Gregory XIII

The Julian calendar assumed a year of 365 days and six hours. The true tropical year is about eleven minutes shorter, so the calendar gained roughly three days every four centuries against the sun. By the sixteenth century the spring equinox was falling around 11 March rather than 21 March, the date the Council of Nicaea had worked from when it fixed the rule for calculating Easter in 325. The problem was liturgical before it was astronomical: the church's greatest feast was drifting out of its intended season, and successive councils had asked for a correction without producing one.

The scheme Gregory XIII adopted came from a proposal by the Calabrian physician and astronomer Aloysius Lilius, refined after his death by a commission in which the Jesuit mathematician Christopher Clavius did the decisive work. The bull Inter gravissimas, issued in February 1582, did two things. It removed ten days once, so that Thursday 4 October 1582 was followed directly by Friday 15 October, restoring the equinox to 21 March. And it changed the leap rule going forward: century years would be leap years only when divisible by four hundred, which drops three leap days every four hundred years and holds the calendar close to the sun indefinitely.

Adoption followed confessional lines rather than scientific ones. Catholic Spain, Portugal, Poland and the Italian states changed in October 1582; Protestant Germany and the Dutch republic held out for over a century, Britain and its colonies until 1752, Russia until 1918, Greece until 1923. Orthodox churches largely kept the Julian reckoning for the liturgical year, which is why Christmas is still celebrated in January in parts of the Christian world.

Gregory XIII was a canon lawyer who had taught at Bologna and served at the Council of Trent, and he read the council's decrees as an instruction to rebuild the clergy through education. His answer was institutional. He endowed the Roman College of the Society of Jesus on a scale that let it become the model Catholic university of the age, and it carries his name still as the Pontifical Gregorian University. Around it he founded or funded a ring of national colleges in Rome, among them the English, German and Hungarian, Greek, Maronite and Armenian foundations, each training priests to be sent back into territory the church had lost or never held. The English College in particular supplied the missionary priests who entered Elizabethan England at the risk of execution.

Against that record stands the reaction to the news from Paris. On and after 24 August 1572, weeks into his pontificate, thousands of French Protestants were killed in the Saint Bartholomew's Day massacre, beginning in Paris and spreading to provincial towns. Rome received the killings as a deliverance of the church from a Huguenot conspiracy. Gregory XIII had a Te Deum sung, ordered a jubilee of thanksgiving, struck a commemorative medal, and commissioned Giorgio Vasari to paint the events in the Vatican.

Historians differ over how much he understood of the scale and character of the killings when he responded, and the medal and frescoes are sometimes read as a reaction to a reported political victory rather than to a massacre of civilians. What is not in dispute is that the celebration happened and that it was public, and it is the fact most often set beside the calendar when his reign is weighed.