As early as 1232 John of Holy Wood, an English astronomer known usually by the Latin translation of his name, Sacro Bosco, observed that the Julian calendar was becoming increasingly inaccurate over time. It wasn’t until roughly 300 years later that Christopher Clavius, a German Jesuit, formulated the improved Gregorian calendar that was adopted by Pope Gregory XIII and instituted on Friday, October 15, 1582. This is the astronomical work for which Clavius is best known but he was a polymath who wrote on various branches of science and mathematics. Born in 1538, two years before St. Ignatius Loyola’s Society of Jesus was approved by the Catholic Church, Clavius became one of the earliest members of the Order that would have considerable influence on the history of astronomy and mathematics, particularly in the 16th and 17th centuries. The title page shown on the cover, with its armillary sphere, is from one of Clavius’s most important works, the In Sphaeram Ioannis de Sacro Bosco Commentarius (Rome, 1570), which not only presented the much earlier work of the 13th century Sacro Bosco but included evidence of Clavius’s awareness of the Copernican theory as described in the De Revolutionibus Orbium Cœlestium of 1543 (though Clavius remained loyal to the theory of Ptolemy). It also provides evidence of Clavius’s long time correspondence with Galileo, in which at one point he provided confirmation of some of Galileo’s calculations on Venus and Saturn. Galileo was sick in bed when he received Clavius’s confirming data and J. MacDonnell pointed out that “the letter brought him so much joy, it occasioned his immediate recovery” [1]. During that period Clavius was also corresponding with Viete and Kepler. As an author of scientific treatises as well as textbooks widely used in Jesuit schools, he was influential in introducing notation or promulgating that of others; e.g., the decimal point, the radical, applications of logarithms, and parentheses to group together algebraic symbols. His books were on topics in astronomy, spherical geometry and practical geometry, on Euclid (where he called attention to the problem involving the fifth postulate), as well as practical arithmetic. George Sarton referred to Clavius as “the most influential teacher of the Renaissance”. His reputation in science prompted the forming of the Clavius Mathematics Group in 1963, an organization of lay and religious mathematicians who organize four-week research meetings each summer in settings on campuses as widespread as Georgetown, the IAS at Princeton, McGill, Notre Dame, UC Berkeley, and the IHES outside Paris, among others.
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Oct 1, 2013
Silvio Maracchia 
Open Access
