Cristoph Clavius (krĬs´tôf klā´vēəs), 1537–1612, German astronomer and mathematician. He entered the Jesuit order in 1555 and studied at Coimbra and Rome. He taught mathematics at the Collegio Romano from 1565. In a commentary on Euclid (1574) and other works, Clavius collected mathematical knowledge, adding some proofs and methods of his own; he helped spread elements of modern algebraic notation such as the plus sign and parentheses. His Commentary on the Sphere of Sacrobosco (1581), which had many editions, was the standard astronomical text of his time. In 1582 his proposed reform of the calendar was adopted by Pope Gregory XIII. In the last years of his life he confirmed Galileo's telescopic observations, although he did not accept them as proof of the Copernican theory.