Christopher Clavius, S.J., was a celebrated 16th-century mathematician and astronomer. In 1582, upon request from Pope Gregory XIII, Clavius helped develop the Gregorian calendar, which is still in use today. For 45 years, Clavius was a university professor at the Jesuit-run Collegio Romano, now known as the Pontifical Gregorian University, where he advocated master classes to position Jesuit trainees at the forefront of European scholarship. One of his students, Matteo Ricci, S.J., later translated Clavius’ works into Chinese, which ushered in a period of scientific innovations in China. Widely considered the most influential scholar and teacher of the Renaissance, Clavius was an early advocate of Galileo Galilei, whose heliocentric model of the universe upended the longstanding Ptolemaic system.