I will emphasize certain parts of St. Thomas's words here by underlining them to help make finding the Scholastic "bottom line" on the subject of astrology much easier. 

To the surprise, even shock, of many -- especially Catholics who've converted from Fundamentalist or Evangelical sects -- Astrology, in se, is not forbidden. To sum things up in a few sentences, a Catholic may believe that the heavenly bodies affect our inclinations and our lower natures. But they cannot control the will. Any form of Astrology that denies free will, that implies that a given individual will do X because of this or that alignment of the planets, is erring. Any Catholic who treats the planets as gods or as having some pretenatural power of their own rather than seeing any influence they might have as simply a part of God's natural world, is erring. A Catholic is in no way compelled to believe in Astrology at all, but, by the same token, no Catholic should accuse another Catholic of "dabbling in the occult" or of being "New Agey" or what have you if that Catholic does consider as possible, or true, the idea of celestial bodies' influences over our inclinations, passions, and lower natures.

Article 3. Whether the heavenly bodies are the cause of what is produced in bodies here below?

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.



Article 4. Whether the heavenly bodies are the cause of human actions?

Objection 1. It would seem that the heavenly bodies are the cause of human actions. For since the heavenly bodies are moved by spiritual substances, as stated above (110, 3), they act by virtue thereof as their instruments. But those spiritual substances are superior to our souls. Therefore it seems that they can cause impressions on our souls, and thereby cause human actions.

Objection 2. Further, every multiform is reducible to a uniform principle. But human actions are various and multiform. Therefore it seems that they are reducible to the uniform movements of heavenly bodies, as to their principles.

Objection 3. Further, astrologers often foretell the truth concerning the outcome of wars, and other human actions, of which the intellect and will are the principles. But they could not do this by means of the heavenly bodies, unless these were the cause of human actions. Therefore the heavenly bodies are the cause of human actions.

On the contrary, Damascene says (De Fide Orth. ii, 7) that "the heavenly bodies are by no means the cause of human actions."

I answer that, The heavenly bodies can directly and of themselves act on bodies, as stated above (3). They can act directly indeed on those powers of the soul which are the acts of corporeal organs, but accidentally: because the acts of such powers must needs be hindered by obstacles in the organs; thus an eye when disturbed cannot see well. Wherefore if the intellect and will were powers affixed to corporeal organs, as some maintained, holding that intellect does not differ from sense; it would follow of necessity that the heavenly bodies are the cause of human choice and action. It would also follow that man is led by natural instinct to his actions, just as other animals, in which there are powers other than those which are affixed to corporeal organs: for whatever is done here below in virtue of the action of heavenly bodies, is done naturally. It would therefore follow that man has no free-will, and that he would have determinate actions, like other natural things. All of which is manifestly false, and contrary to human habit. It must be observed, however, that indirectly and accidentally, the impressions of heavenly bodies can reach the intellect and will, forasmuch, namely, as both intellect and will receive something from the inferior powers which are affixed to corporeal organs. But in this the intellect and will are differently situated. For the intellect, of necessity, receives from the inferior apprehensive powers: wherefore if the imaginative, cogitative, or memorative powers be disturbed, the action of the intellect is, of necessity, disturbed also. The will, on the contrary, does not, of necessity, follow the inclination of the inferior appetite; for although the passions in the irascible and concupiscible have a certain force in inclining the will; nevertheless the will retains the power of following the passions or repressing them. Therefore the impressions of the heavenly bodies, by virtue of which the inferior powers can be changed, has less influence on the will, which is the proximate cause of human actions, than on the intellect.

To maintain therefore that heavenly bodies are the cause of human actions is proper to those who hold that intellect does not differ from sense. Wherefore some of these said that "such is the will of men, as is the day which the father of men and of gods brings on" (Odyssey xviii 135). Since, therefore, it is manifest that intellect and will are not acts of corporeal organs, it is impossible that heavenly bodies be the cause of human actions.

Reply to Objection 1. The spiritual substances, that move the heavenly bodies, do indeed act on corporeal things by means of the heavenly bodies; but they act immediately on the human intellect by enlightening it. On the other hand, they cannot compel the will, as stated above (111, 2).

