Copernicus was the last of the Aristotelians among the great men of science. In their attitude to nature, men like Roger Bacon, Nicolas of Cusa, William of Ockham and Jean Buridan, who predated him by a century or two, were "moderns" compared to Copernicus. The Ockhamist school in Paris, which flourished in the fourteenth century, and to which I have briefly referred before, had made considerable advances in the study of motion, momentum, acceleration and the theory of falling bodies – all of which are basic problems of the Copernican universe. They had shown that Aristotelian physics with its "unmoved movers", its "natural" and "violent" motion et cetera, was empty verbiage; and they had come very close to formulating Newton's Law of Inertia. In 1337 Nicolas of Oresme had written a Commentary on Aristotle De Coelo – in fact, a refutation of it – in which he attributed the daily round of the heavens to the rotation of the earth, and based his theory on much sounder physical grounds than Copernicus, as an Aristotelian, could do. Copernicus was not acquainted with the discoveries in dynamics of the Paris school (which seem to have been ignored in Germany); but my point is that at Merton College and at the Sorbonne, a century and a half before him, a succession of men of lesser fame than Copernicus had shaken off the authority of Aristotelian physics to which he remained a life-long slave.
It was this almost hypnotic submission to authority which became Copernicus' undoing, both as a man and a scientist. As Kepler was to remark later on, " Copernicus tried to interpret Ptolemy rather than nature". His absolute reliance not only on the physical dogmata, but on the astronomic observations of the ancients was the main reason for the errors and absurdities of the Copernican system. When the Nuremberg mathematician Johannes Werner published a treatise "On the Motion of the Eighth Sphere", in which he permitted himself to question the reliability of certain observations by Ptolemy and Timocharis, Copernicus attacked him with venom:
"... It is fitting for us," he wrote, "to follow the methods of the ancients strictly and to hold fast to their observations which have been handed down to us like a Testament. And to him who thinks that they are not to be entirely trusted in this respect, the gates of our Science are certainly closed. He will lie before that gate and spin the dreams of the deranged about the motion of the eighth sphere; and he will get what he deserved for believing that he can lend support to his own hallucinations by slandering the ancients." 18
This was not the outburst of a youthful fanatic – Copernicus wrote this in 1524, when he was past fifty. Departing from his habitual caution and restraint, the unexpected vehemence of language stems from a desperate need to cling to his faith in the ancients which was already shaken. Ten years later he was to confide to Rheticus that the ancients had cheated him, that "they had not shown disinterestedness, but had arranged many observations to fit their personal theories about the movements of the planets." 19
Apart from the twenty-seven observations of his own, the entire Copernican system was based on the observational data of Ptolemy, Hipparchus and other Greek and Arab astronomers, whose statements he had uncritically accepted as Gospel truth, never pausing to consider the possibility of errors committed by careless scribes and translators in those notoriously corrupt texts, nor of mistakes and the doctoring of figures by the ancient observers themselves. When, at last, he realized the unreliability of the data on which he had built, he must have felt that the bottom had fallen out of his system. But by then it was too late to do anything about it. 20 Apart from his fear of ridicule, it must have been this realization of its basic unsoundness which made him so reluctant to publish the book. He did believe that the earth really moved. But he could no longer believe that either the earth, or the other planets, really moved in the manner, and along the orbits, which his book assigned to them.
The tragedy of blind faith in ancient authority, which makes Copernicus such a pathetic figure, is illustrated by a curious example. The point is highly technical, and I must simplify it. Trusting a handful of very precarious data on alleged observations by Hipparchus, Menelaus, Ptolemy and Al Battani, dispersed over two thousand years, Copernicus was led to believe in a phenomenon that does not exist – a periodic change in the rate of the wobble of the earth's axis. 21 In reality the wobble goes on at the same, steady rate; the figures of the ancients were simply wrong. As a result, Copernicus felt obliged to construct an incredibly laborious theory, which attributed two independent oscillatory motions to the earth's axis. But oscillations along a straight line are "violent" motions forbidden by Aristotelian physics; hence Copernicus devotes a whole chapter 22 to show how this motion in a straight line can be produced by a combination of two 'natural", that is circular, motions. The result of this phantom-chase was that he had to ascribe four more circular motions to the earth, in addition to the already existing five.
Towards the end of this painful chapter, where Copernicus' obsession with circles reaches its climax, as it were, the manuscript contains the lines: "It should be noticed, by the way, that if the two circles have different diameters, other conditions remaining unchanged, then the resulting movement will not be a straight line but ... an ellipse." * This is actually not true, for the resulting curve will be a cycloid merely resembling an ellipse – but the odd fact is that Copernicus had hit on the ellipse which is the form of all planetary orbits – had arrived at it for the wrong reasons and by faulty deduction – and having done so, promptly dropped it: the passage is crossed out in the manuscript, and is not contained in the printed edition of the Revolutions. The history of human thought is full of lucky hits and triumphant eurekas; it is rare to have on record one of the anti-climaxes, the missed opportunity which normally leaves no trace.
