Today, as I’m sure you’re all aware, is Leap Day. As such, this is a perfect opportunity to talk about a subject I recently had reason to look into a bit, namely the transition from the Julian to the Gregorian calendar in early-modern Europe.
It’s widely known that the main difference between the two calendars is that in the Julian calendar every year divisible by 4 is a leap year, while in the Gregorian years divisible by 100 but not by 400 are not (so, for example, 2000 was a leap year but 2100 will not be). This brings the Gregorian calendar significantly closer to the true length of the solar year, which is slightly less than 365.25 days.
What’s less well-known is that the primary reason for the change was not to bring the calendar back into alignment with the sun but to fix the date of Easter as close as possible to the date that had been set at the Council of Nicaea in 325. The date of Easter was calculated based on a combination of the phase of the moon with the date of the vernal equinox, which the Julian calendar set on March 21. Unfortunately, since the calendar was not perfectly aligned with the lunar year, March 21 began to drift further and further from the actual vernal equinox, bringing the calculated date of Easter further and further from where it had been placed at Nicaea.
By the sixteenth century the calendar was off by a full ten days, and the Council of Trent in 1563 decided that it needed to be fixed, not just by bringing it back in line but by changing it to more closely approximate the solar year so that it wouldn’t get as far off again. The scheme that was adopted was developed by Aloysius Lilius and popularized by Christopher Clavius. It involved skipping ten days to counteract the drift so far and replacing the old leap-year rule with the new one. On February 24, 1582, Pope Gregory XIII issued the bull Inter gravissimas, which decreed that October 4, 1582, would be followed immediately by October 15, and that henceforth the new leap year rule would be used.
Note, however, that Gregory was a pope, not a king. His authority outside of his own dominions, the Papal States, extended only to religious matters, and moreover was only recognized in Catholic countries. He didn’t have the authority even in those countries to change the civil calendar, which was under the control of the civil authorities. It therefore took a long time for the Gregorian calendar to be adopted throughout Europe. It was only adopted on the date specified in the bull in Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of the states of Italy (including, of course, the Papal States). France adopted it in December 1582, as did the Dutch provinces of Holland and Zeeland, the first Protestant states to do so.
Other Protestant countries resisted the change for a long time, while other Catholic countries fell into line quickly. The Catholic parts of Germany and Switzerland adopted the new calendar in 1583, as did Hungary in 1587. The remaining Dutch provinces, along with Norway, Denmark, and the Protestant parts of Germany, made the change in 1700.
Sweden also made the decision to change in 1700, but unlike all the other countries it decided to make the change gradually rather than all at once, by skipping all the leap days from 1700 through 1740. This enormously confusing and poorly administered transition was abandoned in 1712, but instead of adopting the normal Gregorian calendar then the Swedes went back to the Julian calendar, which they kept until finally switching to the Gregorian in 1753.
Joining Sweden in bringing up the tail end of adoption of the new calendar in western Europe was Great Britain, which made the switch in 1752. At the same time the British switched the beginning of the civil year from March 25, which it had been since the Middle Ages, to January 1, a change that most other European countries had made even before adopting the Gregorian calendar.
The last countries in Europe to change to the new calendars were those in eastern Europe that were dominated by the Eastern Orthodox churches (all of which except the Finnish Orthodox Church still use some form of the Julian calendar for liturgical purposes). The civil authorities in these countries, including Russia, Greece, Bulgaria and Romania, finally made the change in the early twentieth century, generally in the context of the massive political upheavals in eastern Europe at that time.
The Gregorian calendar is now so widely accepted and obviously superior to the Julian that we barely even think about it, but it actually took a very long time to be widely accepted. This was largely because it originated during a time when Europe was in throes of intense political and religious conflict, and even something as simple and beneficial as a calendar reform was vigorously resisted by the opponents of the side that introduced it. Something to ponder on this special day.