Tomb of Pope Gregory XIII

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Tomb of Pope Gregory XIII by Camillo Rusconi, 1723
St. Peter's Basilica, Vatican City
Rome, June 2019

“Pope Gregory XIII is best known for his commissioning of the calendar after being initially authored by the doctor/astronomer Aloysius Lilius and with the aid of Jesuit priest/astronomer Christopher Clavius who made the final modifications. The reason for the reform was that the average length of the year in the Julian calendar was too long – as it treated each year as 365 days, 6 hours in length, whereas calculations showed that the actual mean length of a year is slightly less (365 days, 5 hours and 49 minutes). As a result, the date of the 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. This was verified by the observations of Clavius, and the new calendar was instituted when Gregory decreed, by the papal bull Inter gravissimas of 24 February 1582, that the day after Thursday, 4 October 1582 would be not Friday, 5 October, but Friday, 15 October 1582. The new calendar duly replaced the Julian calendar, in use since 45 BC, and has since come into nearly universal use. Because of Gregory's involvement, the reformed Julian calendar came to be known as the Gregorian calendar. The switchover was bitterly opposed by much of the populace, who feared it was an attempt by landlords to cheat them out of a week and a half's rent. However, the Catholic countries of Spain, Portugal, Poland, and Italy complied. France, some states of the Dutch Republic and various Catholic states in Germany and Switzerland (both countries were religiously split) followed suit within a year or two, Austria and Hungary followed in 1587.” (Pope Gregory XIII, Wikipedia)