Editing Pope Gregory XIII (section)

===The Gregorian calendar===
{{Main|Gregorian calendar}}
[[Image:Gregorianscher Kalender Petersdom.jpg|thumb|300px|right|Detail of the [[List of extant papal tombs|tomb of Pope Gregory XIII]] celebrating the introduction of the Gregorian calendar.]]

Pope Gregory XIII is best known for commissioning the [[Gregorian calendar]], initially authored by the physician/astronomer [[Aloysius Lilius]]<ref>{{cite web |title=Who Invented the Calendar We Have Today? |url=https://www.whoinventedit.net/who-invented-the-calendar-we-have-today.html |website=Who Invented It |date=1 September 2018}}</ref><ref>{{cite web |title=CATHOLIC ENCYCLOPEDIA: Aloisius Lilius |url=http://www.newadvent.org/cathen/09247c.htm |website=newadvent.org}}</ref> and aided by Jesuit priest/astronomer [[Christopher Clavius]], who made the final modifications. This calendar is more accurate than the [[Julian calendar]], which treats each year as 365 days and 6 hours in length, even though the actual length of a year is slightly less (365 days, 5 hours, and 49 minutes). As a result, the date of the [[March equinox|vernal equinox]] had slowly, over the course of 13 centuries, slipped to 10 March, while the [[computus]] (calculation) of the date of Easter still followed the traditional date of 21 March. Clavius verified this phenomenon.

Gregory subsequently decreed, by the [[papal bull]] ''[[Inter gravissimas]]'' of 24 February 1582, that the day after Thursday, 4 October 1582 would be the fifteenth, not the fifth, of October. The new calendar replaced the Julian calendar, which had been used since 45 BC. Because of Gregory's involvement, the new calendar came to be known as the Gregorian calendar and has been almost universally adopted.

Much of the populace bitterly opposed this reform; they feared it was an attempt by landlords to cheat them out of a week and a half's rent. However, the Catholic countries of [[Habsburg Spain|Spain]], [[Kingdom of Portugal|Portugal]], [[Polish–Lithuanian Commonwealth|Poland-Lithuania]], and the Italian states complied. [[Kingdom of France|France]], some states of the [[Dutch Republic]] and various Catholic states in the [[Holy Roman Empire]] and [[Old Swiss Confederacy|Switzerland]] (both countries were religiously split) followed suit within a year or two. [[Archduchy of Austria|Austria]] and [[Kingdom of Hungary (1526-1867)|Hungary]] followed in 1587.

However, more than a century passed before [[Protestant]] Europe accepted the new calendar. [[Denmark-Norway]], the remaining states of the Dutch Republic, and the Protestant states of the Holy Roman Empire and Switzerland adopted the Gregorian reform in 1700–01. By that time, the calendar trailed the seasons by 11 days. [[Kingdom of Great Britain|Great Britain]], its [[British North America|American colonies]] and [[Kingdom of Ireland|Ireland]] adopted the reformed calendar in 1752, where Wednesday 2 September 1752 was immediately followed by Thursday 14 September 1752; they were joined by the last Protestant holdout, [[Sweden]], on 1 March 1753.

The Gregorian calendar was not accepted in [[eastern Christendom]] for several hundred years, and then only as the civil calendar.<ref>Henry, Jonathan. "Chapter 3". ''Earth Science''. Clearwater, Fl: Clearwater Christian College, 2010. Print.</ref>