Three-section orbs

[nablator]

Where is this boundary supposed to be? In BNF Fr. 565 this wavy boundary is beyond the stars. I've checked my collection of planetary charts, this one from You are not allowed to view links. Register or Login to view. shows very similar boundary among other bands immediately surrounding the Earth, represented by some kind of T-O map with separate polar regions (?).

These bands represent the concentric spheres of the elements: earth, water, air, fire.

[The Aristotelian] system held that earth was the heaviest element, with the strongest movement towards the center, thus water formed a layer surrounding the sphere of Earth. The tendency of air and fire, on the other hand, was to move upwards, away from the center, with fire being lighter than air. Beyond the layer of fire, were the solid spheres of aether in which the celestial bodies were embedded. They were also entirely composed of aether.

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There are spheres above the 8th in Sacrobosco's Sphaera mundi, so it makes sense to put a "cosmic boundary" (same symbol, different meaning) in the simplified diagram of BNF Fr. 565 above the sphere of the stars (8th), whether it represents the 9th or 10th sphere, or both, or a separation between the spheres and the Empyrean is left to the imagination of the reader.

The total number of celestial spheres was not fixed. In this 16th-century illustration, the firmament (sphere of fixed stars) is eighth, a "crystalline" sphere (posited to account for the reference to "waters ... above the firmament" in Genesis 1:7) is ninth, and the Primum Mobile is tenth. Outside all is the Empyrean, the "habitation of God and all the elect".

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