Copernicus, the Man, the Work, and Its History

Author(s): Willy Hartner

Source: Proceedings of the American Philosophical Society, Vol. 117, No. 6, Symposium on
Copernicus (Dec. 31, 1973), pp. 413-422
Published by: American Philosophical Society
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 COPERNICUS, THE MAN, THE WORK, AND ITS HISTORY
WILLY HARTNER
Professor of the History of Science, Johann Wolfgang Goethe University, Frankfurt;
President, International Academy of the History of Science, Paris
(Read April 20, 1973, in Symposium on Copernicus)

To THE MODERN SCIENTIST, the primary cri- observations are made that apparently or ac-
terion for the quality of a theory is its capability tually invalidate the theory. There are two
of representing known experimental and obser- ways to overcome the dilemma: either to
vational facts and of predicting new facts, modify-tacitly in most cases-the master's
unknown at the time when the theory was con- theory, or else simply to take no account of
ceived. Of two competitive theories advanced the new disturbing phenomena and to go on as
to explain the same phenomena, that one is though nothing had happened. Depending on
considered superior which yields the better the case, scientists of later ages have had
agreement with observation and permits us to recourse to both expedients. To illustrate this,
make the more exact predictions. The value of may I start with a survey of the main stages
any theory is relative. The idea of perfection is of pre-Copernican astronomy; it will prove
ipso facto excluded. On the basis of new ex- indispensable for an appreciation of the Coper-
perimental evidence, a theory regarded as un- nican accomplishment.
shakable until this day may be superseded by From observation we learn that all fixed
a better one tomorrow. stars, revolving eternally with uniform motion,
It might seem superfluous to state that the describe perfect circles about the celestial poles.
ancient or medieval scientist's attitude towards On the other hand, geometry teaches us that
theory on the one hand, and experiment and of all two-dimensional figures the circle is the
observation on the other, was essentially dif- most perfect one, and similarly, of all solids,
ferent. Everybody knows that Galileo had to the sphere has the highest degree of perfection.
overcome prejudices and to fight arguments Thus the geometrical ideal of the circle is found
adduced against him by his opponents that realized in the celestial revolutions, and this
were not taken from the realm of empirical fact makes the firmament the visible symbol of
facts, but stemmed from other domains of perfection and incorruptibility, characterized by
human thought claimed to be and actually the principle of eternal uniform circular motion.
accepted by a great majority of his contem- However, there are seven celestial bodies: the
poraries as far more important: religious dog- sun, the moon, and the five planets, the two
matism and scientific traditionalism. former of which violate the principle of uni-
Naturally, the importance of observational formity, traveling sometimes faster, sometimes
and experimental facts has at all times been more slowly through the constellations, while
duly recognized because any attempt to de- the planets, in addition, have standstills and
scribe nature must be based on observa- retrogradations, thus describing characteristic
tion of natural phenomena. Thus no theory loops of varying size in certain parts of their
can be set forth before a sufficient amount of orbit. Now, accepting the axiom of circular
individual observations is available. But the uniform motion, it becomes a philosophical
observations alone do not of course suffice. necessity to explain also these irregularities as
To establish a theory, it is necessary to accept due to the combined effect of circles or spheres
as valid certain hypotheses and axioms that are revolving with constant velocity. No motion
taken, not from the domain of experience, but other than the circular is possible. In particu-
from that of philosophical speculation. Now lar, according to Aristotle, rectilinear motion
if the founder of the theory is one of the great has to be strictly excluded. It belongs exclu-
masters of learning, such as Aristotle, Ptolemy, sively to the four terrestrial elements, of which
or Galen, there arises a conflict when new earth and water have a natural downward, and
PROCEEDINGS OF THE AMERICAN PHILOSOPHICAL SOCIETY, VOL. 117, NO. 6, DECEMBER 1973
413
414 WILLY HARTNER [PROC. AMER. PHIL. SOC.

