How Leap Years Work: Astronomical Math & Leap Day Explained

A calendar year is normally 365 days long. However, Earth's actual orbit around the Sun takes approximately 365.2422 days (a tropical solar year). Without periodic adjustments, our calendar would drift out of sync with natural seasons by roughly 24 days every century.

The 3 Rules for Determining a Leap Year

Under the modern Gregorian calendar established in 1582, a year is a leap year if:

1. Rule 1: The year is evenly divisible by 4 (e.g. 2024, 2028).
2. Rule 2: If divisible by 100, it is NOT a leap year (e.g. 1900, 2100).
3. Rule 3: Unless it is also divisible by 400, then it IS a leap year (e.g. 1600, 2000, 2400).

Why Do We Need the 400-Year Exception?

If we simply added a leap day every 4 years (Julian Calendar), the calendar year would average 365.25 days. But since the solar year is 365.2422 days, that 0.0078-day difference adds up to about 3 extra days every 400 years. Skipping 3 leap years every four centuries keeps the calendar accurate to within 1 day every 3,236 years!