For Aristotle, the so-called sublunary sphere consists of the classical four elements, fire-water-earth-air. It is the realm of ever-changing things in contrast to the part of the universe which begins with the moon and stretches to the fixed stars and in which everything is made from the fifth element, the quintessence or aether, and so is incorruptible and eternal, never deviating from its regular and periodic motion.
But there is some counter-evidence to this theory that can be gained by unaided observations of the night sky.
If we look at the full moon with the naked eye, in normal circumstances, we can see at least the lunar maria, maybe, if we have good eyesight, even a big crater like Clavius.
How did Aristotle explain that the lunar maria are darker than the highlands if the whole moon is made from aether? How can an object made out of this perfect material have such blemishes?
And then there are shooting "stars", which are transient and changing. Yes, they are "sublunary", of course. But how could Aristotle have known this?
Visible comets are not "sublunary" and while they have regular orbits (but with very long periods), there is no such cyclical change in brightness (as observed with the Moon).
Finally, there are variable stars (stars whose brightness as seen from Earth changes, an extreme example would be a supernova). As stars they would have been located in the outermost celestial sphere, and so clearly made out of aether. There is some evidence that variable stars were observed in 1200 BCE by Egyptian astronomers.
Was there some criticism of the "aether theory" along those lines by Aristotle's contemporaries? Did Aristotle try to give justifications how this counter-evidence is reconcilable with his theory?