air and fire, a natural upward motion. Conse- With one exception, to be discussed towards
quently, for the celestial world the existence the end of the present lecture,3 the dimensions
of a fifth element, the Aristotelian ether (Latin resulting were accepted as real by all later
quinta essentia) has to be postulated; it is devoid astronomers down to the time of Tycho Brahe.
of gravity, neither heavy nor light, and has The first model was devised by the great
a natural eternal circular motion.1 As a further mathematician Eudoxus of Cnidos. It con-
consequence there results that the terrestrial sisted of sets of three or four spheres, one set
and the celestial worlds cannot possibly have for each planet, concentric with the earth and
anything in common, whence the question as revolving about different axes, by the inter-
to a law of nature valid for both becomes action of which the planet's motion was rep-
meaningless. It will be well to remember that resented. Although not applicable to all of
less than 300 years have gone since Isaac the planets-the model did not work, for in-
Newton, by his law of general gravitation stance, in the case of Mars-and not fit for
(1687), put an end to this dogma of the two explaining the changes in luminosity observable
different worlds, laying therewith the founda- above all in the cases of Mars and Venus, this
tion of a new discipline: celestial mechanics, system of "homocentric spheres" was never-
which marks the beginning of modern science. theless made the basis of Aristotle's universe,
Everything prior to this, from the first at- as explained in Book 12 of his Metaphysics.4
tempts made in Babylonia and in Greece down A century after, Apollonius of Perga intro-
to the seventeenth century, is not celestial duced two new conceptions of great conse-
mechanics, but celestial kinematics, that is, a quence: the eccentric circle and the epicycle
pure description of the phenomena of motion, whose center is carried about in the circum-
disregarding, with the exception of the very ference of a concentric circle, proving at the
last phase, the question as to the action of same time the mathematical equivalence of both.
natural forces regulating and maintaining plane- These two new devices, mostly in the combina-
tary motion. tion of an eccentric deferent carrying an epi-
While the Babylonians evidenced remarkable cycle, were to form the basis of Ptolemy's
skill in finding numerical solutions to the chief planetary theory, in his "Great Composition,"
problem of astronomy, that is, the prediction which we are used to call by its Arabic name,
of a planet's place within the zodiacal belt, Almagest. Thus the postulate of rendering ce-
the Greeks pursued a totally different aim. lestial motion by combined circular motion was
After the Pythagoreans, who had devised a fulfilled, though with a significant modification
cosmos in which the earth, contrary to appear- contrary to Platonic or Aristotelian thought:
ance, revolves together with the two luminaries it was found, namely, that observations, even
and the planets about an hypothetical central within the wide limits of accuracy then valid,
fire, the time of Plato witnessed the beginning could be represented satisfactorily only by
of a new period that was to last two millennia; abandoning the postulate of uniform motion in
it is characterized by the attempt to contrive the case of the eccentric deferent. In the new
mechanical models capable of rendering quan- model the revolution of the epicycle along the
titatively the planetary motions. deferent appears uniform only if seen, not from
It cannot be said in one word whether these the center, but from a fictitious point, called
models were regarded as actual images of the equant, situated on the straight line joining the
mechanisms producing the motions of the plan- earth with the center of the deferent in such
ets. While opinions may have differed in the a way that the center of the deferent bisects
beginning, it is certain that Ptolemy, in his the distance earth-equant. Thus each planet is
Hypotheseis (composed later than the Alma- characterized by a direction in space, called
gest), ascribed to his model indubitable reality, "line of apsides," and three equidistant points:
which caused him to draw from it erroneous
conclusions concerning the distances of the langes Alexandre Koyre, I: L'Aventure de la Science (Paris,
planets and of the vault of the fixed stars.2 1964), pp. 254-282; reprinted in W. Hartner, Oriens-
Occidens (Hildesheim, 1968), pp. 319-348, and B. R.
1 See Aristotle, De caelo ("On the Heavens"), Book I, Goldstein, "The Arabic Version of Ptolemy's Planetary
chs. 3 and 4 (269b 18-271a 34). Hypotheses," Trans. Amer. Philos. Soc. 57, 4 (1967).
2 See W.
Hartner, "Mediaeval Views on Cosmic Di- 3 See below, p. 422.
4 1073b 16-1074a 14.
mensions and Ptolemy's Kitab al-Manshfirat," in Me-
VOL. 117, NO. 6. 1973] COPERNICUS, THE MAN AND THE WORK 415

