In the geocentric model of the universe, there is nine spheres surrounding the Earth, each supporting some celestial objects. Listing them from the outer one to the one closest to the Earth, we find:
- Sphere of the Primum Mobile.
- Sphere of the Zodiac.
- Sphere of Saturn.
- Sphere of Jupiter.
- Sphere of Mars.
- Sphere of the Sun.
- Sphere of Venus.
- Sphere of Mercury.
- Sphere of the Moon.
Each of these spheres is animated by circular motion. Celestial objects are supposed to be perfect. And everything that is perfect is animated by circular motion. The origin of the motion of a particular sphere is an Intelligence. There is one Intelligence associated to each sphere.
There is no vacuum. The space between the spheres is filled by an Element, the Ether. Being perfect, this Element is also animated by circular motion.
What lays under the Moon is imperfect and subject to change and degeneration. Imperfect objects are either motionless or animated by straight line motion.
The Earth is at the center of the geocentric universe. It is motionless, so has no associated Intelligence.
Below the Moon are found the four imperfect Elements: Fire, Air, Water and Earth. Being imperfect they are animated by straight line motion. Air and Fire move upward. Water and Earth move downward. The symbols for Air and Fire are triangles pointing upward. The symbol for Earth and Water are triangles pointing downward.