Math of the Gregorian Adjustment
To correct this drift, we add one extra day (24 hours) every four years (4 × 0.2422 ≈ 0.9688 days). This adjustment keeps our calendar stable. However, adding a day every four years actually overcorrects slightly (adding 1 full day is slightly larger than the accumulated 0.9688 fractional days). To adjust for this overcorrection, the Gregorian calendar applies three rules:
- A year is a leap year if it is evenly divisible by 4.
- Except if it is divisible by 100, in which case it is not a leap year.
- Unless it is also divisible by 400, in which case it is a leap year.
For example, the year 2000 was a leap year because it is divisible by 400. However, the years 1700, 1800, and 1900 were not leap years because they were divisible by 100 but not 400. This fine-tuning ensures our calendar remains aligned with Earth's orbit for thousands of years.