the earth, the center of the deferent, and the

equant (see fig. 1). The similarity with the
ellipse is obvious if we remember that for small
eccentricities the "empty focus" in the Keple-
rian ellipse may with a pretty high degree of
accuracy be regarded as the center of uniform
motion. But neither Ptolemy nor the Arabs
nor Copernicus ever thought of the possibility
of giving up the axiom of circular motion. It
was Kepler who took the decisive step.
It is well known that four hundred years
before Ptolemy, in the third century B.C.,
Aristarchus of Samos had conceived of a uni-
verse practically infinite in size, in which the
planetary loops are explained as due to the
revolution of the earth about the sun. It is
less well known, though geometrically evident,
that the equivalence of the heliocentric with
the geocentric system results directly from that
of the eccentric with the epicyclic model and
that Apollonius and Ptolemy have doubtless
been aware of this.
Now the question arises why Aristarchus's
model, simpler as it was, never succeeded in
supplanting the geocentric but fell into com-
plete oblivion after Ptolemy. Several reasons
can be adduced for this seemingly strange fact. FIG. 1. Model of planetary motion (Venus and superior
First of all a religious reason because, according planets). E = Earth. F = Center of eccentric de-
ferent. E' = punctum aequans ("equant"). H =
to Plutarch,5 a certain Cleanthes claimed it to Center of epicycle.
be the duty of the Greeks to indict Aristarchus
on the charge of impiety for having put the
earth-the "hearth of the universe"-in motion smaller than the distance from earth to sun
and attributed to her a twofold revolution. according to our modern knowledge. In such
The second stems from Aristotelian philosophy. a little universe, the earth's revolution about
Ptolemy discusses it thoroughly (Alm. I 7) and the sun must cause a parallactic change of the
all later astronomers, the Arabs, Peuerbach, places of the fixed stars in the amount of 6-7?,
Regiomontanus, and even Galileo in his early that is, in the order of magnitude of the loops
phase repeat the same argument: the earth described by Saturn. Moreover, the constella-
occupies the center of the universe and is tions of the zodiac, in the course of a year;
motionless because all heavy bodies fall verti- would appear enlarged or reduced in size by
cally down towards its center, while all light 6 per cent in relation to a mean value. But
ones ascend vertically. If the earth rotated experience tells us that these phenomena do not
about her axis, all falling bodies would deviate occur. It was for this reason that Aristarchus
westward, there could be no clouds moving or had assumed the universe to be of practically
birds flying eastward, etc. The third reason, infinite size, but this, again, was incompatible
however, evident though not found formulated with Aristotelian philosophy and with the imagi-
in words, has special weight: the Ptolemaic native faculty of ordinary man.
universe, according to the Hypotheseis has a Even with his much improved instruments,
radius of 20,000 terrestrial radii, or only 17 which were accurate to about one minute of
times the distance from earth to sun (according arc, Tycho Brahe was incapable of finding any
to Ptolemy's wrong estimate). In other words, parallactic effect, which caused him to reject
it was believed to be about 80 million miles the Copernican system. It was not until 1838
that the first stellar parallaxes were measured.
5 De facie in orbe lunae, ch. 6.
They were in the order of magnitude of 0.8
416 WILLY HARTNER [PROC. AMER. PHIL. SOC,

tions were not based on new observations, but

derived from philosophical considerations. Here
I mention Jabir ibn Aflab of Sevilla, in Latin
called Geber (not to be confused with the
alchemist Geber) and after him al-Bitrfiji (Latin
Alpetragius) who, doubtless unaware of the
Eudoxian model, made a new attempt to rep-
resent planetary motion by the interaction of
homocentric spheres.6 His work marked a re-
gression rather than progress. Nevertheless it
exerted an astonishing influence on scholastic
and early Renaissance astronomers, which puts
in evidence that their aim was also not to attain
better agreement with observation but exclu-
sively to devise models in better accord with
the Aristotelian axioms.
The same is true of the mathematically fas-
cinating new lunar theory of the Persian as-
tronomer Nasir al-Din of Tis, based strictly
I,.s
J} 0 on the Aristotelian principles of motion.7 In-
stead of eliminating the enormous variation of
FIG. 2. The "Tfsi couple." The center of the small
the moon's diameter as resulting from Ptolemy's
circle revolves on the dotted circle while the small
circle revolves with twice the angular speed in the theory (which would have been an easy task)
opposite direction. Then the point K moves rec- he concentrates all his efforts on preserving it
tilinearly from Ko through Ki, C . . . to K,,o and in his model, so as to force the moon as well
back to Ko. as could be done into its Ptolemaic orbit. For
this purpose he invents a new mechanism:
and 0.4 second of arc, corresponding to a dis- a circle revolving in a second circle twice as
tance of 24 and 48 trillion (1012) miles for our large, by which he succeeds in making a point
nearest neighbors among the stars, or to a uni- in the circumference of the small circle move
verse whose radius was 300,000 to 600,000 times rectilinearly up and down the diameter of the
greater than Ptolemy's. great circle (see fig. 2). Here I put special
I have mentioned that Ptolemy had to in- stress on the word rectilinear, reminding you
troduce in his planetary theory certain modifi- that Aristotle ascribes natural rectilinear mo-
cations, first and foremost the equant, incom- tion only to the four terrestrial elements and
patible with the Aristotelian axiom of uniform excludes it from the celestial regions. The
circular motion. On the other hand, it results philosophical consequences to be drawn from
from his lunar theory that the moon when this "Tusi couple," as E. S. Kennedy has
standing in her first and last quarters would termed this two-circle mechanism in English,
have to appear twice as large as the full moon, are extremely serious because it shows that the
which is strictly contrary to appearance. Hence 6 For al-Bitruiji, see B. R. Goldstein, Al-Bifrufi: On
the model could not possibly be correct, and the Principles of Astronomy (2 v., New Haven and London,
this cannot have escaped astronomers' atten- 1971).
tion. If it nevertheless remained the basis of 7 See W. Hartner, "Nadir al-Din al-Tuisi's Lunar
astronomical computations for more than a Theory," Physis 11 (1969): pp, 287-304, and W. Hartner,
"Trepidation and Planetary Theories. Common Features
thousand years, it was for the sole reason that in Late Islamic and Early Renaissance Astronomy,"
it represented the moon's motion in longitude Oriente e Occidente nel Medioevo: Filosofia e Scienze [Acca-
with astounding accuracy. This illustrates in demia dei Lincei (13? Convegno Volta), Rome, 1971].
a striking way the fact mentioned before that I accept with pleasure a casual remark made in a private
observations not fitting into a certain scheme conversation by Professor Noel Swerdlow that leaving
aside philosophical principles it would seem sufficient to
could be tacitly ignored. state that a sphere revolving with variable speed about
It was in the twelfth century, in western its axis is mechanically inconceivable. Nevertheless,
Islam, that the first serious critics of Ptolemaic philosophical reasons are adduced by Arab astronomers
astronomy appeared. However, their objec- as well as by Copernicus (De rev., I 4).
VOL. 117, NO. 6, 1973] COPERNICUS, THE MAN AND THE WORK 417

