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The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely used civil calendar. It is named for Pope Gregory XIII, who refined the Julian calendar.

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 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 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, taxes 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 Gregorian calendar improves the approximation made by the Julian calendar by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long. This approximation has an error of about one day per 3,300 years with respect to the mean tropical year. However, because of the precession of the equinoxes, the error with respect to the vernal equinox (which occurs, on average, 365.24237 days apart near 2000) is 1 day every 7,700 years. By any criterion, the Gregorian calendar is substantially more accurate than the 1 day in 128 years error of the Julian calendar (average year 365.25 days).

The Catholic Church tried to popularize the new calendar. A bronze commemorative medal, issued in 1582 proves this. On the reverse of the 39 mm diameter 29.1 gram weight medal the right facing portrait of Pope Gregory XIII is visible. The Pope's ornate investiture shows one miracle of Jesus, his walking on the water. Legend around is "GREGORIVS XIII · PONT[ifex] · OPT[imus] · MAXIMVS", i. e. Gregory XIII highest and best priest. Below the portrait is the name of medalist: "·L·PARM·".

Calendar 1 reverseCalendar 1 obverse

The reverse shows the event. In the middle a head of a ram with five stars, from the horns a hanging wreath, and legend above "·ANNO·RESTITVTO·MDLXXXII", i. e. the year of restoration 1582 is visible. The whole image is surrounded by a magnificent dragon biting its tale. The meaning of the head of the ram is easy to understand, and the stars on it also help us. The calendar reform put the date of the vernal equinox to 21th March, thus it is always in Aries astrological sign. (This is not equal to the Aries constellation. Because of the precession the vernal equinox moves around in every 26000 year, and today it is not in the Aries, but in the Aquarius. But it always happens on 21th March, or around it, and that is the first day of the Aries astrological sign.) The common sign of the vernal equinox is a pair of ram-horn also.

In 1891, Pope Leo XIII issued a Motu Proprio re-founding the Specola Vaticana (Vatican Observatory) and a new observatory was built on the walls at the edge of the Vatican. For this occasion a silver medal was issued. On the obverse of the 43.5 mm diameter and 37 gram medal the building of the observatory with its dome is visible on the center right. However more dominating the scene the standing Urania, the muse of astronomy holding a star-globe in her hand. She is leaning on a column that has a figure and text ("ANNO RESTITVTO MDLXXXII") that refers to the calendar reform that was promulgated in 1582, and to the design of the former medal. Actually the first observatory in the Holy See was established in 1578 and later Jesuit astronomers and mathematicians were called on to study the scientific data and implications involved in the reform of the calendar. A beautiful astrolabe is visible at the feet of Urania. Legend around is: "REI · ASTRONOM · HONOR · IN · VAT · INSTAVRATVS · ET · AVCTVS", i. e. established and expanded for respecting astronomical work in Vatican. Below is the date "MDCCCXCI", i. e. 1891.

Vatican 1 reverseVatican 1 obverse

The reverse shows the left facing portrait of the re-founder, Pope Leo XIII in his rich ornamented investiture. In legend is: "LEO · XIII · PONT · MAX · AN · XIV ·".

The calendar reform was again remembered in 1982 on its 4th centenary. A heavy bronze medal was issued depicting Pope Gregory XIII on its obverse. Legend above is: "BULLE INTER GRAVISSIMAS" and "GREGOIRE XIII". Legend below is: "CALENDRIER GREGORIAN 1582". The medalist was Martha Schwenck, whose signature can be discovered near the left rim of the medal.

Calendar 2 reverseCalendar 2 obverse

The reverse shows a beautiful armillary sphere, the moon, the sun and the stars.

The second 80 mm diameter, and 153 gram weight bronze medal was made by Wolfgang Günzel. On its reverse the half left right portrait of the Pope can be found. Legend is in five lines: "PAPST GREGOR XIII. 1502 1582".

Calendar 3 reverseCalendar 3 obverse

The reverse is only text in seven lines: "400 JAHRE GREGORIANISCHER KALENDER 1 · 2 · 3 · 4 · 15 OCTOBER 1582", i. e. 400 years of Gregorian calendar. The line of numbers 1, 2, 3, 4, 15 represents the beginning days of October 1582, when 4th of October was followed by 15th October.