Christophorus Clavius, Italian Astronomer

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Christophorus Clavius (March 25, 1538 - February 6, 1612) was a German Jesuit mathematician and astronomer who was the main architect of the modern Gregorian calendar. Very little is known about his early life. He joined the Jesuit order in 1555. 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, 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. 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 he held strictly to the geocentric model of the solar system, in which all the heavens rotate about the Earth. He opposed the heliocentric model of Copernicus, but recognized problems with the orthodox model. He was treated with great respect by Galileo, who visited him in 1611 and discussed the new observations being made with the telescope. He died in 1612 at the age of 73.