In Cael. 2.12 Aristotle explains tentatively why the apparent motion of the sun and the moon is produced by fewer spheres (and is thus less complex) than the apparent motion of the planets. The question he attempts to answer suggests that he operates with a cosmological model based on Eudoxus’ theory of homocentric spheres which explains the apparent motion of the sun, the moon and each planet as the resultant of uniform and circular motions. However, his argument for the indestructibility of aether in Cael. 1.3-4 conflicts with fundamental assumptions in this model which thus cannot have been developed at the time he put forth the argument in Cael. 1.3-4.
