Copernicus did not question it, Ptolemy could not.
Given the conceptual context within which ancient thought thrived, how could anyone have questioned this principle?
The reasons for this are partly observational, partly philosophical, and reinforced by other aesthetic and cultural factors.

First, the observational reasons.
The obvious natural fact to ancient thinkers was the diurnal rotation of the heavens.
Not only did constellations like Draco, Cepheus, and Cassiopeia spin circles around the pole, but stars which were not circumpolar rose and set at the same place on the horizon each night.
Nor did a constellation's stars vary in brightness during the course of their nocturnal flights.
The conclusion -- the distances of the constellations did not vary and their paths were circular.
Moreover, the sun's path over earth described a segment of a great circle;
this was clear from the contour of the shadow traced by a gnomon before and after noon.

As early as the 6th century B.C. the earth was seen to be spherical.
Ships disappear hull-first over the horizon;
approaching shore their masts appeared first.
Earth, being at the center of the universe, would have the same shape as the latter;
so, e.g. did Aristotle argue, although this may not be an observational reason in favor of circularity.
The discoid shapes of sun and moon were also felt to indicate the shape of celestial things.

In light of all this, one would require special reasons for saying that the paths of the heavenly bodies were other than circular.
Why, for example, should the ancients have supposed the diurnal rotation of the heavens to be elliptical?
Or oviform?
Or angular?
There were no reasons for such suppositions then.
This, conjoined with the considerations above, made the circular motions of heavenly bodies appear an almost directly observed fact.

Additional philosophical considerations, advanced notably by Aristotle, supported further the circularity principle.
By distinguishing superlunary (celestial) and sublunary (terrestrial) existence, and reinforcing this with the four-element physics of Empedocles, Aristotle came to speak of the stars as perfect bodies, which moved in only a perfect way, viz. in a perfect circle.

Now what is perfect motion?
It must, apparently, be motion without termini.
Because motion which begins and ends at discrete places would (e.g. for Aristotle) be incomplete.
Circular motion, however, since it is eternal and perfectly continuous, lacks termini.
It is never motion towards something.
Only imcomplete, imperfect things move towards what they lack.
Perfect, complete entities, if they move at all, do not move towards what they lack.
They move only in accordance with what is in their natures.
Thus, circular motion is itself one of the essential characteristics of completely perfect celestial existence.

To return now to the four-element physics, a mixture of muddy, frothy water will, when standing in a jar, separate out with earth at the bottom, water on top, and the air on top of that.
A candle alight in the air directs its flame and smoke upwards.
This gives a clue to the cosmical order of elements.
Thus earth has fallen to the center of the universe.
It is covered (partly) with water, air is atop of that.
Pure fire (the stars) is in the heavens.
When combined with the metaphysical notion that pure forms of this universe are best appreciated when least embodied in a material substratum, it becomes clear that while earth will be dross on a scale of material-formal ratios, celestial bodies will be of a subtle, quickened, ethereal existence, in whose embodiment pure form will be the dominant component and matter will be absent or remain subsidiary.

The stars constitute an order of existence different from what we encounter on earth.
This is clear when one distinguishes the types of motion appropriate to both regions.
A projectile shot up from earth returns rectlinearly to its 'natural' place of rest.
But the natural condition for the heavenly bodies is neither rest, nor rectilinear motion.
Being less encumbered by material embodiments they partake more of what is divine.
Their motion will be eternal and perfect.

Let us re-examine the publicized contrasts between Ptolemaic and Copernican astronomy.
Bluntly, there never was a Ptolemaic system of astronomy.
Copernicus' achievement was to have invented systematic astronomy.
The Almagest and The Hypotheses outline Ptolemy's conception of his own task as the provision of computational tables, independent calculating devices for the prediction of future planetary perturbations.
Indeed, in the Halma edition of Theon's presentation of The Hypotheses there is a chart setting out (under six distinct headings) otherwise unrelated diagrams for describing the planetary motions.
No attempt is made by Ptolemy to weld into a single scheme (a-la-Aristotle), these independent predicting-machines.
They all have this in common: the earth is situated near the center of the deferent.
But that one should superimpose all these charts, run a pin through the common point, and then scale each planetary deferent larger and smaller (to keep the epicycles from 'bumping'), this is contrary to any intention Ptolemy ever expresses.
He might even suppose the planets to move at infinity.
Ptolemy's problem is to forecast where, against the inverted bowl of night, some particular light will be found at future times.
His problem concerns longitudes, latitudes, and angular velocities.
The distances of these points of light is a problem he cannot master, beyond crude conjectures as to the orderings of the planetary orbits viewed outward from earth.
But none of this has prevented scientists, philosophers, and even historians of science, from speaking of the Ptolemaic system, in contrast to the Copernican.
This is a mistake.
It is engendered by confounding the Aristotelian cosmology in The Almagest with the geocentric astronomy.

Ptolemy recurrently denies that he could ever explain planetary motion.
This is what necessitates the nonsystematic character of his astronomy.
So when textbooks, like that of Baker set out drawings of the 'Ptolemaic System', complete with earth in the center and the seven heavenly bodies epicyclically arranged on their several deferents, we have nothing but a misleading 20th-century idea of what never existed historically.

