The Gregorian calendar

The story

A calendar that was wrong

The calendar Europeans used at the end of the 16th century had been introduced by Julius Caesar in 46 BC, and for this reason it is called the Julian calendar. It counted 365 days in a year, plus one extra day every four years, in leap years, giving an average of 365 and a quarter days. But the astronomical year, that is, the time the Earth takes to go around the Sun, is a little shorter: about 365 days, 5 hours, 48 minutes and 46 seconds.

The difference is only about eleven minutes a year, but over the centuries it adds up: one day about every 128 years. By the 16th century the error had reached ten days. The spring equinox, which the Council of Nicaea in 325 had fixed at around 21 March, now fell around 11 March. For the Church this was a serious problem, because the date of Easter depends on the spring equinox and the full moon.

Who reformed it

It had been discussed for a long time, and the Council of Trent had also asked the pope to deal with it. Pope Gregory XIII, elected in 1572, appointed a commission of experts, including the Calabrian doctor and astronomer Luigi Giglio (Aloysius Lilius) and the German Jesuit Christopher Clavius, a professor of mathematics in Rome. Giglio died before the reform was approved, but his proposal was taken up and perfected by Clavius.

The solution was elegant. First, ten days had to be skipped, to bring the equinox back to 21 March. Second, to stop the error from coming back, the rule for leap years was changed: years divisible by four stayed leap years, but not the years that begin a century, unless they were divisible by four hundred. So 1600 and 2000 are leap years, while 1700, 1800 and 1900 were not. In this way the average length of the year becomes 365.2425 days, very close to the true one.

From Thursday the 4th to Friday the 15th

On 24 February 1582 Gregory XIII signed the bull “Inter gravissimas”, introducing the new calendar. It was decided that after Thursday 4 October 1582 the next day would be Friday 15 October: ten days, from the 5th to the 14th, simply did not exist. No day of the week was skipped, so the sequence of weekdays carried on. It was a big change: there were doubts, some people feared losing ten days of their life or of their wages, and some rents and interest payments had to be recalculated.

Italy, Spain, Portugal and some other Catholic states adopted the new calendar at once. In Protestant and Orthodox countries, however, acceptance was very slow, because people did not want to obey a decree of the pope. England and its American colonies adopted it in 1752, when the difference had grown to eleven days, and so they had to jump from 2 to 14 September. Sweden, Russia, Japan, China and Greece adopted it even later. In Russia it was only done in 1918, after the revolution.

Today's calendar

The new calendar also reached the countryside and the villages: parish priests had to announce the change from the church, and merchants had to redo their calculations of deadlines. In border areas between states that had adopted the calendar in different years, two neighbouring towns could have different dates.

The Gregorian calendar is still the one we use. Some Orthodox Churches still follow the Julian calendar for their festivals, and for this reason Orthodox Christmas, in Russia and in other countries, falls on 7 January. The Gregorian calendar is not perfect: in about three thousand years it will have built up an error of one day. But for now it is good enough.