Christopher Clavius (25 March 1538 – 3 February 1612) was a
German Jesuit mathematician and
astronomer who modified the proposal of the modern
Gregorian calendar after the death of its primary author,
Aloysius Lilius. Clavius would later write defences and an explanation of the reformed calendar, including an emphatic acknowledgement of Lilio's work. In his last years he was probably the most respected astronomer in
Europe and his textbooks were used for astronomical education for over fifty years in and even out of Europe.