It is the chief merit in Copernicus' work that all his planetary calculations are interdependent.
He cannot, e.g. compute the retrograde arc traveled by Mars, without also making suppositions about the earth's own motion.
He cannot describe eclipses without entertaining some form of a three-body problem.
In Ptolemaic terms, however, eclipses and retrograde motion were phenomena simpliciter, to be explained directly as possible resultants of epicyclical combinations.
In a systematic astronomy, like that of Copernicus, retrogradations become part of the conceptual structure of the system;
they are no longer a puzzling aspect of intricately variable, local planetary motions.

Another contrast stressed when discussing Ptolemaic vs. Copernican astronomy, turns on the idea of simplicity.
It is often stated that Copernican astronomy is 'simpler' than Ptolemaic.
Some even say that this is the reason for the ultimate acceptance of the former.
Thus, Margenau remarks: "A large number of unrelated epicycles was needed to explain the observations, but otherwise the (Ptolemaic) system served well and with quantitative precision.
Copernicus, by placing the sun at the center of the planetary universe, was able to reduce the number of epicycles from eighty-three to seventeen.
Historical records indicate that Copernicus was unaware of the fundamental aspects of his so-called 'revolution', unaware perhaps of its historical importance, he rested content with having produced a simpler scheme for prediction.
As an illustration of the principle of simplicity the heliocentric discovery has a peculiar appeal because it allows simplicity to be arithmetized;
it involves a reduction in the number of epicycles from eighty-three to seventeen".

Without careful qualification this can be misleading.
If in any one calculation Ptolemy had had to invoke 83 epicycles all at once, while Copernicus never required more than one third this number, then (in the sense obvious to Margenau) Ptolemaic astronomy would be simpler than Copernican.
But no single planetary problem ever required of Ptolemy more than six epicycles at one time.
This, of course, results from the non-systematic, 'cellular' character of Ptolemaic theory.
Calculations within the Copernican framework always raised questions about planetary configurations.
These could be met only by considering the dynamical elements of several planets at one time.
This is more ambitious than Ptolemy is ever required to be when he faces his isolated problems.
Thus, in no ordinary sense of 'simplicity' is the Ptolemaic theory simpler than the Copernican.
The latter required juggling several elements simultaneously.
This was not simpler but much more difficult than exercises within Ptolemy's astronomy.

Analogously, anyone who argues that Einstein's theory of gravitation is simpler than Newton's, must say rather more to explain how it is that the latter is mastered by student-physicists, while the former can be managed (with difficulty) only by accomplished experts.

In a sense, Einstein's theory is simpler than Newton's, and there is a corresponding sense in which Copernicus' theory is simpler than Ptolemy's.
But 'simplicity' here refers to systematic simplicity.
The number of primitive ideas in systematically-simple theories is reduced to a minimum.
The Axioms required to make the theoretical machinery operate are set out tersely and powerfully, so that all permissible operations within the theory can be traced rigorously back to these axioms, rules, and primitive notions.
This characterizes Euclid's formulation of geometry, but not Ptolemy's astronomy.
There are in The Almagest no rules for determining in advance whether a new epicycle will be required for dealing with abberations in lunar, solar, or planetary behavior.
The strongest appeal of the Copernican formulation consisted in just this: ideally, the justification for dealing with special problems in particular ways is completely set out in the basic 'rules' of the theory.
The lower-level hypotheses are never 'ad hoc', never introduced ex post facto just to sweep up within the theory some recalcitrant datum.
Copernicus, to an extent unachieved by Ptolemy, approximated to Euclid's vision.
De Revolutionibus is not just a collection of facts and techniques.
It is an organized system of these things.
Solving astronomical problems requires, for Copernicus, not a random search of unrelated tables, but a regular employment of the rules defining the entire discipline.

Hence, noting the simplicity achieved in Copernicus' formulation does not provide another reason for the acceptance of De Revolutionibus, another reason beyond its systematic superiority.
It provides exactly the same reason.

1543 A.D. is often venerated as the birthday of the scientific revolution.
It is really the funeral day of scholastic science.
Granted, the cosmological, philosophical, and cultural reverberations initiated by the De Revolutionibus were felt with increasing violence during the 300 years to follow.
But, considered within technical astronomy, a different pattern can be traced.

In what does the dissatisfaction of Copernicus-the-astronomer consist?
What in The Almagest draws his fire?
Geocentricism per se?
No.
The formal displacement of the geocentric principle far from being Copernicus' primary concern, was introduced only to resolve what seemed to him intolerable in orthodox astronomy, namely, the 'unphysical' triplication of centric reference-points: one center from which the planet's distances were calculated, another around which planetary velocities were computed, and still a third center (the earth) from which the observations originated.
This arrangement was for Copernicus literally monstrous: "With (the Ptolemaists) it is as though an artist were to gather the hands, feet, head and other members for his images from divers models, each part excellently drawn, but not related to a single body;
and since they in no way match each other, the result would be a monster rather than a man".

Copernicus required a systematically integrated, physically intelligible astronomy.
His objective was, essentially, to repair those aspects of orthodox astronomy responsible for its deficiencies in achieving these ends.
That such deficiencies existed within Ptolemy's theory was not discovered de novo by Copernicus.
The critical, rigorous examinations of Nicholas of Cusa and Nicholas of Oresme provided the context (a late medieval context) for Nicholas Copernicus' own work.
The latter looked backward upon inherited deficiencies.
Without abandoning too much, Copernicus sought to make orthodox astronomy systematically and mechanically acceptable.
He did not think himself to be firing the first shot of an intellectual revolution.