Pope Gregory XIII Issues Gregorian Calendar Bull

Context

By the sixteenth century the Julian calendar, introduced in 45 BCE, had accumulated an error of roughly ten days because its assumption of a 365.25-day year exceeded the true tropical year by about eleven minutes annually. This drift had shifted the vernal equinox backward from its nominal March 21 date, undermining the Church’s calculation of Easter, which depended on the equinox and the subsequent full moon. Earlier calls for reform had come from medieval scholars such as Roger Bacon and from the Fifth Lateran Council, but concrete action waited until the Council of Trent authorized Pope Paul III in 1545 to restore the equinox to its position at the time of the Council of Nicaea in 325.

What Happened

In the 1570s a Calabrian physician and astronomer, Aloysius Lilius, submitted a detailed proposal that reduced the number of leap years by omitting three century years in every four hundred unless they were divisible by four hundred. Jesuit mathematician Christopher Clavius refined Lilius’s tables and prepared the supporting calculations. On February 24, 1582, Pope Gregory XIII, then in his tenth year as pontiff, signed and promulgated the bull Inter gravissimas from the Vatican. The document ordered Catholic clergy and states to advance the calendar by ten days in October—Thursday, October 4 would be followed directly by Friday, October 15—and instituted the revised leap-year rule together with adjusted lunar tables for Easter.

Aftermath

Catholic kingdoms including the Papal States, Spain, Portugal, and parts of Italy and France implemented the change on schedule in October 1582, producing the first dates recorded under the new system. Protestant realms rejected the reform on religious and political grounds, maintaining the Julian calendar for centuries; Britain and its colonies waited until 1752, while Orthodox countries adopted the civil Gregorian calendar even later. Contemporary documents often carried dual dates labeled “Old Style” and “New Style” to avoid confusion during the transition.

Legacy

The Gregorian calendar gradually became the worldwide civil standard, adopted for international commerce, diplomacy, and science even in regions that retained the Julian reckoning for religious purposes. Its 400-year cycle of 97 leap years produces an average year length of 365.2425 days, keeping the equinoxes aligned for millennia with only minor future adjustments. Historians view the reform as both a technical triumph of Renaissance astronomy and a marker of the religious divisions that delayed its acceptance across Europe.