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Christopher Clavius was a Jesuit astronomer who helped Pope Gregory XIII to introduce what is now called the Gregorian calendar.Clavius entered the Jesuit Order in 1555 and received his education within the Order. He attended the University of Coimbra in Portugal. Following this he went to Italy and studied theology at the Jesuit Collegio Romano in Rome.
He remained at the Collegio Romano were 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.
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.
Viète 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.
Although Clavius produced little mathematics of his own, he did more than any other German scholar of the 16th Century to promote a knowledge of mathematics. He was the first, however, to use the decimal point.
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.
The picture above is from a engraving made from a portrait from life.
References (12 books/articles)
References elsewhere in this archive:
Tell me about the errors in the Julian calendar
There is a Crater Clavius on the moon. You can see a list of lunar features named after mathematicians.
Other Web sites:
Rice University, USA
Clarke University, USA