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*
My italics.
4. The Genesis of the Copernican System
The figure of Copernicus, seen from the distance, is that of an intrepid, revolutionary hero of thought. As we come closer, it gradually changes into that of a stuffy pedant, without the flair, the sleepwalking intuition of the original genius; who, having got hold of a good idea, expanded it into a bad system, patiently plodding on, piling more epicycles and deferents into the dreariest and most unreadable among the books that made history.
To deny that Copernicus was an original thinker may sound paradoxical or blasphemous. Let us try to retrace the process of reasoning which led Nicolas Koppernigk to the Copernican system. It is a much-debated problem, and of a certain interest both to the psychology of discovery and the history of human thought.
Our starting point is his first astronomical treatise, the Commentariolus. It opens, characteristically:
"Our ancestors assumed a large number of celestial spheres for a special reason: to explain the apparent motion of the planets by the principle of regularity. For they thought it altogether absurd that a heavenly body should not always move with uniform velocity in a perfect circle."
Having stated his credo, Copernicus turns to Ptolemy, whose system, he says, is consistent with the observed facts, but ... and here follows a revealing passage which explains the reason that started Copernicus on his quest. It is his shocked realization of the fact that in Ptolemy's universe a planet moves on perfect circles, but not really at uniform speed. More precisely, the planet does not cover equal distances at equal times when seen from the centre of its circle – it only appears to do so when observed from a different point specially chosen for that purpose. This point is called the punctum equans, or "equant" for short. Ptolemy invented this trick to save the principle of uniform motion – his punctum equans enabled him to say that there exists, after all, a point in space where an observer could enjoy the illusion that the planet's motion is a steady one. But, Copernicus remarks indignantly, "a system of this sort seemed neither sufficiently absolute nor sufficiently pleasing to the mind." 23
It was the grievance of a perfectionist who could not tolerate this offence against his ideal of circular uniform motion. It was an imaginary grievance, for in reality the planets did not
move in circles anyway, but on the epicycles of epicycles, producing oval curves; and whether uniformity was "saved" relative to the centre of the imaginary epicycle, or to the equally imaginary equant, made hardly any difference except to an obsessional mind. Yet, as Copernicus himself explains, it was this grievance which started the whole chain-reaction:
"Having become aware of these defects, I often considered whether there could perhaps be found a more reasonable arrangement of circles ... in which everything would move uniformly about its proper centre, as the rule of absolute motion requires." 24
Thus Copernicus' first impulse to reform the Ptolemaic system originated in his urge to remove a minor blemish from it, a feature which did not strictly conform to conservative Aristotelian principles. He was led to reversing the Ptolemaic system by his desire to preserve it – like the maniac who, pained by a mole on his beloved's cheek, cut off her head to restore her to perfection. Yet it happened not for the first time in history that a puritan reformer started by attacking a minor imperfection, and ended by realizing that it was a symptom of a deeprooted and irremediable disease. Ptolemy's equants were nothing to get excited about, but they were symptomatic of the jarring artificiality of the system.
Once he had started to take the Ptolemaic clockwork to pieces, he was on the lookout for some useful hint how to rearrange the wheels in a different order. He did not have to look for long:
"I therefore went to the trouble of reading anew the books of all philosophers on which I could lay hands, to find out whether someone did not hold the opinion that there existed other motions of the heavenly bodies than assumed by those who taught the mathematical sciences in the schools. And thus I found first in Cicero that Hiketas had held the belief that the earth moves. Afterwards I found in Plutarch * that others have also held this opinion. I shall put down his own words so that everybody can read them:
'But others hold that the earth moves; thus Philolaus the Pythagorean held that it revolves round the Fire in an oblique circle like the sun and moon. Herakleides of Pontus and Ekphantus the Pythagorean also suppose the earth to move, though not in a progressive motion, but after the manner of a wheel, turning upon an axle about its own centre from west to east.'
And so, taking occasion from this, I too began to think about the mobility of the earth. And although it seemed an absurd opinion, yet, because I knew that others before me had been granted the liberty of supposing whatever orbits they chose in order to demonstrate the phenomena of the stars, I considered that I too might well be allowed to try whether sounder demonstrations of the revolutions of the heavenly orbs, might be discovered by supposing some motion of the earth." 25
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*
He refers in fact to the pseudo-Plutarch work De Placiti Philosophorum III, 13.
