# De Sphera

Commentary

De Sphera

Accession number:

BAV, MS Pal. Lat. 1414, fol. 35r

Collection:

Vatican Library

About half the surviving manuscripts of

The semi-circular diagram illustrates the opening section of the text of

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,

The text begins with the outermost semicircle (ACB) and works inwards. The largest, outermost semicircle represents the ‘prime mover’ (

The remaining three semicircles create four further spheres: three of the four elements (

*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**).