De sphaera mundi is a medieval introduction to the basic elements of astronomy written by Johannes de Sacrobosco, circa 1230. Johannes de Sacrobosco (1195 - 1256) was a scholar, monk and astronomer. He wrote an essay on the Hindu-Arabic numeral system which became the most widely read introduction to that subject in the later medieval centuries. 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.