De sphaera mundi

De sphaera mundi (Latin title meaning On the Sphere of the World, sometimes rendered The Sphere of the Cosmos; the Latin title is also given as Tractatus de sphaera, Textus de sphaera, or simply De sphaera) is a medieval introduction to the basic elements of astronomy written by Johannes de Sacrobosco (John of Holywood) c. 1230. Based heavily on Ptolemy's Almagest, and drawing additional ideas from Islamic astronomy, it was one of the most influential works of pre-Copernican astronomy in Europe.

Sacrobosco's De sphaera mundi was the most successful of several competing thirteenth-century textbooks on this topic. It was used in universities for hundreds of years and the manuscript copied many times before the invention of the printing press; hundreds of manuscript copies have survived. The first printed edition appeared in 1472 in Ferrara, and at least 84 editions were printed in the next two hundred years. The work was frequently supplemented with commentaries on the original text. The number of copies and commentaries reflects its importance as a university text.

The 'sphere of the world' is not the earth but the heavens, and Sacrobosco quotes Theodosius saying it is a solid body. It is divided into nine parts: the "first moved" (primum mobile), the sphere of the fixed stars (the firmament), and the seven planets, Saturn, Jupiter, Mars, the sun, Venus, Mercury and the moon. There is a 'right' sphere and an oblique sphere: the right sphere is only observed by those at the equator (if there are such people), everyone else sees the oblique sphere. There are two movements: one of the heavens from east to west on its axis through the Arctic and Antarctic poles, the other of the inferior spheres at 23° in the opposite direction on their own axes.

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