Why Pope Gregory XIII reformed the calendar in 1582

On 24 February 1582, Pope Gregory XIII issued the bull Inter gravissimas, replacing the Julian calendar with the one most of the world uses today. His motive was Easter. The Julian year of 365.25 days ran about 11 minutes too long, so the calendar slipped against the Sun by roughly a day every 128 years, and by the 1500s the spring equinox that anchors Easter was arriving around 11 March.

The target was not Julius Caesar’s original calendar but the Church’s own benchmark: the Easter rules standardised at the First Council of Nicaea in 325. About ten days had built up since then, so ten days were cut: Thursday 4 October 1582 was followed by Friday 15 October.

Dropping ten days fixed the past; a new leap-year rule, the one that did the real work, fixed the future. Century years stopped being leap years unless divisible by 400, so 1700, 1800 and 1900 lost their 29 February while 1600 and 2000 kept it. Trimming three leap days every four centuries gives an average year of 365.2425 days, close enough that the error builds to a full day only after about 3,000 years.

The fix was a long time coming. The Church had debated reform for more than a century, consulting astronomers including Regiomontanus and Copernicus. The workable scheme came in the 1570s from Aloysius Lilius and, after his death, Christoph Clavius, whose massive book on the reform followed.

Not everyone accepted a Roman decree. In Protestant England the mathematician John Dee devised a more exact calendar for Elizabeth I, but it never gained wide use, and Britain did not switch until 1752. The Gregorian calendar is now used in nearly every country.