neat Aristotelian distinction between the ter- our common history that has our interest, for
restrial and the celestial worlds is no longer the reason that it yields us an incomparable
possible. Nevertheless, neither Nasir al-Din insight into the liberties and the limits of
nor anybody after him ever alluded to this: thought of a man of the very highest importance
"If a fact disturbs you, ignore it," the pre- and at the same time a typical representative
scription reads. of one of the most fascinating and most com-
It was only in the last phase of Islamic plex periods of European history. Here I avoid
astronomy, in the fourteenth century, that Qutb repeating the epithet "great." Copernicus's
al-Din of Shiraz and, above all, Ibn al-Shatir greatness cannot be doubted, but a biographer's
of Damascus8 devised new, very complicated aim ought to be not hero-worship but an ob-
models for the moon and the planets destined jective appreciation of the hero's merits and
to eliminate certain inconsistencies with obser- deficiencies.
vation, above all the intolerable variation of After their father's untimely death, the ma-
the moon's apparent diameter as resulting from ternal uncle, Lucas Watzelrode, later Bishop
Ptolemy's lunar theory, under strict observance of Varmia, a man of high intelligence and
of the Aristotelian axioms. By introducing a erudition, was entrusted with the education of
second epicycle ("epi-epicycle"), Ibn al-Shatir the four children, of whom Nicolaus was eldest.
succeeded in making the moon's distance from Watzelrode played an important role in Coper-
the earth vary within reasonable limits, while nicus's life, giving him support and advice
in his planetary theory a philosophically in- whenever there was need for it. In 1491
contestable construction, of considerable mathe- Copernicus enrolled in the University of Cracow,
matical interest, replaces the embarrassing of high repute at the time, and the only uni-
equant, though without of course abolishing it. versity in Central Europe that possessed a chair
The treatises containing these innovations were of astronomy. The matriculation roll is still
never translated into Latin. Nevertheless all preserved; it carries the entry Nicolaus Nicolai
essential features mentioned here are found de Thuronia solvit totum (Nicolaus, the son of
again in the work of the great man whose Nicolaus from Thorn has paid in full). This
quincentenary we are celebrating this year. is the first time Copernicus's name appears in
Nicolaus Copernicus was born in Thorn an official document.
(Polish Torunf) on February 19, 1473. As this At a remarkably early time Cracow had
city, founded by Teutonic Knights in the become a center of the new currents commonly
thirteenth century, had come under the sover- called Renaissance and Humanism. Albert
eignty of the King of Poland in the year 1466, Brudzewski, himself a leading humanist, had
he was born a Polish subject. From his letters founded there a widely known center for the
written in German it results that he had a study of mathematics and astronomy. It is
perfect mastery of the German language, and not certain, though probable, that Copernicus
we may assume as probable if not certain that still had the opportunity of getting into personal
German was his mother tongue. No conclu- touch with him. However, in view of the
sions as to his extraction or "nationality" circumstance that Brudzewski, judging from his
(a modern notion hardly applicable to his time) commentary on Peurbach's Theoricae planeta-
should be drawn from this, nor from the lack rum, can hardly be termed a severe critic of
of Polish documents written by his hand. For Ptolemy, the assertion that he may have been
a century or more Polish and German historians the first to call Copernicus's attention to errors
have wasted their energy on this question. found in the Almagest seems unfounded. From
Today, on both sides, we regard him no longer documents still in existence we know that
as an object of debate but rather as a con- Copernicus, during the years 1491-1495, at-
necting link. It is his work and its place in tended courses on Sacrobosco's Sphaera, on
8 See Victor Roberts, "The Solar and Lunar philosophy, on Euclid's Elements, on eclipse
Theory computation, on the calendar, on geography,
of Ibn ash-Shatir," Isis 48 (1957): pp. 428-432; E. S.
Kennedy and Victor Roberts, "The Planetary Theory and, not least, on astrology, in particular
of Ibn al-Shatir," Isis 50 (1959): pp. 227-235; Fuad Ptolemy's Tetrabiblos.9
Abbud, "The Planetary Theory of Ibn al-Shatir: Reduc-
tion of the Geometric Models to Numerical Tables," 9 According to Nicholas Copernicus
Isis 53 (1962): pp. 492-499. (Mikolaj Kopernik)
1473-1543, prepared by J. Rudnicki, translated from the
418 WILLY HARTNER [PROC. AMER. PHIL. SOC.

