|Inscription on the grave of Gregory XIII, St. Peter’s Basilica
honoring the Gregorian Calendar
By a papal decree signed on 24 February 1582 by Pope Gregory XIII. the days from October 5, 1582 to October 14, 1582 never happened. This was, because the actually used calendar was out of tune with the mechanics of the heavens. The Julian calendar, named after Iulius Caesar, did not provide sufficient precision to keep in tune for more than 15 centuries with the effect that the most important liturgic festivals and holidays lost synchronicity with heavenly events.
The then used Julian calendar created an error of one day every 128 years, and reaching the 16th century, the lunar calendar calculated after the Moon was already out of phase by ten days in contrast to the ‘real’ Moon. In 1563, the plan for correcting the errors was approved by the Council of Trend, a council of the Roman Catholic Church with the task to react to demands of the reformation.
The reformation of the calendar was to be executed in two stages. At first the correct length of the solar year had to be worked out. Secondly, it was important to provide an accurate as well as simple calendar. The formula was designed by astronomer and philosopher Aloysius Lilius. It indicated 97 leap years out of 400 to solve the problem that the solar year actually lasts 365.2425 days, which means that after four years, the calendar would be off one day without the leap years. Furthermore, every year that can be divided by 100 has to be a leap year while years that are exactly divisable by 400 must not be a leap year, leading to the most possible accuracy. One of this theory’s opponents was the mathematician Giambattista Benedetti, claiming that Easter had to be computed from the real lunar movement, but he did not find many sympathizers.
The new calendar was in general well adopted in between the “borders” of the Catholic Church, but had some difficulties beyond. Overall it took about five centuries until all Christians used the same calendar. Especially the members of the Protestant Church refused to use the new calendar fearing a plot by the Catholics. Most people on the globe were not happy with the ten days that were to be erased like Sweden. The country’s officials decided to transform rather gradually which led to severe confusions. By the time the last adherents of the Julian calendar in Eastern Europe (Russia and Greece) changed to the Gregorian system in the 20th century, they had to drop 13 days from their calendars, due to the additional difference between the two calendars accumulated after 1582.
At yovisto you might watch a video from Khan Academy explaining the various notations of dates and calendars.
References and Further Reading:
- The Mathematics of the Gregorian Calendar
Dave Braverman, 2009