Inter gravissimas was a papal bull issued by Pope Gregory XIII on February 24, 1582. The document, written in Latin, reformed the Julian calendar. The reform came to be regarded as a new calendar in its own right and came to be called the Gregorian calendar, which is used in most countries today.
The intention expressed by the text of this bull was "to restore" the calendar so that seasonal events critical for the calculation of Easter dates would be back in their "proper places" and would be prevented from being moved away again. The idea of reform as such is not otherwise mentioned. The bull identifies "three necessary" things for the correct determination of Easter dates: correct placement of the northern vernal equinox; correct identification of the "14th day of the moon" (effectively full moon) that happens on or next after the vernal equinox, and the first Sunday that follows that full moon. The first two items were the ones that received attention; the third, about choosing the next following Sunday, was not identified as causing any problem, and was not further mentioned.
By "restore", Gregory meant two things. First, he adjusted the calendar so that the vernal equinox was near March 21, where it had been during the Council of Nicaea (May 20 – August 25, 325). This required removing ten days of drift. Second, he made the tabular 14th day of the moon correspond with the real full moon, removing "four days and more" of drift. This would restore the dates of Easter to near where they were at the time of the Council of Nicaea, although that council had not specified where in the calendar the vernal equinox should fall and had not adopted any particular type of lunar tables. The practices of the Roman Catholic Church that had become traditional by 1582 for calculating the Easter and lunar calendars became settled when Dionysius Exiguus translated the rules of the Church of Alexandria from Greek into Latin in 525. (Northumbria adopted them at Whitby in 664, the Welsh around 768, and France around 775. Before this, France and Rome had used Victorius's less exact 457 translation of the Alexandrian calendar; Britain and Rome before Victorius had used Augustalis's 84-year cycle.)
Gregory also made changes to the calendar rules, intending to ensure that, in the future, the equinox and the 14th day of the Paschal moon, and consequently Easter Sunday, would not move away again from what the bull called their proper places.
The changes (relative to the Julian calendar) were as follows:
- Reduction of the number of leap years - centennial years, such as 1700, 1800, and 1900 ceased to be leap years, but years that can be divided by 400, such as 1600 and 2000 continued to be;
- Turning back extra days - October 4, 1582, was to be followed by October 15, 1582, and these 10 missing days were not to be counted in calculating end days of loans, taxes etc.;
- Easter was to be computed with reference not only to the new March 21, but also by the use of new Paschal tables.
The name of the bull consists of the first two words of the bull, which starts: "Inter gravissimas pastoralis officii nostri curas…" ("Among the most serious duties of our pastoral office…").
The bull refers to "the explanation of our calendar" and to a canon related to the dominical letter. To accompany the bull there were six chapters of explanatory rules ('canons'), and some of these (canons 1, 2, 4) refer to a book entitled Liber novæ rationis restituendi calendarii Romani  for a fuller explanation of the tables than that contained in the canons (or the bull). Because the bull and canons refer to each other, they must have been written at roughly the same time, printed at the same time (March 1), and distributed to the several countries together.
These canons enabled the computation of Easter dates in the reformed ('restored') Gregorian calendar, and gave two calendar-listings saints' days, one for the 'year of correction' (1582) and another for the entire new Gregorian year. The bull, canons, and calendars were reprinted as part of the principal book explaining and defending the Gregorian calendar, Christoph Clavius, Romani calendarii a Gregorio XIII. P. M. restituti explicatio (1603), which is tome V in his collected works Opera Mathematica (1612).
The version of "Inter gravissimas" included by Christoph Clavius in his work explaining the Gregorian calendar contained these dating clauses: "Anno Incarnationis Dominicae M. D. LXXXI. Sexto Calend. Martij, Pontificatus nostri Anno Decimo. ... Anno à Natiuitate Domini nostri Iesu Christi Millesimo Quingentesimo Octuagesimo secundo Indictione decima,". These clauses include four years:
- "Anno Incarnationis Dominicae M. D. LXXXI." (In the year of the Incarnation of the Lord 1581) is the year beginning March 25, 1581. March 25 is the traditional date of the conception, annunciation, and incarnation of Jesus.
- "Pontificatus nostri Anno Decimo" (In the tenth year of our pontificate) is the year beginning May 13, 1581. Gregory XIII was elected pope on May 13, 1572 (in terms of the Julian calendar).
- "Anno à Natiuitate Domini nostri Iesu Christi Millesimo Quingentesimo Octuagesimo secundo" (In the year from the Nativity of our Lord Jesus Christ 1582) is the year beginning December 25, 1581 using the modern January 1 beginning of the year. It marks the beginning of year 1582 if that year began on December 25, the traditional date of the birth or nativity of Jesus.
- "Indictione decima" (Indiction 10) is the year beginning January 1, which agrees with modern reckoning.
All of these years agree that the bull was dated February 24, 1582, using the modern January 1 beginning of the year.
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Gregory's reform was enacted in the most solemn of forms available to the Church, but the bull had no authority beyond the Catholic Church and the Papal States. The changes which Gregory was proposing included changes to the civil calendar over which Gregory had no authority (except in the Papal States). The text of the bull recognized this by giving what amounted to orders to the clergy and those "presiding over churches": but in contrast, where the text addresses the civil authorities ("kings, princes and republics"), it "asks", "exhorts" and "recommends" the new calendar changes. The changes required adoption by the civil authorities in each country to have legal effect.
The bull Inter gravissimas was immediately adopted by the major Catholic powers of Europe, but the Protestant countries refused to adopt it until the 18th century, and Eastern European countries adopted it only during or after World War I (the last European country to adopt it was Greece, in 1923).
Most Orthodox Churches have not adopted it at all and continue to reckon their ecclesiastical years by the Julian calendar, even though their home countries use the Gregorian calendar for civil purposes.
- Inter Gravissimas Bill Spencer's (in English) translation of the Inter Gravissimas, 1999, revised 2002, based on Rodolphe Audette's transcription of the (in Latin) text. Also includes Rodolphe Audette's translation into (in French).
- Inter Gravissimas prepared for ISO TC 154 (in English) (in Latin) (in French)
- Les textes constitutifs du calendrier grégorien (in Latin) (in French) (bull, canons, and calendars)
- G Moyer (1983), "Aloisius Lilius and the 'Compendium novae rationis restituendi kalendarium'", pages 171-188 in G.V. Coyne (ed.), The Gregorian Reform of the Calendar: Proceedings of the Vatican conference to commemorate its 400th anniversary (Vatican City: Specola Vaticana), 1983.
- "Inter gravissimas" in Christoph Clavius, Romani calendarii a Gregorio XIII. P. M. restituti explicatio, 1603. (in Latin) The bull begins on page 53, counting from the front cover. The title page is page 5.
- Opera Mathematica of Christoph Clavius (in Latin) Go to Page, Go To Specific Page, Work: Roman Calendar of Gregory XIII, Page: Calendar – Page 13. The fifth volume contains his works on the Gregorian calendar.