Medieval diagram of an armillary sphere. 1230 artwork from De sphaera mundi, by the Parisian astronomer and monk Johannes de Sacrobosco (circa 1195-1256). De sphaera mundi is a short astronomy textbook that was widely read and influential in Europe as an introduction to astronomy. An armillary sphere is a model of objects in the sky (in the celestial sphere), consisting of a spherical framework of rings, centred on Earth, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic. As such, it differs from a celestial globe, which is a smooth sphere whose principal purpose is to map the constellations. Medieval depiction of a lunar eclipse.