Copernicus and Newton both deny the Aristotelian theory of two kinds of natural motion, but they do so in opposite ways. Aristotle had argued that there are two kinds of natural motion. There is one kind for the corruptible things below the sphere of the moon namely in a straight line (up or down), and another for the supposedly incorruptible bodies above the moon (in a circle). The difference makes some sense because corruptible things have a beginning and an end (like a straight line), and incorruptible things don’t (like a circle).
Copernicus denies the distinction, and claims that there is only one kind of natural motion: circular motion. Straight line motion is only a corrective that happens when something has been removed from its proper place. Thus Revolutions, I,8:
A simple body possesses a simple movement—this is first verified in the case of circular movement—as long as the simple body remain in its unity in its natural place. In this place, in fact, its movement is none other than the circular, which remains entirely in itself, as though at rest. Rectilinear movement, however, is added to those bodies which journey away from their natural place or are shoved out of it or are outside it somehow… Therefore rectilinear movement belongs only to bodies which are not in the right condition and are not perfectly conformed to their nature when they are separated from their whole and abandon its unity. Furthermore, bodies which are moved upward or downward do not possess a simple, uniform, and regular movement—even without taking into account circular movement… Therefore, since circular movement belongs to wholes and rectilinear to parts, we can say that the circular movement stands with the rectilinear, as does animal with sick.
He thus elevates the earth, as it were, to the status of a heavenly body.
Newton thinks the symmetrical opposite: motion is as it were “natural” when it is in a straight line. Curved motion comes about by the composition of straight motions. He thus brings the heavenly bodies down to the level of terrestrial things. Thus Principia III,4: the moon is a thrown rock.
Modern astronomy thus begins by elevating the terrestrial (Copernicus), but reaches its classical form by degrading the celestial (Newton).