Medieval manuscript. 1230 manuscript by the Parisian astronomer and monk Johannes de Sacrobosco (circa 1195-1256), with an artwork depicting the opening of the 'Compotus'. Sacrobosco is notable for writing a short introduction to the Hindu-Arabic numeral system, which became the most widely read introduction to the subject in the later mediaeval centuries. He also wrote a short astronomy textbook, De sphaera mundi, a medieval introduction to the basic elements of astronomy, which was widely read and influential in Europe as an introduction to astronomy. He also correctly described the defects of the then-used Julian calendar and recommended what was essentially the Gregorian calendar.