Owing to Watzelrode's intercession, Coper- he had actually mentioned Aristarchus, but

nicus, in 1495, was appointed canon at the then inadvertently obliterated his name while
Chapter of Frauenburg near the Baltic. In- blotting out for other reasons the passage
stead of making use of this prebend, however, containing it.
he set out on a prolonged trip to Italy in order His promotion to the dignity of a Doctor of
to study canon law at the famous law school Canon Law took place in Ferrara, in 1503.
of Bologna. There he pursued also his as- Thereafter he went for another year to Padua
tronomical studies, encouraged and supported to study medicine. Having returned to Var-
by ,the learned Domenico Maria da Novara, mia, finally, he kept his prebend waiting for
who allegedly (it seems insufficiently attested) five or six more years, taking residence at the
through his own observations had started to bishop's palace in Heilsberg (Lidzbark), where
doubt the correctness of the Ptolemaic theory. he served as his uncle's medical and legal
My own doubts as to the correctness of this adviser. About 1510, two years before Watzel-
assertion are based on the circumstance, to be rode's death, eventually, he moved to Frauen-
discussed hereafter, that even Copernicus's revo- burg to take possession of his office, kept open
lutionary criticism is founded almost exclusively to him for no less than fifteen years.
on philosophical arguments, not on observa- In all probability it was about this time
tional facts.10 In his De revolutionibus, Coper- that he wrote the first draft of his new theory,
nicus relies on no more than twenty-seven ob- the so-called Commentariolus, destined only for
servations of his own. Of these an occultation the use of some of his close friends. While
of the star Aldebaran, carried out in Bologna this little work, which gives us a precious
in 1497, is the earliest. insight into the gradual maturation of his ideas,
After three years' stay in Bologna, Coper- never went to print, another one, of a totally
nicus went to Rome in the Jubilee Year 1500, different character, was printed in Cracow in
where he served temporarily as a professor of 1509: a Latin translation of the idyllic and
mathematics. Nothing definite seems known erotic letters of Theophylactus Simocatta, a Byz-
about this activity. Early in 1501 he returned antine author of the seventh century. No other
home in order to enter formally upon his office, work of his ever appeared in print during his
but took leave again immediately so as to finish lifetime.
his studies of canon law in Padua. There he Already in Heilsberg, as Watzelrode's at-
learned Greek, studied Plato and other authors, tendant and adviser, Copernicus had had the
including Cicero and Plutarch, in whose writ- difficult task to intervene in the continuous
ings, according to his dedication to Pope Paul III quarrels between the Polish government and
of his magnum opus, he found references to the Teutonic Knights, for whom the bishop
men who (citing his words literally) "had been showed no sympathy whatsoever. The same
of opinion that there exist other motions of the task fell to his share, on many occasions, during
celestial bodies than assumed by those who the subsequent years. From 1516 to 1519 he
have taught mathematical science at school." was ordered by his superiors to reside in Allen-
He mentions the Pythagoreans Philolaus and stein as an "administrator," with a wide range
Hicetas, also Heracleides and Ecphantus. But of duties and responsibilities. Open war broke
the true founder of the heliocentric system, out in 1520 and was ended by an armistice
Aristarchus of Samos, is not mentioned in the in 1521. Thereafter Copernicus composed sev-
printed edition of his work. However, this eral memoirs, of high interest to the historian
was due to a mere slip: as attested by his of economics, in which he discussed the de-
original manuscript," which has fortunately plorable financial plight of the devastated
survived the vicissitudes of half a millennium, country and made practical proposals, most of
which were accepted by the Prussian Estates,
Polish by B. W. A. Massy, with a foreword by Sir Arthur
for the restabilization of the monetary unit.
Eddington; published by the Copernicus Quatercentenary
Celebration Committee (London, 1943), p. 3. While Copernicus showed no sympathy for
10As I am demonstrating in a forthcoming paper, Luther's reformation (Luther in turn called him
Copernicus's theory of Mercury, based exclusively on a fool who wished to destroy astronomy) he
the axiom of uniform circular motion and differing widely was everything but narrow-minded in his inter-
from Ptolemy's, yields exactly the same results as the
latter and takes no account of new observations. course with personal friends and disciples.
l Fol. 1 lb. This is shown above all by his friendly terms
VOL. 117, NC. 6, 1973] COPERNICUS, THE MAN AND THE WORK 419

