Gregorian calendar

===Background===
Because the [[date of Easter]] is a function{{snd}}the ''[[computus]]''{{snd}}of the date of the [[march equinox|spring equinox]] in the northern hemisphere, the [[Catholic Church]] considered unacceptable the increasing divergence between the [[Canon law of the Catholic Church|canonical date]] of the equinox and observed reality. Easter is celebrated on the Sunday after the [[ecclesiastical full moon]] on or after 21 March, which was adopted as an approximation to the March equinox.{{sfnp|Richards|2013|page=599}} European scholars had been well aware of the calendar drift since the early medieval period.

[[Bede]], writing in the 8th century, showed that the accumulated error in his time was more than three days. [[Roger Bacon]] in {{Circa|1200}} estimated the error at seven or eight days. [[Dante]], writing {{Circa|1300}}, was aware of the need for calendar reform. 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. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome.<ref>{{cite book | first1=Ari |last1=Ben-Menahem |title= Historical Encyclopedia of Natural and Mathematical Sciences | volume=1 |year=2009| url= https://books.google.com/books?id=9tUrarQYhKMC&pg=PA863 | page= 863|publisher=Springer |isbn=9783540688310 }}</ref> 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,{{sfnp|Carabias Torres|2012|page=241}} but the project was not taken up again until the 1540s, and implemented only under Pope Gregory XIII (r. 1572–1585).