Summary of Plato’s understanding of the stars from Origen and the Life of the Stars: A History of an Idea by Alan Scott (Oxford Early Christian Studies, Clarendon Press, 1994):
Plato is less concerned with how things happen than with why they happen, and for this reason he regards astronomy as only of secondary importance. Though Plato does associate wisdom and purity with gazing upon heaven, his ideal is not the astronomer but the philosopher. Like geometry, astronomy is a discipline in which knowledge of what is eternally true can be available, but such knowledge is of no use unless it is first subordinated to philosophy. Plato has little interest in observational astronomy: true astronomy is not concerned merely with what is seen in heaven but with the understanding of what lies behind what is seen. Even if the Greater Hippias is not a genuine Platonic work, it is faithful to Plato in depicting the learned, pompous, and intellectually shallow Hippias as particularly expert in astronomy. The destiny of the soul is not to look upon the sensible heaven but upon the ’superheavenly place’, which is not possible for physical eyes but only for the soul. The stars, inasmuch as they are visible, do not embody exact knowledge, which can only be grasped by the mind and thought. For Plato, as also for the Pythagoreans, astronomy was useful chieﬂy as a means of understanding what was purely rational. To the mind which understood properly, there was true harmony in heaven even if this was not possible for the material bodies of heaven, even as there is exactness in geometry though it is not part of any merely visible diagram. This is the understanding of sun, moon, and stars enjoyed by the inhabitants of the ‘true earth’ in the Phaedo. Thus geometry and astronomy are part of the necessary training for insight into what was immutable and eternal.
Just as Plato accepts elements of the latest astronomical research but not the philosophical and religious implications it was sometimes thought to have, so too before his later writings he can accept the popular veneration of the heavens without taking it altogether seriously. In the Republic, Plato does say that the craftsman of heaven, like Daedalus, fashioned the courses of the stars with the greatest beauty possible, and at one point Plato even goes so far as to refer casually to ‘the gods in heaven’, one of which is the sun, and yet he also openly doubts that the visible stars are eternal and immutable. Even in his ‘middle period’ Plato shows little interest in the visible stars and planets and with observational astronomy. In this again he was similar to Socrates, who by all accounts avoided the investigation of the heavens and concerned himself mainly with ethical questions.
…The astral soul is either immanent or transcendent; if it is immanent it acts directly on the body, if transcendent, it acts either through the intermediary of a special material body which it provides itself, or through some unknown agency. Plato does not make clear at this point the number of souls in heaven: his usual assumption is that each heavenly body has its own soul and is a god, but if in heaven soul transcends its body there might be only one heavenly soul. It is also not clear in the Laws (as it was in the Timaeus) if stars are gods as well as planets: the Laws only explicitly refers to the divinity of the planets (which is the view found in the Statesman).
One thing which is clear is that the astral soul itself is invisible: we do not look upon the soul, we only calculate its movements mathematically. As Plato had said earlier in the Republic, it is not what is seen in heaven which is important, but what is intelligible. Thus, strictly speaking, one would expect Plato to assert that the heavenly bodies are not gods, but are merely controlled by gods in some way. More speciﬁcally, one might expect him to say that the visible star or planet is a body joined eternally to a soul, which is how he says he imagines the gods in the Phaedrus myth. But Plato is very elusive in matters of religion, and in the end his real opinion is never clear. What is clear is that he has no objection to calling the planets (and sometimes the stars) gods and worshipping them, just as he includes devotion to images in the religion of the state.
…The author of [Epinomis] tells us as Plato did that most people regard the stars as lifeless because of their uniform motion, but that this is in fact a clear sign of their intelligence. The planets do not ‘wander’, and youths should learn enough astronomy to avoid such an error. Mathematical training is combined with astronomical theory, for number is a divine gift which has been granted to humanity to be learned through the observation of heavenly revolution, and is a prerequisite of wisdom. Their precise movement is a proof of universal divine providence and of the priority of soul to body, as it was also in the Laws. The divinity of the stars and of the seven planets is both presumed and stated throughout the dialogue, as it is in much of the Platonic corpus.
This last point that “most people regard the stars as lifeless because of their uniform motion, but that this is in fact a clear sign of their intelligence” is the same one that G.K. Chesterton makes here:
People feel that if the universe was personal it would vary; if the sun were alive it would dance. This is a fallacy even in relation to known fact. For the variation in human affairs is generally brought into them, not by life, but by death; by the dying down or breaking off of their strength or desire. A man varies his movements because of some slight element of failure or fatigue. He gets into an omnibus because he is tired of walking; or he walks because he is tired of sitting still. But if his life and joy were so gigantic that he never tired of going to Islington, he might go to Islington as regularly as the Thames goes to Sheerness.