Objection 1. It would seem that the heavenly bodies are not the cause of what is produced in bodies here below. For Damascene says (De Fide Orth. ii, 7): "We say that they"--namely, the heavenly bodies--"are not the cause of generation or corruption: they are rather signs of storms and atmospheric changes."
Objection 2. Further, for the production of anything, an agent and matter suffice. But in things here below there is passive matter; and there are contrary agents--heat and cold, and the like. Therefore for the production of things here below, there is no need to ascribe causality to the heavenly bodies.
Objection 3. Further, the agent produces its like. Now it is to be observed that everything which is produced here below is produced through the action of heat and cold, moisture and dryness, and other such qualities, which do not exist in heavenly bodies. Therefore the heavenly bodies are not the cause of what is produced here below.
Objection 4. Further, Augustine says (De Civ. Dei v, 6): "Nothing is more corporeal than sex." But sex is not caused by the heavenly bodies: a sign of this is that of twins born under the same constellation, one may be male, the other female. Therefore the heavenly bodies are not the cause of things produced in bodies here below.
On the contrary, Augustine says (De Trin. iii, 4): "Bodies of a grosser and inferior nature are ruled in a certain order by those of a more subtle and powerful nature." And Dionysius (Div. Nom. iv) says that "the light of the sun conduces to the generation of sensible bodies, moves them to life, gives them nourishment, growth, and perfection."
I answer that, Since every multitude proceeds from unity; and since what is immovable is always in the same way of being, whereas what is moved has many ways of being: it must be observed that throughout the whole of nature, all movement proceeds from the immovable. Therefore the more immovable certain things are, the more are they the cause of those things which are most movable. Now the heavenly bodies are of all bodies the most immovable, for they are not moved save locally. Therefore the movements of bodies here below, which are various and multiform, must be referred to the movement of the heavenly bodies, as to their cause.
Reply to Objection 1. These words of Damascene are to be understood as denying that the heavenly bodies are the first cause of generation and corruption here below; for this was affirmed by those who held that the heavenly bodies are gods.
Reply to Objection 2. The active principles of bodies here below are only the active qualities of the elements, such as hot and cold and the like. If therefore the substantial forms of inferior bodies were not diversified save according to accidents of that kind, the principles of which the early natural philosophers held to be the "rare" and the "dense"; there would be no need to suppose some principle above these inferior bodies, for they would be of themselves sufficient to act. But to anyone who considers the matter aright, it is clear that those accidents are merely material dispositions in regard to the substantial forms of natural bodies. Now matter is not of itself sufficient to act. And therefore it is necessary to suppose some active principle above these material dispositions.
This is why the Platonists maintained the existence of separate species, by participation of which the inferior bodies receive their substantial forms. But this does not seem enough. For the separate species, since they are supposed to be immovable, would always have the same mode of being: and consequently there would be no variety in the generation and corruption of inferior bodies: which is clearly false.
Therefore it is necessary, as the Philosopher says (De Gener. ii, 10), to suppose a movable principle, which by reason of its presence or absence causes variety in the generation and corruption of inferior bodies. Such are the heavenly bodies. Consequently whatever generates here below, moves to the production of the species, as the instrument of a heavenly body: thus the Philosopher says (Phys. ii, 2) that "man and the sun generate man."
Reply to Objection 3. The heavenly bodies have not a specific likeness to the bodies here below. Their likeness consists in this, that by reason of their universal power, whatever is generated in inferior bodies, is contained in them. In this way also we say that all things are like God.
Reply to Objection 4. The actions of heavenly bodies are variously received in inferior bodies, according to the various dispositions of matter. Now it happens at times that the matter in the human conception is not wholly disposed to the male sex; wherefore it is formed sometimes into a male, sometimes into a female. Augustine quotes this as an argument against divination by stars: because the effects of the stars are varied even in corporeal things, according to the various dispositions of matter.
Source: Summa Theologica (New Advent)