Inter gravissimas[1] was a papal bull issued by Pope Gregory XIII on February 24, 1582. The document reformed the Julian calendar and created a new calendar which 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 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 to move the vernal equinox close to March 21, near to where it had been during the Council of Nicaea (May 20 - August 25, 325), (removing ten days of drift), and to make the tabular 14th day of the moon correspond with the real full moon (removing "four days and more" of drift) – even though the Council of Nicaea 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 calulating the Easter and lunar calendars were of much later date than than the Council of Nicaea. The lunar tables became settled when Dionysius Exiguus translated the rules of the Church of Alexandria from Greek into Latin in 525. (Britain adopted them at the Council of Whitby in 664 and France adopted them about 775. Before these years, France and Rome used the tables of Victorius of Aquitaine, which were published in 457. Britain before 664 and Rome before 457 used an 84-year Paschal 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 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;

2. 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, etc.;

3. 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[1][2] 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 entitled Liber novæ rationis restituendi calendarii Romani (not extant) for a fuller explanation of the tables than that contained in the canons (or the bull). Because the bull, canons, and book all 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 calendarij à Gregorio XIII. P. M. restituti explicatio (1603), which is tome V in his collected works Opera Mathematica (1612).[4]


The version of "Inter gravissimas" included by Christoph Clavius in his work explaining the Gregorian calendar[4] contained these dating clauses: "Anno Incarnationis Dominicae M. D. LXXXI. Sexto Calend. Martij, Pontificatus nostri Anno Decimo. ... Anno à Nativitate Domini nostri Jesu Christi Millesimo Quingentesimo Octvagesimo 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
  • "Pontificatus nostri Anno Decimo" (Our pontificate in the year 10) is the year beginning May 13, 1581
  • "Anno à Nativitate Domini nostri Jesu Christi Millesimo Quingentesimo Octvagesimo secundo" (In the year from the Nativity of our Lord Jesus Christ 1582) is the year beginning December 25, 1581
  • "Indictione decima" (Indiction 10) is the year beginning January 1, 1582

All of these years agree that the bull was dated, according to the modern numbering beginning the year with January 1, February 24, 1582.


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 he was proposing included changes to the civil calendar over which he had no authority. 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.

For dates on which various countries adopted the Gregorian reforms, see Gregorian calendar.

The Nicene Council of 325 sought to devise rules whereby all Christians would celebrate Easter on the same day. In fact it took a very long time before Christians achieved that objective (see Easter for the issues which arose). However, the bull Inter gravissimas, which was not immediately adopted by many European countries, became the law of the Catholic Church. It was not recognised, however, by Protestant Churches nor by Orthodox Churches and others. Consequently, the day on which Easter was celebrated by different Christian Churches again diverged.


  1. 1.0 1.1 See Wikisource English translation of the (Latin) 1582 papal bull 'Inter gravissimas' instituting Gregorian calendar reform
  2. Inter Gravissimas (Latin) (French) (English)
  3. Les textes constitutifs du calendrier grégorien (Latin) (French) (bull, canons, and calendars)
  4. 4.0 4.1 Opera Mathematica of Christoph Clavius (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)

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