Christopher Clavius (25 March 1538 – 3 February 1612) was a German Jesuit mathematician and astronomer who modified the proposal of the modern Gregorian calendar after the death of its primary author, Aloysius Lilius. Clavius would later write defences and an explanation of the reformed calendar, including an emphatic acknowledgement of Lilio's work. In his last years he was probably the most respected astronomer in Europe and his textbooks were used for astronomical education for over fifty years in and even out of Europe.
Little is known about Clavius' early life other than the fact that he was born in Bamberg in either 1538 or 1537. His given name is not known to any great degree of certainty—it is thought by scholars to be perhaps Christoph Clau or Klau. There are also some who think that his taken name, "Clavius", may be a Latinization of his original German name, suggesting that his name may have been "Schlüssel" (German for "key", which is "clavis" in Latin).
Clavius joined the Jesuit order in 1555. He attended the University of Coimbra in Portugal, where it is possible that he had some kind of contact with the famous mathematician Pedro Nunes (Petrus Nonius). Following this he went to Italy and studied theology at the Jesuit Collegio Romano in Rome. In 1579 he was assigned to compute the basis for a reformed calendar that would stop the slow process in which the Church's holidays were drifting relative to the seasons of the year. Using the Prussian Tables of Erasmus Reinhold and building on the work of Aloysius Lilius, he proposed a calendar reform that was adopted in 1582 in Catholic countries by order of Pope Gregory XIII and is now the Gregorian calendar used worldwide.
Within the Jesuit order, Clavius was almost single-handedly responsible for the adoption of a rigorous mathematics curriculum in an age where mathematics was often ridiculed by philosophers as well as fellow Jesuits like Benito Pereira. In logic, Clavius' Law (inferring of the truth of a proposition from the inconsistency of its negation) is named after him.
He used the decimal point in the goniometric tables of his astrolabium in 1593 and he was one of the first who used it in this way.
As an astronomer Clavius held strictly to the geocentric model of the solar system, in which all the heavens rotate about the Earth. Though he opposed the heliocentric model of Copernicus, he recognized problems with the Ptolemaic model. He was treated with great respect by Galileo, who visited him in 1611 and discussed the new observations being made with the telescope; Clavius had by that time accepted the new discoveries as genuine, though he retained doubts about the reality of the mountains on the Moon. Later, a large crater on the Moon was named in his honor.
- Novi calendarii romani apologia. Rome, 1588
- Romani calendarii a Gregorio XIII P.M. restituti explicatio. Rome, 1603 (An explanation of the Gregorian calendar)
- Romani calendarii a Gregorio XIII P.M. restituti explicatio. (European Cultural Heritage Online)
- Romani calendarii a Gregorio XIII P.M. restituti explicatio. (University of Notre Dame)
- Commentary on Euclid, 1574
- Elementorum Libri XV. Cologne, 1627 (Published online by the Sächsischen Landesbibliothek - Staats- und Universitätsbibliothek Dresden)
- Treatise of gnomonics, 1581
- Refutatio cyclometriae Iosephi Scaligeri. Mainz, 1609
- Clavius, Christoph (1992). Corrispondenza Edizione critica a cura di Ugo Baldini e Pier Daniele Napolitani. Pisa: Università di Pisa – Dipartimento di Matematica. (Critical edition of his correspondence)
Clavius' complete mathematical works (5 volumes, Mainz, 1612) are available online.