There are further references 26 to "the Pythagoreans Herakleides and Ekphantus", and to "Hiketas of Syracuse, who let the earth rotate in the centre of the world". Then, in Book I, chapter 10, called About the order of the heavenly orbits, Copernicus gives us his own version of the Genesis of his system:
"Therefore it seemed to me that it would be wrong to ignore certain facts well-known to Martianus Capella, who wrote an encyclopaedia, and of some other Latins. He believed that Venus and Mercury do not go round the earth like other planets, but turn round the sun as their centre and therefore cannot go farther away from the sun than the sizes of their orbits permit. What else does this mean but that the sun is the centre of their orbits and that they turn round him? Thus the sphere of Mercury would be enveloped by that of Venus which is twice as large and would find sufficient space inside it. If we seize the opportunity to refer Saturn, Jupiter and Mars to the same centre [i.e. the sun] ... then their motions will fall into a regular and explainable order... And as now all of them are arranged round the same centre, so it becomes necessary that the space which is left between the convex surface of the sphere of Venus and the concave sphere of Mars should be filled in by the earth and the moon which accompanies it and by all the matter to be found in the sublunary sphere... Therefore we do not hesitate to state that moon and earth describe annually a circular orbit placed between the outer and the inner planets round the sun, which rests immobile in the centre of the world; and that everything which appears as a motion of the sun is in truth a motion of the earth."
Now all this is familiar ground to us. Copernicus refers first to the so-called "Egyptian" system of Herakleides, * that "halfway house", in which the two inner planets circle round the sun, while the sun itself, and the outer planets, still go round the earth. He then takes the second step (of letting the outer planets too circle round the sun) which in antiquity was taken either by Herakleides or by Aristarchus; and finally, the third step to the complete heliocentric system, where all the planets, including the earth, revolve round the sun, as suggested by Aristarchus of Samos.
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*
See Part One, Chapter III, 2.
There can be no doubt that Copernicus was acquainted with Aristarchus' idea, and that he was following in his footsteps. The proof of this is to be found in Copernicus' own manuscript of the Revolutions, where he refers to Aristarchus – but, characteristically, this reference is crossed out in ink. Thus Aristarchus' forerunners are given credit in the book, but not Aristarchus himself – just as the names of Rheticus, of Brudzewski and Novara, the teachers to whom Copernicus owed most, are omitted. He had to mention the fact that the heliocentric idea was known to the ancients, in order to prove its respectability, as it were; yet he confused the trail, as was his habit, by leaving out the most important among them. 27
It is highly unlikely, though, that Copernicus should have stumbled on his idea simply by browsing through the ancient philosophers. Talk of a moving earth, of the earth as a planet or star, was becoming increasingly frequent in the days of his youth. We have seen (p. 99 f) that during the later Middle Ages the system of Herakleides had been favoured by most of the scholars who took an interest in astronomy. From the thirteenth century onward the influence of Ptolemy had reasserted itself, simply because there existed no other planetary theory as detailed and comprehensive as the Almagest; but soon afterwards a strong current of criticism and opposition arose. Earlier already, Averroes, the greatest Arab philosopher in Europe (1126-1198) had commented: "The Ptolemaic astronomy is nothing so far as existence is concerned; but it is convenient for computing the non-existent." 28 He had no better alternative to offer; but his epigram could serve as a motto for the growing discontent with the prevailing double-think in cosmology.
This metaphysical malaise flared into open revolt in the first half of the century into which Copernicus was born. Nicolas of Cusa (1401- 1464), a German ecclesiastic, the son of a boatsman on the Moselle, who rose to the rank of Cardinal, was the first to kick against the lid of the medieval universe. In his Learned Ignorance, 29 written in 1440, printed in 1514, twenty years before the Revolutions, he asserted that the world had no boundaries, and consequently neither a periphery nor a centre. It was not infinite, merely "interminate", that is unbounded, and everything in it was in flux:
"Since, then, the earth cannot be the centre, it cannot be entirely devoid of motion... It is clear to us that the earth is really in motion though this may not be apparent to us, since we do not perceive motion except by comparison with something fixed." 30
Earth, moon and planets all move round a centre, which is not defined; but Cusa expressly denies that they move either in perfect circles, or at uniform speed:
"Moreover, neither the sun, nor the moon, nor any sphere – though to us it seems otherwise – can in [its] motion describe a true circle, because they do not move around a fixed base. Nowhere is there a true circle such that a truer one would not be possible, nor is [anything] ever at one time [exactly] as at another, neither does it move in a precisely equal [manner], nor does it describe an equally perfect circle, thou
gh we are not aware of it." 31
By denying that the universe has either a centre or a periphery, Cusa also denied its hierarchic structure, denied the lowliness of the earth's position in the Chain of Being, denied that mutability is an evil confined to the sub-lunary sphere. "The earth is a noble star," he proclaimed triumphantly, "it is not possible for human knowledge to determine whether the region of the earth is in a degree of greater perfection or baseness in relation to the regions of the other stars..." 32
Lastly, Cusa was convinced that the stars were made of the same stuff as the earth, and that they were inhabited by beings neither better nor worse than man, but simply different:
"It cannot be said that this place of the world [is less perfect because it is] the dwelling place of men, and animals, and vegetables that are less perfect than the inhabitants of the region of the sun and of the other stars... It does not seem that, according to the order of nature, there could be a more noble or more perfect nature than the intellectual nature which dwells here on this earth as in its region, even if there are in the other stars inhabitants belonging to another genus: man indeed does not desire another nature, but only the perfection of his own." 33