Reply to Objection 2. Just as the multiformity of corporeal movements is reducible to the uniformity of the heavenly movement as to its cause: so the multiformity of actions proceeding from the intellect and the will is reduced to a uniform principle which is the Divine intellect and will.

Reply to Objection 3. The majority of men follow their passions, which are movements of the sensitive appetite, in which movements of the heavenly bodies can cooperate: but few are wise enough to resist these passions. Consequently astrologers are able to foretell the truth in the majority of cases, especially in a general way. But not in particular cases; for nothing prevents man resisting his passions by his free-will. Wherefore the astrologers themselves are wont to say that "the wise man is stronger than the stars" [Ptolemy, Centiloquium, prop. 5], forasmuch as, to wit, he conquers his passions.



Article 5. Whether heavenly bodies can act on the demons?

Objection 1. It would seem that heavenly bodies can act on the demons. For the demons, according to certain phases of the moon, can harass men, who on that account are called lunatics, as appears from Matthew 4:24 and 17:14. But this would not be if they were not subject to the heavenly bodies. Therefore the demons are subject to them.

Objection 2. Further, necromancers observe certain constellations in order to invoke the demons. But these would not be invoked through the heavenly bodies unless they were subject to them. Therefore they are subject to them.

Objection 3. Further, heavenly bodies are more powerful than inferior bodies. But the demons are confined to certain inferior bodies, namely, "herbs, stones, animals, and to certain sounds and words, forms and figures," as Porphyry says, quoted by Augustine (De Civ. Dei x, 11). Much more therefore are the demons subject to the action of heavenly bodies.

On the contrary, The demons are superior in the order of nature, to the heavenly bodies. But the "agent is superior to the patient," as Augustine says (Gen. ad lit. xii, 16). Therefore the demons are not subject to the action of heavenly bodies.

I answer that, There have been three opinions about the demons. In the first place the Peripatetics denied the existence of demons; and held that what is ascribed to the demons, according to the necromantic art, is effected by the power of the heavenly bodies. This is what Augustine (De Civ. Dei x, 11) relates as having been held by Porphyry, namely, that "on earth men fabricate certain powers useful in producing certain effects of the stars." But this opinion is manifestly false. For we know by experience that many things are done by demons, for which the power of heavenly bodies would in no way suffice: for instance, that a man in a state of delirium should speak an unknown tongue, recite poetry and authors of whom he has no previous knowledge; that necromancers make statues to speak and move, and other like things.

For this reason the Platonists were led to hold that demons are "animals with an aerial body and a passive soul," as Apuleius says, quoted by Augustine (De Civ. Dei viii, 16). And this is the second of the opinions mentioned above: according to which it could be said that demons are subject to heavenly bodies in the same way as we have said man is subject thereto (4). But this opinion is proved to be false from what we have said above (51, 1): for we hold that demons are spiritual substances not united to bodies. Hence it is clear that they are subject to the action of heavenly bodies neither essentially nor accidentally, neither directly nor indirectly.

Reply to Objection 1. That demons harass men, according to certain phases of the moon, happens in two ways.

Firstly, they do so in order to "defame God's creature," namely, the moon; as Jerome (In Matt. iv, 24) and Chrysostom (Hom. lvii in Matt.) say.

Secondly, because as they are unable to effect anything save by means of the natural forces, as stated above (114, 4, ad 2) they take into account the aptitude of bodies for the intended result. Now it is manifest that "the brain is the most moist of all the parts of the body," as Aristotle says [De Part. Animal. ii, 7: De Sens. et Sensato ii: De Somn. et Vigil. iii]: wherefore it is the most subject to the action of the moon, the property of which is to move what is moist. And it is precisely in the brain that animal forces culminate: wherefore the demons, according to certain phases of the moon, disturb man's imagination, when they observe that the brain is thereto disposed.

Reply to Objection 2. Demons when summoned through certain constellations, come for two reasons.

Firstly, in order to lead man into the error of believing that there is some Divine power in the stars.

Secondly, because they consider that under certain constellations corporeal matter is better disposed for the result for which they are summoned.

Reply to Objection 3. As Augustine says (De Civ. Dei xxi, 6), the "demons are enticed through various kinds of stones, herbs, trees, animals, songs, rites, not as an animal is enticed by food, but as a spirit by signs"; that is to say, forasmuch as these things are offered to them in token of the honor due to God, of which they are covetous.



