I decided to read a little of Copernicus's De Revoluionibus Orbium Coelestium to see whether philosophical bias in any way inspired his researches. To me his goal seems consistent with a desire to explain the apparent irregular motion of the planets and the Sun and Moon in terms the Aristotelian assumption that away from the Earth is a realm of perfection which is reflected in circular motion on celestial spheres. One can judge for oneself what his thought processes are. The entire tract of which I include a few excerpts is wonderful reading.
This is from his introduction. He gives a reason that the method of epicycles and eccentrics has been unsatisfactory.
"... those who devised the eccentrics seem thereby in large measure to have solved the problem of the apparent motions with appropriate calculations. But meanwhile they introduced a good many ideas which apparently contradict the first principles of uniform motion. Nor could they elicit or deduce from the eccentrics the principal consideration, that is, the structure of the universe and the true symmetry of its parts. On the contrary, their experience was just like some one taking from various places hands, feet, a head, and other pieces, very well depicted, it may be, but not for the representation of a single person; since these fragments would not belong to one another at all, a monster rather than a man would be put together from them. Hence in the process of demonstration or "method", as it is called, those who employed eccentrics are found either to have omitted something essential or to have admitted something extraneous and wholly irrelevant. This would not have happened to them, had they followed sound principles. For if the hypotheses assumed by them were not false, everything which follows from their hypotheses would be confirmed beyond any doubt. "
Here he states the desire to have a unified theory of the Universe.
"In the first book I set forth the entire distribution of the spheres together with the motions which I attribute to the earth, so that this book contains, as it were, the general structure of the universe. Then in the remaining books I correlate the motions of the other planets and of all the spheres with the movement of the Earth so that I may thereby determine to what extent the motions and appearances of the other planets and spheres can be saved if they are correlated with the Earth's motions."
By “saved' in the sentence above I take this to mean seen as circular motion. It seems possible that Copernicus is attempting to”save” the Aristotelian model of the Universe that models the heavens as unchangeable spheres whose only possible motion is rotation and which carry the celestial bodes along with them. These therefore must traverse circles on these spheres. Planetary motion was not apparently circular but Copernicus finds this a flaw in models of the Heavens.""THE MOTION OF THE BEAVENLY BODIES Chapter 4 IS UNIFORM, ETERNAL, AND CIRCULAR OR COMPOUNDED OF CIRCULAR MOTIONS
I shall now recall to mind that the motion of the heavenly bodies is circular, since the motion appropriate to a sphere is rotation in a circle. By this very act the sphere expresses its form as the simplest body, wherein neither beginning nor end can be found, nor can the one be distinguished from the other, while the sphere itself traverses the same points to return upon itself."
Speaking of the irregularities in planetary motion and of nearby celestial objects such as the Sun and Moon he concludes
"We must acknowledge, nevertheless, that their motions are circular or compounded of several circles, because these nonuniformities recur regularly according to a constant law. This could not happen unless the motions were circular, since only the circle can bring back the past. Thus, for example, by a composite motion of circles the sun restores to us the inequality of days and nights as well as the is four seasons of the year. Several motions are discerned herein, because a simple heavenly body cannot be moved by a single sphere nonuniformly. For this nonuniformity would have to be caused either by an inconstancy, whether imposed from without or generated from within, in the moving force or by an alteration in the revolving body. From either alternative, however, the intellect shrinks. It is improper to conceive any such defect in objects constituted in the best order.
It stands to reason, therefore, that their uniform motions appear nonuniform to us. The cause may be either that their circles have poles different [from the earth's] or that the Earth is not at the center of the circles on which they revolve. To us who watch the course of these planets from the earth, it happens that our eye does not keep the same distance from every part of their orbits, but on account of their varying distances these bodies seem larger when nearer than when farther away (as has been proved in optics). Likewise, in equal arcs of their orbits their motions will appear unequal in equal times on account of the observer's varying distance. Hence I deem it above all necessary that we should carefully scrutinize the relation of the Earth to the heavens lest, in our desire to examine the loftiest objects, we remain ignorant of things nearest to us, and by the same error attribute to the celestial bodies what belongs to the earth."
Note also that uniform circular motion is considered to be the natural motion of the sphere - and therefore for bodies moving on them - and deviations from it , what he calls “ non uniformity “ he considers “improper to conceive” in objects “constituted in the best order” So he implicitly assumes that uniform circular motion is perfect and must therefore characterize the Divine realm of celestial spheres.
Interestingly, This reminds me of the idea of inertial motion except in the celestial realm uniform circular motion he considers to be inertial rather than linear motion. This force free natural motion for celestial objects is apparently characteristic of the Heavens as opposed to the Earth which seems disturbed by non-uniform forces.