Portrait of Christopher Clavius (1538 - 1612), German Jesuit mathematician and astronomer. Clavius modified the proposal of the modern Gregorian calendar. He adhered to the geocentric model of the solar system, and although he opposed the Copernican heliocentric model, he recognized the inadequacies of the Ptolemaic model. Galileo visited him in 1611 and discussed new observations being made with the telescope. Clavius accepted many of the new discoveries as genuine. He was greatly respected as an astronomer and his textbooks were widely used for astronomical education.