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De sphaera mundi is a medieval introduction to the basic elements of astronomy written by Johannes de Sacrobosco, circa 1230. Johannes de Sacrobosco (circa 1195 - circa 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, De sphaera mundi, which was widely read and influential in Europe during the later medieval centuries as an introduction to astronomy. In De Anni Ratione, 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.
