Johannes de Sacrobosco

Johannes de Sacrobosco, also written Ioannis de Sacro Bosco (c. 1195 – c. 1256), was a scholar, monk and astronomer who was a teacher at the University of Paris. He wrote a short introduction to the Hindu-Arabic numeral system which became the most widely read introduction to that subject in the later medieval centuries (judging from the number of manuscript copies that survive today). He also wrote a short astronomy textbook, Tractatus de Sphaera, which was widely read and influential in Europe during the later medieval centuries as an introduction to astronomy. In his longest and most original book, written more than three centuries before the introduction of the Gregorian calendar, Sacrobosco correctly described the defects of the then-used Julian calendar, and with a pretty good degree of precision he recommended what was essentially the Gregorian calendar.