with Joachim Rheticus, a young Protestant of "principle" or "assumption," when speaking

theologian of Wittenberg who, on occasion of about his new system.'2 He gives no philo-
a two years' visit (1539-1541), was the first to sophical definition of the terms "true" and
be introduced to his new theory, and whom he "probable" but it ought not be overlooked
authorized to publish a preliminary report, that in his dedication he uses the word "truth"
titled Narratio prima, and printed in Danzig only on one occasion, viz., in his statement
in the year 1540. With this, Copernicus broke that the philosopher's aim is the search for
the silence which he had kept, as he says him- truth in all things, "insofar as God has per-
self, paraphrasing a Horatian word, "not only mitted this to human intelligence." With this
nine years, but well into the fourth novennium," restriction he wants to say that in investigating
that is to say, thirty years or more. It was terrestrial and celestial phenomena there can be
due to Rheticus and Copernicus's friend Tiede- no question of finding absolute truth, but only
mann Giese, Bishop of Culm, that he consented various degrees of probability. This corre-
to the printing also of the great work itself. sponds entirely to the scholastic distinction
It appeared in Nuremberg in 1543 under the between truth and probability: truth, veritas,
title De revolutionibus orbium coelestium, "On belongs only to God; according to his will, it
the Revolutions of the Celestial Spheres," may be revealed to us, in whole or in part,
whereas the title given to it by Copernicus thus becoming a religious dogma inaccessible
himself read only De revolutionibus. Bishop to criticism. On the other hand, what human
Giese tells us that the first copy of the book ar- ratiocination can attain is only probabilitas,
rived in Frauenburg on May 24, 1543, the very that is, not probability in the modern trite
day of Copernicus's death. sense, but in the pregnant meaning of id quod
There is no doubt that Copernicus was con- probari potest, "that which can be proved." 13
vinced of the objective reality of his theory. Hence it is up to us, in the case of competitive
He says so in clear words in his dedication to theories, to establish criteria for finding the
Pope Paul III, anticipating at the same time most probable one, but strictly avoiding the
objections of critics "who may dare to attack term "true," which is not applicable. I may
my work by referring themselves to a verse call your attention to the fact that in this
from the Bible which they have badly distorted" respect, but only in this, scholastic thought is
(Joshua 10.12: "Sun, stand thou still upon in perfect agreement with modern scientific
Gibeon; and thou, Moon, in the valley of thought.
Ajalon"). Against them he asks the Pope's Of the many criteria serving to decide on
protection. the degrees of probability of two or more
The anonymous preface, composed by An- theories, simplicity ranks foremost. That one
dreas Osiander, who saw the work through the is most probable which operates with a mini-
press, reads differently. According to him, mum of axioms and hypotheses to "save the
these "hypotheses" are not necessarily true, phenomena," that is, to represent observations
not even probable; it suffices that they yield correctly. It is the "principle of economy"
results in accordance with observation. And (lex parsimoniae) which decides, expressed in
he concludes with the words: "Nobody should the words quod potest fieri per pauca, non fiat
expect, with regard to these hypotheses, any- per multa, "what can be rendered by few prin-
thing certain of astronomy, which cannot pos- ciples should not be rendered by many." It
sibly fulfill such expectations." No doubt Co- sounds convincing, but a simple example already
pernicus, had he known this preface, would mentioned demonstrates its fallacy. We have
have been infuriated by it, as has been the seen that the eccentric circle and the concentric
case of uncounted later generations down to with an epicycle are geometrically equivalent
fairly recent times. However, our modern and yield identical results. But if looked at
view of scientific truth is different and comes from the point of view of economy, the former
close to Osiander's standpoint, despite the fact
that a man like Kepler would scorn us for it. 12 See E. Rosen, Three
Copernican Treatises (2nd ed.,
As for the word "hypothesis," considered the Dover Publications, New York, 1959), p. 28f.
13See for instance Anneliese Maier, Metaphysische
primary offense, we find that Copernicus him- Hintergriinde der spdtscholastischen Naturphilosophie
self, not always consistent in his terminology, (Rome, 1955), ch. I: "Das Prinzip der doppelten
uses it on many occasions, often as a synonym Wahrheit."
420 WILLY HARTNER [PROC. AMER. PHIL. SOC.

