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 ? ?
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 .