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, Sacrobosco correctly described the defects of the then-used Julian calendar, and, three centuries before its implementation, recommended a solution much like the modern Gregorian calendar. Very little is known about the education and biography of Sacrobosco. For one thing, his year of death has been guessed at 1236, 1244, and 1256, each of which is plausible and each lacking adequate evidence Less