Medieval depiction of the Solar System. 1230 extract from De sphaera mundi, by the Parisian astronomer and monk Johannes de Sacrobosco (circa 1195-1256), with an artwork depicting the earth in the centre, surrounded by a tail-biting dog, the moon and its phases and the sun on its orbit, correlated to signs of the zodiac. De sphaera mundi is a short astronomy textbook that was widely read and influential in Europe as an introduction to astronomy. Sacrobosco is also known 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 correctly described the defects of the then-used Julian calendar and recommended what was essentially the Gregorian