Three-section orbs

[VViews]

I'd like to add that the illustration JKP presented on the previous page,( BNF Fr 565) is from an exemplar of Oresme's De Spera.
In the same vein:
BNF Anc 8014(2), 15thC France, Christine de Pisan.
The volume contains various treaties on the virtues, astrology and on astronomy, including a section reprising Nicolas Oresme's Treaty of the Sphere, where we find the following:
Detail from f75:




There does seem to be a rather consistent link between the inverted T-O and Oresme's treaty. The shape does not however always connote the earth, but rather celestial phenomena.