The medievals believed that the spheres were the remote source of change on earth and that the spheres moved continuously because angels caused them to do so. If you want to precise, then you must say that this account is not so much supernaturalist as it is preternaturalist, for angelic causality in the physical world is preternatural rather than supernatural.
Furthermore, Aquinas would argue that the activity of such an angel would itself need a cause, even though no physical thing "pushes" the angel into "pushing" the sphere. In positing the need for such a cause, they would be reasoning in a manner similar to Aristotle, who saw it fit to posit a first unmoved mover (i.e., a first cause of the unmoved moving-actions of the intelligences that moved the speres).
If we assume that it is no embarrassement to medieval metaphysics to claim that the angels' transitive yet unmoved motion upon the sphere(s) needs a cause, then it is no embarrassment to a post-Galilean theist to argue that uniform movements (i.e., momentum) need a cause as well.
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