Article 6. Whether heavenly bodies impose necessity on things subject to their action?

Objection 1. It would seem that heavenly bodies impose necessity on things subject to their action. For given a sufficient cause, the effect follows of necessity. But heavenly bodies are a sufficient cause of their effects. Since, therefore, heavenly bodies, with their movements and dispositions, are necessary beings; it seems that their effects follow of necessity.

Objection 2. Further, an agent's effect results of necessity in matter, when the power of the agent is such that it can subject the matter to itself entirely. But the entire matter of inferior bodies is subject to the power of heavenly bodies, since this is a higher power than theirs. Therefore the effect of the heavenly bodies is of necessity received in corporeal matter.

Objection 3. Further, if the effect of the heavenly body does not follow of necessity, this is due to some hindering cause. But any corporeal cause, that might possibly hinder the effect of a heavenly body, must of necessity be reducible to some heavenly principle: since the heavenly bodies are the causes of all that takes place here below. Therefore, since also that heavenly principle is necessary, it follows that the effect of the heavenly body is necessarily hindered. Consequently it would follow that all that takes place here below happens of necessity.

On the contrary, The Philosopher says (De Somn. et Vigil. [De Divin. per Somn. ii): "It is not incongruous that many of the signs observed in bodies, of occurrences in the heavens, such as rain and wind, should not be fulfilled." Therefore not all the effects of heavenly bodies take place of necessity.

I answer that, This question is partly solved by what was said above (4); and in part presents some difficulty. For it was shown that although the action of heavenly bodies produces certain inclinations in corporeal nature, the will nevertheless does not of necessity follow these inclinations. Therefore there is nothing to prevent the effect of heavenly bodies being hindered by the action of the will, not only in man himself, but also in other things to which human action extends.

But in natural things there is no such principle, endowed with freedom to follow or not to follow the impressions produced by heavenly agents. Wherefore it seems that in such things at least, everything happens of necessity; according to the reasoning of some of the ancients who supposing that everything that is, has a cause; and that, given the cause, the effect follows of necessity; concluded that all things happen of necessity. This opinion is refuted by Aristotle (Metaph. vi, Did. v, 3) as to this double supposition.

For in the first place it is not true that, given any cause whatever, the effect must follow of necessity. For some causes are so ordered to their effects, as to produce them, not of necessity, but in the majority of cases, and in the minority to fail in producing them. But that such cases do fail in the minority of cases is due to some hindering cause; consequently the above-mentioned difficulty seems not to be avoided, since the cause in question is hindered of necessity.

Therefore we must say, in the second place, that everything that is a being "per se," has a cause; but what is accidentally, has not a cause, because it is not truly a being, since it is not truly one. For (that a thing is) "white" has a cause, likewise (that a man is) "musical" has not a cause, but (that a being is) "white-musical" has not a cause, because it is not truly a being, nor truly one. Now it is manifest that a cause which hinders the action of a cause so ordered to its effect as to produce it in the majority of cases, clashes sometimes with this cause by accident: and the clashing of these two causes, inasmuch as it is accidental, has no cause. Consequently what results from this clashing of causes is not to be reduced to a further pre-existing cause, from which it follows of necessity. For instance, that some terrestrial body take fire in the higher regions of the air and fall to the earth, is caused by some heavenly power: again, that there be on the surface of the earth some combustible matter, is reducible to some heavenly principle. But that the burning body should alight on this matter and set fire to it, is not caused by a heavenly body, but is accidental. Consequently not all the effects of heavenly bodies result of necessity.

Reply to Objection 1. The heavenly bodies are causes of effects that take place here below, through the means of particular inferior causes, which can fail in their effects in the minority of cases.

Reply to Objection 2. The power of a heavenly body is not infinite. Wherefore it requires a determinate disposition in matter, both as to local distance and as to other conditions, in order to produce its effect. Therefore as local distance hinders the effect of a heavenly body (for the sun has not the same effect in heat in Dacia as in Ethiopia); so the grossness of matter, its low or high temperature or other such disposition, can hinder the effect of a heavenly body.

Reply to Objection 3. Although the cause that hinders the effect of another cause can be reduced to a heavenly body as its cause; nevertheless the clashing of two causes, being accidental, is not reduced to the causality of a heavenly body, as stated above.