Inter Gravissimas (1582) — The Day Ten Days Were Stolen

On February 24, 1582, Pope Gregory XIII issued the papal bull Inter gravissimas, reforming the Julian calendar. The Julian calendar added a leap day every four years, but the solar year is 365.2422 days, not 365.25 — an error of 11 minutes per year. By 1582, this had accumulated to 10 days: the calendar date was 10 days ahead of the astronomical seasons. Gregory's solution: drop 10 days entirely (October 4 was followed by October 15, 1582), and adjust the leap year rule: century years are not leap years unless divisible by 400. This keeps the calendar accurate to one day in 3,030 years. The mathematics of calendar reform is a study in best rational approximations to an irrational ratio.