Inter gravissimas (English: "Among the most serious...") was a papal bull issued by Pope Gregory XIII on 24 February 1582. [1] [2] 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 (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 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 Coptic 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:
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'), [3] and some of these (canons 1, 2, 4) refer to a book entitled Liber 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-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), [5] which is tome V in his collected works Opera 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:
All of these years agree that the bull was dated 24 February 1582, using the modern 1 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 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 Eastern Orthodox Churches and Oriental 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.
The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year. The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh people.
A leap year is a calendar year that contains an additional day compared to a common year. The 366th day is added to keep the calendar year synchronized with the astronomical year or seasonal year. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day, a leap day, or month, a leap month, into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected.
Pope Gregory XIII, born Ugo Boncompagni, was head of the Catholic Church and ruler of the Papal States from 13 May 1572 to his death in April 1585. He is best known for commissioning and being the namesake for the Gregorian calendar, which remains the internationally accepted civil calendar to this day.
The Revised Julian calendar, or less formally the new calendar and also known as the Milanković calendar, is a calendar proposed in 1923 by the Serbian scientist Milutin Milanković as a more accurate alternative to both Julian and Gregorian calendars. At the time, the Julian calendar was still in use by all of the Eastern Orthodox Churches and affiliated nations, while the Catholic and Protestant nations were using the Gregorian calendar. Thus, Milanković's aim was to discontinue the divergence between the naming of dates in Eastern and Western churches and nations. It was intended to replace the Julian calendar in Eastern Orthodox Churches and nations. From 1 March 1600 through 28 February 2800, the Revised Julian calendar aligns its dates with the Gregorian calendar, which had been proclaimed in 1582 by Pope Gregory XIII.
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As a moveable feast, the date of Easter is determined in each year through a calculation known as computus. Easter is celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or after 21 March. Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the Julian or Gregorian calendar. The complexity of the algorithm arises because of the desire to associate the date of Easter with the date of the Jewish feast of Passover which, Christians believe, is when Jesus was crucified.
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The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.
The year 1582 in science and technology included a number of events, some of which are listed here. This year sees the introduction of the Gregorian calendar, promulgated by Pope Gregory XIII in the Papal bull Inter gravissimas on February 24 and based largely on the work of Christopher Clavius. Under the Habsburg monarchy in Spain, Portugal and Italy, together with the Polish–Lithuanian Commonwealth, the year continues under the Julian calendar as normal until Thursday October 4, the next day becoming Friday October 15; France follows two months later, letting Sunday December 9 be followed by Monday December 20. Other countries switch in later years.
The adoption of the Gregorian Calendar was an event in the early modern history of most cultures and societies, marking a change from their traditional dating system to the modern dating system – the Gregorian calendar – that is widely used around the world today. Some states adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between. A few still have not, but except for these, the Gregorian calendar is now the world's civil calendar universally, although in many places an old style calendar remains used in religious or traditional contexts. During – and for some time after – the change between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them.