would have a higher probability than the latter, ferent inclinations to the ecliptic, pass through
and there is no accessory criterion to help us the sun. Ptolemy had made them pass through
out of the dilemma. Thus the decision be- the earth, which caused him to introduce com-
comes a matter of practical considerations or plicated mechanisms, naturally based on the
of personal taste. As long as scholastic thought axiom of circular motion, to account for the
prevailed, things remained at that. It was changing latitudes, i.e., angular distances from
only with Kepler, profoundly convinced as he the ecliptic. Now, with Copernicus, the planes
was of the possibility of finding the divine laws of planetary orbits pass through the center of
governing the universe, that the situation the earth's orbit, the axioms remaining the same.
changed. His approach may be called an at- There necessarily results a no less clumsy ap-
tempt to fuse religion, taken in the widest paratus to render the variations in latitude,
sense, and science into one inseparable whole, explained at length in the sixth Book of De
which marks a complete break with scholastic revolutionibus. Thus a revolutionizing idea aris-
tradition. ing from the desire of simplifying and harmo-
When Copernicus, in his young days, con- nizing the universe carries along step by step
ceived his new theory, things may have ap- new complications until, finally, the question as
peared to him as simple as modern textbooks to the relative "economy" of the old and the
and popular biographies take pleasure in rep- new systems becomes meaningless.
resenting it. By introducing an unmoved sun Two errors encountered time and again in
and a moved earth, astronomically indistin- the ocean of Copernican literature require
guishable from the other planets, the loops of rectification.
retrogradation and their different sizes are ex- One is the assertion that the Ptolemaic sys-
plained as parallactic phenomena. The epi- tem in the course of time, because of new
cycles become superfluous, and six circular observations, had become burdened with so
(actually eccentric) orbits about the sun take many complications, additional circles and the
the place of highly complex mechanisms. In like, that a strong need for simplification was
view of the circumstance that the centers of felt, and that this circumstance caused Coper-
the epicycles of M\Iercuryand Venus coincide nicus to ponder about a new solution. Except
with the mean sun and that a similar relation for the criticisms of Nasir al-Din and Ibn
results for the outer planets, a simple trans- al-Shatir, whose probable impact on Copernicus
formation of coordinates could be expected to will be discussed later, the Ptolemaic system,
simplify things in a striking way. However, in despite new observations, remained unchanged
carrying out his task, Copernicus's optimism throughout the ages; apart from this, Coper-
must soon have dwindled. Remember that the nicus saw himself confronted only with the
phenomena mentioned refer not to the true but original text of the Almagest.
to the mean sun which, according to Ptolemy's Another erroneous assertion is that Copernicus
definition, is a fictitious point moving uniformly has contrived to abolish the equant. What lies
in the ecliptic, being at one time ahead of the behind it can easily be explained. As we have
true sun, at another lagging behind it. Since seen, this fictitious point, indispensable as it is,
the phenomena of motion as described in the disturbs the apparent harmony of the Ptolemaic
Almagest have to be preserved, the transforma- model insofar as it violates the circle axiom.
tion of the geocentric to the heliocentric (or I have mentioned, too, that this caused Ibn
rather heliostatic) system will inevitably lead al-Shatir to devise a threefold epicyclic motion
again to a fictitious point near the sun instead in which the center of the third epicycle revolves
of making the sun itself the center of planetary in such a way that its motion appears uniform
motion. Hence the Copernican theory has to if seen from the equant. Thus the equant need
refer all planetary orbits to this fictitious point, no longer be marked on the apseline because it
which is of course the center of the earth's is replaced by an ingenious construction in
eccentric circular orbit. Therewith the earth perfect accord with the circle axiom. Now
assumes an undesired privileged position even exactly the same construction is found again in
in the heliostatic system. De revolutionibus. It is possible indeed that
This entails many disturbing consequences. Copernicus discovered it independently, but it
As one example out of many, I mention that seems more probable that the news of his
the planes of all planetary orbits, having dif- Islamic predecessor's model reached him in
 _

-VOL. 117, NO. 6, 1973] COPERNICUS, THE MAN AND THE WORK 421

!, c j j.jo
'4 ) oj7.L8 jg JJ
oj1^^.

