Clavius
Christopher Clavius (1537-1612) was a German scholar who became a Jesuit and who spent the latter part of his life in Rome. His portrait gives evidence of his varied mathematical activities for the instruments hung on his wall or standing on his desk indicate his interest in astronomy and trigonometry, the drawings under his hand and the compasses be holds show his work in geometry, and the books piled before him may be looked upon as being his arithmetic, his algebra, and his commentaries on the works of earlier writers. Clavius was influential through his power as a teacher and through the popularity of his publications rather than for his discoveries in the field of mathematics. What is perhaps his greatest achievement was in connection with the adoption of our present calendar. Here, again, his role was to interpret and execute the ideas of others. It is unnecessary to discuss the details of the omission of ten days from the year 1582 in order to bring the calendar year into harmony with the astronomical year or the replacing of the Julian calendar sponsored by Julius Caesar by the Gregorian calendar of Pope Gregory XIII, but it is interesting to note that Clavius was summoned to Rome to explain the theory of the innovations. Ball notes* that Clavius rejected the proposal of omitting a leap year day once in 134 years and substituted the omission of three days in a 400 year period.