Before the solar system was realized to be made up of discrete masses traveling in elliptical orbits, before the space between the planets was realized to be a hard vacuum, before the Universe was understood to be infinite (or nearly infinite) in size, a much more complicated system was required to explain the motion of the Heavens. This model, developed by Greeks starting in the 6th Century B.C.E., portrayed all planets, and the Earth’s moon, as orbiting the Earth. Initially, this model was composed of concentric rings, but Plato’s student Eudoxus developed a model that explained the apparent lateral motions of some bodies. This model was composed of concentric shells. It was, naturally, geocentric. Because one of the initial versions of this model was discussed by Ptolemy, it is frequently referred to as the Ptolemaic model. This model was utilized in the search for facts that we would consider today to be of scientific import, such as the distances between the planets. Roger Bacon used the basic theory to calculate, for example, the time necessary to walk from the Earth to the Moon.
Arranged outward from the Earth, lay the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn. Philosophers added to these seven spheres an eighth and ninth. These outer shells were variously populated by God (Prima Mobile) and the angels. The furthest sphere from the Earth was called the Empyrean sphere, or in Christian literature, the Highest Heaven. It may sound strange to our ears that this sphere was also seen as the Firmament, the dwelling place of God. That’s right. The outside boundary of the Universe, essentially, was the solid dwelling place of God. In the books of Job and Isaiah, the Firmament is described as a ceiling of crystalline material.
On a note that may be more pertinent to historians of science than to occultists, when Copernicus and Galileo developed better, solar-centric theories which more completely explained various astronomical observables, this basic explanation of solar system’s geometry was altered only with a replacement of the central body. It remained for Johannes Kepler to demolish the attempted use of the spherical shell model in a rigorously scientific context.
Returning to how this plays into the question at hand, notice firstly that the Universe, according to these models, is spherical and essentially bounded by God. Deep inside, at the juicy center, is all that mortal humans know. More next time... Be thou blessed.