Christopher Clavius
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Christopher Clavius
Christopher Clavius, born Christoph Clau, (March 25, 1538 – February 12, 1612) was a German mathematician and astronomer who was the main architect of the modern Gregorian calendar. In his last years he was probably the most respected astronomer in Europe.
Clavius, born in Bamberg and joined the Jesuit order in 1555. He attended the University of Coimbra in Portugal, where the famous mathematician Pedro Nunes was his teacher. Following this he went to Italy and studied theology at the Jesuit Collegio Romano in Rome. He remained at the Collegio Romano where he taught mathematics. In fact, except for a period in Naples around 1596 and a visit to Spain in 1597, Clavius was to remain Professor of Mathematics at the Collegio Romano for the rest of his life. 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. This calendar was adopted in 1582 in Catholic countries by order of Pope Gregory XIII and is now used worldwide.
The Julian leapyear rule created 3 leap years too many in every period of 385 years. As a result, the actual occurrence of the equinoxes and solstices slowly moved away from their calendar dates. The date of the spring equinox determines the date of Easter so the church began to press for reform.
Clavius proposed that Wednesday, Oct. 4, 1582 (Julian) should be followed by Thursday, Oct. 15, 1582 (Gregorian). He proposed that leap years occur in years exactly divisible by four, except that years ending in 00 must be divisible by 400 to be leap years. This rule is still used today and is so accurate that no further reform of the calendar will be necessary for many centuries.
Vičte did not like Clavius's calendar and the people of Frankfurt rioted against the Pope and mathematicians who, they believed, had conspired together to rob them of 11 days. Clavius wrote Novi calendarii romani apologia (1595) which justified the new calendar reforms defending them against these attacks.
Clavius was a gifted teacher and writer of textbooks. He produced a version of Euclid's Elements in 1574 which contains ideas of his own. Another well written book was Algebra (1608). His arithmetic books were used by many mathematicians including Leibniz and Descartes.
Clavius produced a number of instruments. He worked on an instrument to measure fractions of angles. He also designed sundials and developed a quadrant for use in surveying.
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 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; Clavius had by that time accepted the new discoveries as genuine, though he retained doubts about the reality of the mountains on the Moon. In light of this fact, it is very ironic that a large crater on the moon is named for him.
External links
 The Galileo Project (http://galileo.rice.edu/sci/clavius.html) — biography of Christopher Clavius.it:Christopher Clavius