With the movement of the 'fixed' stars of the eighth sphere, Ptolemy saw need for another sphere which could serve as a backdrop for the eighth, and he came up with the 'Sphere of the Prime Mover'. This giant something was imagined as a giant crystalline something, oftentimes adorned with some adventurous specifications. Beyond this, there was only God.
In the card the giant something is held by an angel, who dances on... well, turtles all the way down.