Three-section orbs

[oshfdk]

The undulating line you mention is often referred to as a cloud band, (previously as a Wolkenband) and it serves the purpose of being a cosmic boundary. It was used by medieval artists to separate the human realm from the divine in illustrations where divinity (Christian or classical) was made manifest. It was also used in cosmic diagrams.

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 (?).

   

I wonder, if three distinct bands surrounding the Earth are named in the text.