Pope Gregory XIII

Pope Gregory XIII led the Catholic Church from 1572 to 1585. He introduced the Gregorian calendar in 1582 and supported Catholic reform and missionary activity.

Gregorian calendar: The solar calendar introduced by Pope Gregory XIII in 1582 to correct drift in the Julian calendar. Gregory’s most enduring reform replaced the Julian calendar across much of the world.
Aloysius Lilius: An Italian physician and astronomer whose proposal formed the basis of the Gregorian calendar reform. His calendar scheme supplied the central proposal Gregory’s commission adopted.
Papal conclave of 1572: The election assembly that chose Ugo Boncompagni as Pope Gregory XIII in May 1572. The conclave brought Gregory to the papacy after the death of Pius V.
Calendar adoption: The gradual acceptance of the Gregorian calendar by states and communities after its introduction in 1582. Catholic states adopted Gregory’s reform first, while Protestant and Orthodox regions followed later or retained alternatives.
Inter gravissimas: The 1582 papal bull that promulgated the Gregorian calendar reform. This bull formally established the calendar changes Gregory commissioned.
Christopher Clavius: A Jesuit mathematician and astronomer who explained and defended the Gregorian calendar reform. He helped calculate, publish, and justify the reform issued under Gregory.
Pius V: Pope from 1566 to 1572 who enforced Tridentine reforms and preceded Gregory XIII. Gregory inherited the reform program and institutions established under his predecessor.
Gregorian chant: A tradition of unaccompanied liturgical singing in the Roman Catholic Church, named after Pope Gregory I. Its name is often confused with Gregory XIII, though it is associated with a different pope.
Julian calendar: The calendar introduced by Julius Caesar in 46 BCE, based on a 365-day year with a leap day every four years. Its gradual drift from the solar year prompted Gregory’s calendar reform.
Tropical year: The time between successive vernal equinoxes, approximately 365.2422 days. The Gregorian calendar more closely matches this seasonal year than the Julian calendar does.

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