The epicycles model represents one arbitrary periodic function as the sum of multiple well understood periodic functions (uniform circular motion, i.e. complex exponentials). This is the same idea we use in a more systematic way today by way of the Fourier series, etc.
It's quite a good method for approximating any arbitrary smooth periodic function.
My best guess is that the originators of this model (perhaps as early as Hipparchus, but at any rate someone by the time of Ptolemy) didn't think there were literally circles being combined, but had a pretty clear idea that they were trying to make an approximate model to fit the data. A previous model, from Eudoxus, was also probably pretty accurate but more cumbersome to compute with, involving approximation of the visible planetary motion by the combination of uniform rotations of some imagined sphere(s) centered on the earth. (We don't know exactly because the books about it don't survive, so all we have are vague descriptions by other people.)
It's almost like the remarkable fact that many things turn out to be turing complete systems that you might not expect (that is, able to perform any operation that any other computer can perform, albiet more slowly). Board games, Chomsky's generative grammar, bacteria (in a sense). Natural languages, programming languages, odd combinations of logical operators.
Any number of things can serve as a basic vocabulary to postulate entities, and do so in a textured way that matches some parts of underlying physical reality.
It's tempting to look at this and make the tragic jump into anti-realism because of the interchangeability of so many forms of representation. But I think as long as you have a combination of pragmatic attitude, treat knowledge claims as provisional and converging on truth, there's a cash value to descriptions that makes them legitimately truth tracking regardless of how they're formed.
It's quite a good method for approximating any arbitrary smooth periodic function.
My best guess is that the originators of this model (perhaps as early as Hipparchus, but at any rate someone by the time of Ptolemy) didn't think there were literally circles being combined, but had a pretty clear idea that they were trying to make an approximate model to fit the data. A previous model, from Eudoxus, was also probably pretty accurate but more cumbersome to compute with, involving approximation of the visible planetary motion by the combination of uniform rotations of some imagined sphere(s) centered on the earth. (We don't know exactly because the books about it don't survive, so all we have are vague descriptions by other people.)