' * ^
. '"- -'-?t -_
L_
__** ._. -1 -
--**~~~~~~
~~~=- -=~

*
I.. ...........
- . . . Ii |i .. . ' .
......... \''....- I |

;: \ /, -r~
\ r-?~ / 1Z0
\/ P
r
j1 "(;Sw
1, ^JZrl.T! <3'j
l -; I

^- ij l^ w o ^J^1-1

e -= -__ __
- .5)
FIG. 3. The Tusi couple in Nasir al-Din's Tadhkira fi cilm al-hay'a, MS. Laleli 2116, fols. 38b-39a. The Arabic
letters along the perpendicular, from top to bottom, are A, H, D, B. The point of contact between the two
circles is designated by G.

some way or other, though nobody knows to may have been re-invented by Copernicus who,
this day through what channels it could have on a later occasion, in his theory of Mercury
been transmitted. I have mentioned before (V 25, where the theorem appears once more)
that the writings of those late Islamic astrono- refers to Proclus's Commentary on Euclid's
mers were never translated into Latin. Elements, in which it is implicitly contained.
There is other evidence to support the bor- However, what proves clearly that we have to
rowing theory. Copernicus's lunar model, where do with a case of borrowing, is the lettering
the intolerable variation of the moon's distance of the diagrams found in the Tfisi manuscripts
resulting from Ptolemy's theory is reduced to and in De revolutionibus. In both the five
reasonable limits, agrees in every detail with letters, a, d, b, g, and h, denote the same char-
Ibn al-Shatir. However, while no unambiguous acteristic points (see figs. 3-5). A reasonable
proof for a direct borrowing has so far been explanation would be that Copernicus, doubtless
adduced in these cases, which were discovered in Italy, saw the diagram in a manuscript of
by E. S. Kennedy and Victor Roberts, I con- Tfisi's astronomical treatise, Tadhkira, and
trived some years ago to uncover a case that asked somebody who knew Arabic to translate
seems indisputable. the passage for him. It may have been years
In his (erroneous) theory of trepidation, pos- later that he found again the note then made
tulating a periodic change of the velocity of and made use of it.
precession, Copernicus makes use of the "Tfisi Another grotesque error has to be pointed
couple," that is, the two rolling circles producing out in this context. In a passage later struck
a straight line, devised by Nasir al-Din for out in the manuscript, Copernicus makes the
his lunar theory.14 Now this mechanism, too, rather trivial remark that other points on the
14 See above, pp. 416-417, and fig. 2. radius of the little circle or its production
422 WILLY HARTNER [PROC. AMER. PHIL. SOC.

oisltiiiarum noralionum acquanaz

i principiodidum fit,motcum ccle
libus ac circularibuscopofitum.

,&agatdimetiesD F G. Oftedcndii
:uloruG HD &c F B cocurretibusino
.
I
i lincaA B hincindc reciprocadore
,M moucriin dwuerfam parte,&duplo
FIG. 4. The Tfisi couple with the same lettering in the FIG. 5. The same in the Editio Princeps of De revolu
MS. of Copernicus's De revolutionibus, fol. 75a. tionibus (Nuremberg, 1543), fol. 67a.

describe "figures called ellipses by mathemati- As its greatest objective merit I point out that
cians." This induced serious scholars like the Copernican theory for the first time in
Curtze, the editor of the Thorn edition of 1873, history has made it possible to compute plane-
and many after him, to claim that Copernicus tary distances objectively and to do away with
with this had adumbrated the possibility of the traditional values resulting from sophisti-
elliptical orbits, in spite of the fact that Book 3, cated speculation. Copernicus himself paid
where the theorem is discussed, contains not a little attention to this fact; it was Kepler who
single word about planetary motion. In point recognized its immense importance and derived
of fact, nothing was farther from Copernicus's from it his third law.
mind than to put in doubt the Aristotelian It has been my aim to show that conserva-
axioms of motion. It was Kepler and nobody tism is the dominating feature of the Copernican
else who after long hesitation contrived to rid revolution. The historian of astronomy who
himself of these age-old prejudices. blames him for this conservatism commits a
It is a cheap mode of entertainment to grave error. He is not aware that only in the
belittle Copernicus's merits because we know course of a long evolution has it been possible
better today. If looked at with the eyes of to rid ourselves step by step of the prejudices
his contemporaries, his work presents itself as of our ancestors. The evolution has not come
the most daring achievement that can be to an end; there will be prejudices as long as
thought of. It demonstrated the possibility of there are people on earth. Nicolaus Copernicus
finding new ways, other than the trodden ones will be remembered as the man who took the
and served as an incitement to later generations. first decisive step.