Christopher Clavius Source List

http://www.gap-system.org/~history/Mathematicians/Clavius.html

http://galileo.rice.edu/sci/clavius.html

http://www.faculty.fairfield.edu/jmac/sj/scientists/clavius.htm

http://www.britannica.com/EBchecked/topic/120629/Christopher-Clavius

H L L Busard, Biography in Dictionary of Scientific Biography (New York 1970-1990).