Gregorian Calendar Adopted

In 1582, Pope Gregory XIII introduced a reformed calendar that corrected a slow drift in the Julian system, which had governed much of Europe since Julius Caesar's reform in 46 BC. The Gregorian calendar, named after him, spread gradually across the world and is now the standard civil calendar in nearly every country. It matters because it keeps the dates of the year aligned with the seasons, and because it gave the globe a largely shared way of reckoning time.

The problem lay in the length of the Julian year, set at 365.25 days through a leap year every four years. The true solar year is slightly shorter, about 365.2422 days. The difference is small, but over centuries it added up to several days. This mattered especially to the Church, which calculated the date of Easter from the spring equinox. By the sixteenth century the equinox was falling around 10 days earlier on the calendar than it had in 325 AD, when the Council of Nicaea set out the principles for dating Easter. Scholars had noted the problem for centuries, and the Council of Trent encouraged reform.

The solution drew on the work of a commission that included the astronomer and physician Aloysius Lilius, whose proposals were developed after his death, and the Jesuit mathematician Christopher Clavius, who became the reform's leading defender. The new rule kept the leap year every four years but dropped it in century years not divisible by 400. Thus 1700, 1800 and 1900 were not leap years, while 1600 and 2000 were. This brought the average year to about 365.2425 days. To restore the equinox to its intended date, ten days were removed from the calendar. In the countries that adopted the reform first, Thursday 4 October 1582 was followed by Friday 15 October.

Adoption was uneven, and it followed religious and political lines more than scientific ones. Catholic states such as Spain, Portugal, Poland-Lithuania and most of Italy switched almost immediately, and France followed within weeks. Many Protestant regions were suspicious of a papal decree and waited, some for well over a century. Britain and its colonies changed in 1752, by which time eleven days had to be skipped. Russia held to the Julian calendar until after the 1917 revolution, and Greece adopted the Gregorian calendar only in 1923. Several Orthodox churches still use the Julian calendar for religious feasts, which is why Christmas and Easter can fall on different dates for different communities.

The result was a long period in which Europeans lived with two dates, and historians must sometimes specify "Old Style" or "New Style" to avoid confusion. The Gregorian calendar is not perfect, as it will drift by about a day over several thousand years, but it is accurate enough that few have seriously pressed for further change.