Christopher Clavius, was a Jesuit German mathematician, head of mathematicians at the Collegio Romano, and astronomer who was a member of the Vatican commission that accepted the proposed calendar invented by Aloysius Lilius, that is known as the Gregorian calendar. Clavius would later write defences and an explanation of the reformed calendar, including an emphatic acknowledgement of Lilius' 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.

Aloysius Lilius, also variously referred to as Luigi Lilio or Luigi Giglio, was an Italian physician, astronomer, philosopher and chronologist, and also the "primary author" who provided the proposal that became the basis of the Gregorian Calendar reform of 1582.