Most school children learn that a leap year occurs every four years by adding February 29th to the calendar. But did you know that rule is incomplete? If we only added a leap day every four years, our calendar would drift out of sync with the seasons by roughly three days every four centuries.
The Astronomical Problem: Solar Year vs Calendar Year
A civil calendar year consists of 365 days. However, the time it takes Earth to complete one full revolution around the Sun—known as a tropical solar year—is approximately 365.24219 days (or roughly 365 days, 5 hours, 48 minutes, and 45 seconds).
If we ignore that extra ~0.2422 of a day, after 100 years the calendar would drift by nearly 24 days, causing summer months to eventually drift into winter over centuries.
The Three-Part Gregorian Leap Year Rule
In 1582, Pope Gregory XIII promulgated the Gregorian calendar, refining the older Julian calendar with a precise mathematical adjustment:
- Rule 1 (The 4 Rule): Every year divisible by 4 is a leap year candidate (e.g. 2020, 2024, 2028).
- Rule 2 (The 100 Exception): If the year is also divisible by 100, it is NOT a leap year (e.g. 1700, 1800, 1900, 2100).
- Rule 3 (The 400 Exception to the Exception): If the year is divisible by 400, it IS a leap year after all (e.g. 1600, 2000, 2400).
Why 2000 Was Special and 2100 Will Not Be a Leap Year
The year 2000 was a rare 400-year centurial leap year. However, the upcoming turn of the century in 2100 will be a common year (365 days) because 2100 is divisible by 100 but not by 400.
Check any year instantly using our Leap Year Checker.