Christopher Clavius

Christopher Clavius (1538–1612) was a German Jesuit mathematician and astronomer who helped explain and defend the Gregorian calendar reform.

Pope Gregory XIII: Pope Gregory XIII led the Catholic Church from 1572 to 1585 and promulgated the Gregorian calendar in 1582. He authorized the reform that Clavius explained and defended.
Gregorian calendar: The Gregorian calendar is the civil calendar introduced by Pope Gregory XIII in 1582 to correct accumulated drift in the Julian calendar. This reform was the central subject of Clavius’s explanations and defenses.
Tropical year: The tropical year is the interval between successive passages of the Sun through the vernal equinox. The calendar’s seasonal alignment depends on approximating this year’s length.
Commentary on the Sphere of Sacrobosco: Clavius’s Commentary on the Sphere of Sacrobosco was a widely used textbook on elementary astronomy, first published in 1570. Its repeated editions spread Clavius’s astronomical teaching across European schools.
Aloysius Lilius: Aloysius Lilius was an Italian physician and astronomer whose proposal formed the basis of the Gregorian calendar reform. His calendar scheme supplied the reform Clavius later expounded.
Computus: Computus is the method used to calculate the date of Easter in a given year. Calendar reform altered the rules and tables used to determine Easter.
Leap year: A leap year contains an extra calendar day to keep the calendar aligned with the solar year. The Gregorian reform changed leap-year frequency to reduce calendar drift.
Gregorian calendar controversy: The Gregorian calendar controversy comprised religious, political, and technical disputes over adopting the 1582 reform. Clavius answered technical criticisms and defended the reform’s astronomical basis.
Robert Bellarmine: Robert Bellarmine was an Italian Jesuit theologian and cardinal who became a leading figure in the Counter-Reformation. Like Clavius, he embodied Jesuit scholarship during the Counter-Reformation.
Julian calendar: The Julian calendar is a solar calendar introduced under Julius Caesar, with a leap year every four years. The Gregorian reform corrected the seasonal drift produced by its leap-year rule.

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