Gregorian calendar
[1][a] 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.
Whilst the reform introduced minor changes, the calendar continued to be fundamentally based on the same geocentric theory as its predecessor.
[4][failed verification] The reform was adopted initially by the Catholic countries of Europe and their overseas possessions.
[5] However, many Orthodox churches continue to use the Julian calendar for religious rites and the dating of major feasts.
It was instituted by papal bull Inter gravissimas dated 24 February 1582 by Pope Gregory XIII,[3] after whom the calendar is named.
An attempt to go forward with such a reform was undertaken by Pope Sixtus IV, who in 1475 invited Regiomontanus to the Vatican for this purpose.
[10] The increase of astronomical knowledge and the precision of observations towards the end of the 15th century made the question more pressing.
Numerous publications over the following decades called for a calendar reform, among them two papers sent to the Vatican by the University of Salamanca in 1515 and 1578,[11] but the project was not taken up again until the 1540s, and implemented only under Pope Gregory XIII (r. 1572–1585).
In 1545, the Council of Trent authorised Pope Paul III to reform the calendar, requiring that the date of the vernal equinox be restored to that which it held at the time of the First Council of Nicaea in 325 and that an alteration to the calendar be designed to prevent future drift.
Some of these experts, including Giambattista Benedetti and Giuseppe Moleto, believed Easter should be computed from the true motions of the Sun and Moon, rather than using a tabular method, but these recommendations were not adopted.
[12] The reform adopted was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio).
He also produced an original and practical scheme for adjusting the epacts of the Moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.
[i] The German mathematician Christopher Clavius, the architect of the Gregorian calendar, noted that the tables agreed neither on the time when the Sun passed through the vernal equinox nor on the length of the mean tropical year.
All values are the same to two sexagesimal places (0;14,33, equal to decimal 0.2425) and this is also the mean length of the Gregorian year.
Lilius proposed that the 10-day drift should be corrected by deleting the Julian leap day on each of its ten occurrences over a period of forty years, thereby providing for a gradual return of the equinox to 21 March.
[20] When the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of 10 days.
A month after having decreed the reform, the pope (with a brief of 3 April 1582) granted to one Antoni Lilio the exclusive right to publish the calendar for a period of ten years.
The Lunario Novo secondo la nuova riforma[k] was printed by Vincenzo Accolti, one of the first calendars printed in Rome after the reform, notes at the bottom that it was signed with papal authorization and by Lilio (Con licentia delli Superiori... et permissu Ant(onii) Lilij).
On 29 September 1582, Philip II of Spain decreed the change from the Julian to the Gregorian calendar.
[22] This affected much of Roman Catholic Europe, as Philip was at the time ruler over Spain and Portugal as well as much of Italy.
For example, the British could not bring themselves to adopt the Catholic system explicitly: the Annexe to their Calendar (New Style) Act 1750 established a computation for the date of Easter that achieved the same result as Gregory's rules, without actually referring to him.
[26] Britain and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752.
On 1 January 1926, the use of the Gregorian calendar was extended to include use for general purposes and the number of the year became the same as in most other countries.
[50] Most Western European countries changed the start of the year to 1 January before they adopted the Gregorian calendar.
For ordinary purposes, the dates of events occurring prior to 15 October 1582 are generally shown as they appeared in the Julian calendar, with the year starting on 1 January, and no conversion to their Gregorian equivalents.
Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar.
It appears in Latin,[73] Italian,[74] French[75] and Portuguese,[76] and belongs to a broad oral tradition but the earliest currently attested form of the poem is the English marginalia inserted into a calendar of saints c. 1425:[77][78][79] Thirti dayes hath novembir April june and Septembir.
The unhelpfulness of such involved mnemonics has been parodied as "Thirty days hath September / But all the rest I can't remember"[80] but it has also been called "probably the only sixteenth-century poem most ordinary citizens know by heart".
ISO 8601, in common use worldwide, starts with Monday=1; printed monthly calendar grids often list Mondays in the first (left) column of dates and Sundays in the last.
This is because the Earth's speed of rotation is gradually slowing down, which makes each day slightly longer over time (see tidal acceleration and leap second) while the year maintains a more uniform duration.
