The primary difficulty with this work is we are so used to a Copernican cosmology that we almost can’t understand what he is saying. His conclusions we can dismiss outright. It might be well, however, to reflect on how he set the stage for cosmology for the next 2,000 years.
His geocentrism appears, and I say it appears because I don’t always understand what he is saying here, to hinge on the argument is that the earth can’t move. We’ll try to unpack that. The heavens cannot move because they are infinite, and an infinite body can’t move in a circle because it would have to move across an infinite range in finite time.
In language anticipating Christian theism, he says heaven is eternal. Well, true. He doesn’t mean by heaven what we mean by heaven, though. What is heaven? Imagine the boundary point between our world and the next (sky, maybe?). Heaven is the substance of the circumference.
Since heaven is eternal, and heaven always has a limit, this means the earth is eternal (283b). This also explains why the earth doesn’t move. If it moves, then it must have begun in time.
He introduces numerous fascinating discussions on the concept of “infinity” that are still in play today (271b). Quite rightly, he notes that an infinite cannot be traversed.
He believes that the earth is a sphere. Sorry, flat-earthers.
He rejects the idea of a plurality of worlds (278a) since only our world contains the entirety of matter.
He says the universe is spherical. I’m not so sure, given big-bang cosmology. It’s more of a funnel-shape.
“We take it for granted that the earth is at rest” (289b).
He says imagine that there are circles within circles. The circles closest within would take longer in the revolution. You don’t need modern science to know this is false. The Greeks ran track. Any runner knows that whoever is on the outer lanes has to run longer.
He rejects the idea of the earth spinning on its axis (296a).