Pope Gregory XIII in the context of "Gregorian Calendar"

The Gregorian calendar is the calendar used in most parts of the world. 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. The principal change was to space leap years slightly differently to make the average calendar year 365.2425 days long rather than the Julian calendar's 365.25 days, thus more closely approximating the 365.2422-day "tropical" or "solar" year that is determined by the Earth's revolution around the Sun.

The rule for leap years is that every year divisible by four is a leap year, except for years that are divisible by 100, except in turn for years also divisible by 400. For example, 1800 and 1900 were not leap years, but 1600 and 2000 were leap years.

The Quirinal Palace (Italian: Palazzo del Quirinale [paˈlattso del kwiriˈnaːle]) is a historic building in Rome, Italy, the main official residence of the President of the Italian Republic, together with Villa Rosebery in Naples and the Tenuta di Castelporziano, an estate on the outskirts of Rome, some 25 km (16 mi) from the centre of the city. It is located on the Quirinal Hill, the highest of the seven hills of Rome in an area colloquially called Monte Cavallo. It has served as the residence for thirty popes, four kings of Italy and twelve presidents of the Italian Republic.

The Quirinal Palace, originally a papal residence built by Pope Gregory XIII, was selected by Napoleon to be his residence par excellence as emperor. However, he never stayed there because of the French defeat in 1814 and the subsequent European Restoration.

The Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh people (also known as the Berbers). For a quick calculation, between 1901 and 2099 the much more common Gregorian date equals the Julian date plus 13 days.

The Julian calendar was proposed in 46 BC by (and takes its name from) Julius Caesar, as a reform of the earlier Roman calendar, which was largely a lunisolar one. It took effect on 1 January 45 BC, by his edict. Caesar's calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years, until 1582 when Pope Gregory XIII promulgated a revised calendar. Ancient Romans typically designated years by the names of ruling consuls; the Anno Domini system of numbering years was not devised until 525, and became widespread in Europe in the eighth century.

The civil calendar is the calendar, or possibly one of several calendars, used within a country for civil, official, or administrative purposes. The civil calendar is almost always used for general purposes by people and private organizations.

The most widespread civil calendar and de facto international standard is the Gregorian calendar. Although that calendar was first declared by Pope Gregory XIII to be used in Catholic countries in 1582, it has since been adopted, as a matter of convenience, by many secular and non-Christian countries, although some countries use other calendars.

Inter gravissimas (English: "Among the most serious...") was a papal bull issued by Pope Gregory XIII on 24 February 1582. The document, written in Latin, reformed the Julian calendar. The reform has conventionally come to be regarded as a new calendar in its own right, called the Gregorian calendar, named after Pope Gregory XIII. In the realm of orthodox christendom whose liturgical cycles did not apply the Inter gravissimas, the reformed calendar is noted as the New Style. The reformed Julian Calendar is used in most countries today, relatively independent of religious orientation of its populaces.

The adoption of the Gregorian Calendar has taken place in the history of most cultures and societies around the world, marking a change from one of various traditional (or "old style") dating systems to the contemporary (or "new style") system – the Gregorian calendar – which is widely used around the world today. Some states adopted the new calendar in 1582, others not before the early twentieth century, and others at various dates between. A few have yet to do so, but except for these, the Gregorian calendar is now the world's universal civil calendar, old style calendars remaining in use in religious or traditional contexts. During – and for some time after – the transition between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them.

The Gregorian calendar was decreed in 1582 by the papal bull Inter gravissimas by Pope Gregory XIII, to correct an error in the Julian calendar that was causing an erroneous calculation of the date of Easter. The Julian calendar had been based upon a year lasting 365.25 days, but this was slightly too long; in reality, it is about 365.2422 days, and so over the centuries, the calendar had drifted increasingly out of alignment with the Earth's orbit. According to Gregory's scientific advisers, the calendar had acquired ten excess leap days since the First Council of Nicaea (which established the rule for dating Easter in AD 325). Consequently, he ruled, the numbering of days must jump by ten, to restore the status quo ante; thus, for example, when the Catholic countries of Europe adopted the new calendar, the day after Thursday, 4 October 1582 was Friday, 15 October 1582. Countries which did not change until the 18th century had by then observed an additional leap year (1700), necessitating the removal of eleven days from the reckoning. Some countries did not change until the 19th or 20th century, necessitating the removal of one or two further days.

The Fontana del Pantheon (English: Fountain of the Pantheon) was commissioned by Pope Gregory XIII and is located in the Piazza della Rotonda, Rome, in front of the Roman Pantheon. It was designed by Giacomo Della Porta in 1575 and sculpted out of marble by Leonardo Sormani.

The Gallery of Maps (Italian: Galleria delle carte geografiche) is a gallery located on the west side of the Belvedere Courtyard in the Vatican containing a series of painted topographical maps of Italy based on drawings by friar and geographer Ignazio Danti.

The gallery was commissioned in 1580 by Pope Gregory XIII as part of other artistic works commissioned by the Pope to decorate the Vatican. It took Danti three years (1580–1583) to complete the 40 panels of the 120 m long gallery. The painter was Antonio Danti, brother of Ignazio.