But we are not certain that in the former they are moving round the earth.

No distinct mention is made in the Republic of the circles of the same and other; although both in the Timaeus and in the Republic the motion of the fixed stars is supposed to coincide with the motion of the whole.

The relative thickness of the rims is perhaps designed to express the relative distances of the planets.
Plato probably intended to represent the earth, from which Er and his companions are viewing the heavens, as stationary in place; but whether or not herself revolving, unless this is implied in the revolution of the axis, is uncertain (Timaeus).

The spectator may be supposed to look at the heavenly bodies, either from above or below.

The earth is a sort of earth and heaven in one, like the heaven of the Phaedrus, on the back of which the spectator goes out to take a peep at the stars and is borne round in the revolution.