Christopher Clavius, German Jesuit priest and mathematician, 1606

Christopher Clavius, German Jesuit priest and mathematician, 1606. Engraving of Clavius (1538-1612) aged 69, who taught mathematics in Rome and reformed the calendar. The Julian leap-year rule made 3 extra leap years in every 385 years. Thus, the equinoxes and solstices were moving away from their calendar dates. As the date of the spring equinox determines Easter, the church wanted reform of the calendar. Clavius proposed that October 4 1582 (Julian) should be followed by October 15 1582 (Gregorian), and that leap years occur in years divisible by four. This rule is still used today and is so accurate that no further reform will be necessary for many centuries. The people of Frankfurt, thinking they had been robbed of 11 days by the Pope and mathematicians, rioted. Clarious was the first to use the decimal point . ©SSPL/Science Museum