Christopher Clavius

Christopher Clavius was the Jesuit mathematician who designed the Gregorian calendar the world still uses. The Julian year was about eleven minutes too long, which had pushed the spring equinox ten days away from where the Council of Nicaea had put it and was steadily moving Easter towards summer. Clavius took the scheme proposed by the physician Luigi Lilio, worked out its consequences, and defended it in print for the rest of his life: ten days were struck out of October 1582, and centurial years stopped being leap years unless divisible by four hundred, which brings the average year to within about twenty-six seconds of the tropical year. Catholic countries adopted it at once, Protestant and Orthodox ones took between one and three and a half centuries, and Britain waited until 1752. He taught at the Collegio Romano for nearly half a century and wrote the mathematics textbooks used across the Jesuit school system, which is how mathematical education reached a great deal of Catholic Europe and, through Matteo Ricci, reached China; Ricci was his pupil and translated Euclid into Chinese from his edition. When Galileo reported the moons of Jupiter in 1610 it was Clavius and the Collegio Romano astronomers whose confirmation made the observations credible, and Galileo went to Rome to be honoured by them. Clavius accepted the observations while continuing to reject the Copernican system itself, holding to a modified Ptolemaic scheme to the end. He died two years later, and the largest crater in the southern lunar highlands carries his name. He argued for the calendar reform in print against Joseph Scaliger and others for three decades after it was enacted. Clavius was not his birth name and its origin is uncertain. He entered the Jesuits at fifteen, having heard Ignatius of Loyola preach.

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