Inter gravissimas (English: "Among the most serious...") was apapal bull issued byPope Gregory XIII on 24 February 1582.^[1][2] The document, written inLatin, reformed theJulian calendar. The reform came to be regarded as a new calendar in its own right and came to be called theGregorian calendar, which isused 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 thevernal equinox was near March 21, where it had been during theCouncil of Nicaea (20 May– 25 August 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 theRoman Catholic Church that had become traditional by 1582 for calculating the Easter and lunar calendars became settled whenDionysius Exiguus translated the rules of theChurch of Alexandria fromGreek intoLatin in 525. (Northumbria adopted them atWhitby in 664, theWelsh around 768, and France around 775. Before this, France and Rome had usedVictorius's less exact 457 translation of theCoptic calendar; Britain and Rome before Victorius had usedAugustalis'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:

1. Reduction of the number ofleap 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;
2. Turning back extra days – 4 October 1582, was to be followed by 15 October 1582, and these 10 missing days were not to be counted in calculating end days ofloans,taxes etc.;
3. Easter was to be computed with reference not only to the new 21 March, but also by the use of newPaschal 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 ourpastoral 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'),^[3] and some of these (canons 1, 2, 4) refer to a book entitledLiber novæ rationis restituendi calendarii Romani^[4] 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 (1 March), 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-listingssaints' 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),^[5] which is tome V in his collected worksOpera Mathematica (1612).^[6]

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,".^[5] These clauses include four years:

• "Anno Incarnationis Dominicae M. D. LXXXI." (In the year of the Incarnation of the Lord 1581) is the year beginning25 March 1581.25 March is the traditional date of the conception,annunciation, andincarnation ofJesus.
• "Sexto Calend. Martij" (On the sixth [day] before the Kalends [1st] of March.) The Romans counted inclusively; modern usage would say the fifth day before 1 March, or 24 February.
• "Pontificatus nostri Anno Decimo" (In the tenth year of our pontificate) is the year beginning13 May 1581.Gregory XIII was elected pope on13 May 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 beginning25 December 1581 using the modern1 January beginning of the year. It marks the beginning of year 1582 if that year began on25 December, the traditional date of the birth ornativity of Jesus.
• "Indictione decima" (Indiction 10) is the year beginning1 January, which agrees with modern reckoning.

All of these years agree that the bull was dated 24 February 1582, using the modern1 January 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 theCatholic Church and thePapal States. The changes which Gregory was proposing included changes to thecivil 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 bullInter 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).

MostEastern Orthodox Churches andOriental 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.

• Wikisource English translation ofInter gravissimas

• Gregorian calendar
• Specific calendars
• 1582 in Christianity
• 16th-century papal bulls
• Documents of Pope Gregory XIII
• 1582 in science
• Date of Easter

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