Gregorian Calendar (1582): Pope Gregory XIII's Reform of Time

The Gregorian calendar is the reform of the Julian calendar promulgated by Pope Gregory XIII in 1582, which dropped ten days from that October and introduced a more precise rule for leap years. Designed to keep the spring equinox, and therefore Easter, in its traditional place, it was adopted first by Catholic countries and gradually by the rest of the world. Today it is the civil calendar used almost everywhere.

Background: why the Julian calendar drifted

The calendar Europe used in the sixteenth century went back to Julius Caesar’s reform of 46 BC, described in the article on the Julian calendar. It set the year at 365 days with an extra day every fourth year, giving an average year of exactly 365.25 days.

The real solar (tropical) year, measured from one spring equinox to the next, is about 365.2422 days. The Julian year was therefore roughly 11 minutes too long. That sounds trivial, but it adds up to about one full day every 128 years.

The error mattered most to the Church. The Council of Nicaea in AD 325 had worked on the basis that the spring equinox fell on 21 March, and Easter was fixed as the first Sunday after the first ecclesiastical full moon on or after that date. By the 1500s the true equinox arrived around 11 March, so Easter was gradually moving away from the season its rules assumed.

Scholars had pointed out the problem for centuries, among them Roger Bacon in the thirteenth century, and the matter had been discussed at the Fifth Lateran Council (1512–1517). The Council of Trent, which closed in 1563, asked the pope to deal with the revision of liturgical books, and calendar reform followed from that mandate.

How the reform was prepared

Pope Gregory XIII (Ugo Boncompagni, pope 1572–1585) set up a commission in Rome to study the question. Its starting point was a proposal by the Calabrian physician and astronomer Aloysius Lilius (Luigi Lilio), who had died in 1576. His brother Antonio presented the plan to the pope.

A summary of Lilius’s scheme, the Compendium novae rationis restituendi kalendarium, was printed in 1577 and sent to Catholic rulers and universities for comment. The commission, whose members included the Jesuit mathematician Christopher Clavius and the Spanish scholar Pedro Chacón, refined it. Clavius later wrote the long technical defence and explanation of the new calendar.

On 24 February 1582 Gregory XIII signed the papal bull Inter gravissimas (“Among the most serious…”), which announced the reform and laid down how it would work.

What the reform changed

The bull made three main changes. First, it removed ten days to bring the equinox back to about 21 March. In the countries that followed it immediately, Thursday 4 October 1582 was followed by Friday 15 October 1582. The cycle of weekdays was not interrupted.

Second, it changed the leap-year rule so the drift would not return. Every year divisible by 4 remained a leap year, except century years, which would be leap years only if divisible by 400. Thus 1600 and 2000 kept their 29 February, but 1700, 1800 and 1900 did not. This removes three leap days every 400 years and gives an average year of 365.2425 days.

Third, the reform revised the tables used to calculate Easter. Lilius replaced the old lunar cycle with a system of “epacts”, numbers that track the age of the Moon at the start of the year and can be corrected for both solar and lunar errors over long periods.

The remaining error is small. The average Gregorian year is about 26 seconds longer than the mean tropical year, so the calendar slips by roughly a day in about 3,000 years.

Adoption across Europe and the world

Spain, Portugal, the Papal States, most Italian states and Poland–Lithuania switched in October 1582. France followed in December 1582, when 9 December was followed by 20 December. The Catholic Southern Netherlands and many Catholic German territories changed during 1582–1584.

Protestant and Orthodox states were suspicious of a reform issued by the pope. The Lutheran astronomer Michael Maestlin wrote against it, and in the mixed-faith city of Augsburg a dispute over the new calendar in 1583–1584 led to serious unrest, showing how quickly a technical change became a confessional symbol. Holland and Zeeland changed as early as 1582–1583, but most Protestant German states, Denmark–Norway and the remaining Protestant Dutch provinces adopted a version of the new calendar around 1700. Sweden tried a gradual transition from 1700, abandoned it and even inserted a 30 February in 1712, before switching outright in 1753.

Great Britain and its colonies, including those in North America, changed in 1752 under the Calendar (New Style) Act 1750. By then the gap had grown to eleven days, so Wednesday 2 September 1752 was followed by Thursday 14 September. The same act moved the official start of the year in England from 25 March to 1 January.

Outside Europe, Japan adopted the Gregorian calendar for civil use in 1873. China’s Republic adopted it in 1912, although it came into consistent use only later. Russia switched after the revolution in 1918, when 31 January was followed by 14 February; Greece followed in 1923 and Turkey from 1926.

Consequences and oddities

The change created confusion for historians and readers. Dates are often marked “Old Style” (Julian) or “New Style” (Gregorian). George Washington was born on 11 February 1731 by the calendar then used in Virginia, a date now usually given as 22 February 1732, because both the eleven-day shift and the new start of the year apply.

Some effects survive in daily life. The British tax year begins on 6 April, a legacy of the old quarter day of 25 March pushed forward by the eleven lost days and by a further day after 1800 was not a leap year. Russia’s “October Revolution” of 1917 took place in November by the Gregorian calendar.

Many Eastern Orthodox churches still calculate Easter by the Julian calendar, and some use it for all feasts, so their Christmas on 25 December Julian falls on 7 January Gregorian.

Myths and facts

Myth: crowds rioted in 1752 demanding “give us our eleven days”. Historians have found little contemporary evidence for such riots. The slogan is best known from William Hogarth’s 1755 painting An Election Entertainment, a satire on electioneering, and the story grew in later retellings.

Myth: people lost eleven days of their lives. No time disappeared; only the numbering of dates changed. Parliament also took care that rents and contracts fell due after the full number of natural days had passed.

Timeline

• AD 325: Council of Nicaea’s Easter reckoning assumes an equinox on 21 March.
• 1563: Council of Trent leaves calendar reform to the papacy.
• 1577: Summary of Lilius’s proposal circulated to Catholic rulers and universities.
• 1582, 24 February: Gregory XIII issues the bull Inter gravissimas.
• 1582, 4/15 October: First countries drop ten days.
• 1700: Most Protestant German states adopt a reformed calendar.
• 1752, 2/14 September: Britain and its colonies change over.
• 1918: Soviet Russia adopts the Gregorian calendar.
• 1923: Greece adopts the Gregorian calendar for civil use.

Why it matters

The Gregorian reform turned a technical problem of astronomy and Church law into the shared timekeeping system of the modern world. Its slow, uneven adoption also shows how religious divisions in Europe shaped even something as basic as the date, and why historians must still check which calendar a document used.