Reform of the Calendar by the pope Gregor XIII: in 1582

With the edict 'Inter gravissimas' from February 24 1582 decreeing a reform of the Christian chronology, Pope Gregor XIII closed a discussion about the inaccuracy of the Julianian Calendar which had lasted for several centuries. The founder of the Julianian Calendar, the roman emperor Julius Caesar, calculated in the year 45 bc the average length of a sunyear to be 365 days and 6 hours based on the calculations of the Alexandrinien scientist Sosigenes. This meant that, in a four-year cycle, three years with 365 days were followed by a leap-year of 366 days. However, in reality the astronomically determined average length of the revolution of the earth around the sun takes only 365 days, five hours, 48 minutes and 14 seconds and, therefore, the sunyear of the Julianian Calendar was 11 minutes and 14 seconds too long - a mistake that sums up to an entire day in 128 years. 

For the Christian Church the Council of Nicaea (325) lay down the rule that Easter was to be celebrated on the Sunday following the first full moon in spring. This created the awkward situation that equinox at the start of spring in reality deviated more and more from the set date of March 21. To even out this discrepancy, Gregor XIII ordered to leave out 10 days in 1582 and to procede from Thursday, October 4, to Friday, October15. To prevent the recurrence of the mistake in the future, every year dividable by 100 (secular years) was only to be a leap year if the year could be divided by four when deleting the last two zeros. Consequently, according to the Calendar of Gregor XIII the year 1600 was a leap year as well as the year 2000 while the 29th of February was left out in the secular years 1700, 1800 and 1900. Thereby, our present chronology is 13 days ahead of the old Julianian Calendar.

By the regular deletion of 3 days within 400 years, the average calendar year only differs by 26 seconds from the actual sunyear and will only deviate for a full day in another 3000 years.

Special thanks to Dr. Ilse-Dore Adler for the translation.