The ten days that vanished: the Gregorian calendar

In 1582 anyone who went to bed on Thursday 4 October woke up on Friday the 15th. Nobody had slept for ten days: the days had been taken out of the calendar. According to tradition, the decision was made on the hill of Monte Porzio Catone, at Villa Mondragone.

Why was a change needed?

The Julian calendar, in use since Julius Caesar, counted the year as 365 and a quarter days. The solar year is a little shorter: the Julian year lasted about 11 minutes and 14 seconds too long, and so lost a day roughly every 128 years. By 1582 the spring equinox fell on 11 March and not on the 21st, as had been fixed.

It was not only a question of astronomy: the main problem was religious. The date of Easter depends on the equinox and the moon, and with the calendar out of step the feast was drifting further and further from its proper time.

Who did the calculations?

Pope Gregory XIII brought together a commission of clergy, mathematicians and astronomers. The basic scheme is the work of the physician and astronomer Luigi Lilio, also known as Giglio. The Jesuit mathematician Christopher Clavius was the reform’s most important defender, and the commission also included Ignazio Danti and Giuseppe Scala.

The reform was decreed with the bull Inter gravissimas, dated 24 February 1582. A short summary was also sent to the universities to explain the method.

What changed

• Ten days fewer. Thursday 4 October 1582 was followed by Friday 15 October.
• A new rule for leap years. End of century years are leap years only if they can be divided by 400: 1600 and 2000 were, 1700, 1800 and 1900 were not. That gives 97 leap years in 400 years instead of 100, and the remaining error is about 26 seconds a year.
• Easter. To calculate it, the system of epacts was used, that is the age of the moon on 1 January.

A note on the sources: one of them gives 5 October as the last Julian day. The others, including the reference entry on the calendar, give the 4th, and that is the date we use.

Who adopted it, and when

The calendar did not change everywhere at once. In 1582 it was adopted by Catholic countries such as Italy, Spain, Portugal and Poland. Within about two years the Catholic regions of France, Germany, Belgium, Switzerland and the Netherlands followed.

Others waited much longer:

• Protestant Germany adopted it in 1700, with a variant based on Kepler’s astronomical tables.
• England and its colonies in 1752, where eleven days were dropped, from 3 to 13 September: in the meantime 1700 had been a leap year in the Julian calendar and the gap had grown.
• Other European countries in the eighteenth century, and nations outside Europe in the twentieth, such as Japan, China, Turkey and Egypt. The Olympic team of imperial Russia in 1908, for example, missed a competition because of the difference in dates.
• The Orthodox Churches still use the Julian calendar for their feasts.

Why so much resistance?

The objections of the Protestant countries were mainly political: they did not want to accept a reform ordered by the pope. Some scientists criticised the simplified calculations, but their objections were minor. The Danish astronomer Tycho Brahe adopted the new calendar straight away, and Kepler supported it.

What does Monte Porzio Catone have to do with it?

According to tradition, reported by Tor Vergata University, which owns Mondragone today, the bull was issued at the villa where Gregory XIII spent his summers. The sources are not uniform: some say it was signed there, others that it was promulgated there, but all of them acknowledge the link. The story of the popes who used the villa is in Villa Mondragone and the popes.

In short

• The problem: the Julian calendar lost about a day every 128 years; in 1582 the equinox fell on 11 March.
• The reform: the bull Inter gravissimas of Gregory XIII, 24 February 1582.
• The jump: from Thursday 4 to Friday 15 October 1582.
• Leap years: end of century years only if divisible by 400.
• Everyone else: Protestant Germany in 1700, England in 1752, other countries up to the twentieth century; the Orthodox Churches still use the Julian calendar for feasts.