The Eighth Sphere, according to Ptolemy, is the sphere of fixed stars that surround the firmament, outside of Saturn's orbit. The stars dotted on the Eighth Sphere were initially thought to be fixed - thus called 'Fixed Stars' - providing the backdrop against which the movements of the planets could be measured. It was dicovered, however, that the 'fixed' stars indeed did move, albeit very, very slowly, but this is a subject for the next card.
The allegory of Octava Spera depicts the sphere held by an angel, who itself is standing on a sphere.