De Sphera

De Sphera
Accession number: 
BAV, MS Pal. Lat. 1414, fol. 35r
Vatican Library
About half the surviving manuscripts of On the Sphere contain a diagram comprised of concentric semicircles, which is closely related to Robert Grosseteste’s text.
The semi-circular diagram illustrates the opening section of the text of On the Sphere, in which Grosseteste describes the spherical form of the machina mundi, or world machine. 
He starts with a definition of a sphere based upon a two-dimensional semicircle: ‘For a sphere is the passage of a semicircle with its diameter fixed until it returns to the place from which it started’.
With the sphere defined, the text goes on to describe a set of five nested spheres, each of which is bounded by a semicircle described by three letters, such as ACB. Each letter marks a point on the semicircle’s circumference. The letters are usually inscribed on either end of a diameter (through O, annotation 1) that provides a baseline, and midway around the arc.
The text begins with the outermost semicircle (ACB) and works inwards. The largest, outermost semicircle represents the ‘prime mover’ (annotation 2). Between this line and the second-largest semicircle (DFE) are the seven planets and the fixed stars (annotations 3 and 4).
The remaining three semicircles create four further spheres: three of the four elements (annotation 5) and the fourth element, Earth, at the centre (annotation 6).