Aquinas has something to say about the movement of heavenly bodies:
"...celestial bodies move because they are moved. Hence, the end of their motion is to attain the divine likeness in both ways. In regard to the way which involves its own perfection, the celestial body comes to be in a certain place actually, to which place it was previously in potency. Nor does it achieve its perfection any less because it now stands in potency to the place in which it was previously. For, in the same way, prime matter tends toward its perfection by actually acquiring a form to which it was previously in. potency, even though it then ceases to have the other form which it actually possessed before, for this is the way that matter may receive in succession all the forms to which it is potential, so that its entire potentiality may be successively reduced to act, which could not be done all at once. Hence, since a celestial body is in potency to place in the same way that prime matter is to form, it achieves its perfection through the fact that its entire potency to place is successively reduced to act, which could not be done all at once."
In other words, their movement needs a cause
"...celestial bodies move because they are moved. Hence, the end of their motion is to attain the divine likeness in both ways. In regard to the way which involves its own perfection, the celestial body comes to be in a certain place actually, to which place it was previously in potency. Nor does it achieve its perfection any less because it now stands in potency to the place in which it was previously. For, in the same way, prime matter tends toward its perfection by actually acquiring a form to which it was previously in. potency, even though it then ceases to have the other form which it actually possessed before, for this is the way that matter may receive in succession all the forms to which it is potential, so that its entire potentiality may be successively reduced to act, which could not be done all at once. Hence, since a celestial body is in potency to place in the same way that prime matter is to form, it achieves its perfection through the fact that its entire potency to place is successively reduced to act, which could not be done all at once."
In other words, their movement needs a cause
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