UNIT 1 COPERNICAN REVOLUTION AND ITS PHILOSOPHICAL IMPLICATIONS

Contents

1.0 Objectives
1.1 Introduction
1.2 Geocentric Theory vs. Heliocentric Theory
1.3 Postulates of Copernican System
1.4 Summary of Book One
1.5 Solution to Retrograde Motion
1.6 Teleology vs. Mechanical Philosophy
1.7 Copernican System Vs Non-Copernican Systems
1.8 Let Us Sum Up
1.9 Key Words
1.10 Further Readings and References
1.11 Answers to Check Your Progress

1.0 OBJECTIVES

This unit will bring you to the threshold of ‘Modern Astronomy’ set in motion by Copernicus.
The principal aim of this unit is to make you familiar with those elements of astronomy which
lay at the root of Copernican system. Therefore in this unit we will focus on aspects on which
Copernicus agrees with his predecessors and aspects on which he dissents. Further, you must be
able to evaluate Copernicus’ contribution to the direction in which astronomy developed by
identifying what exactly constitutes progress in Copernicus’ system and this progress constitutes
the essence of the revolutionary impact for which it is famous for. Therefore this aspect is
highlighted in this unit.

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1.1 INTRODUCTION

The beginnings of modern astronomy are traced to the year 1543 when Copernicus’ De
Revolutionibus Orbium Caelestium (On the Revolutions of the Celestial Spheres) was published.
With the publication of this work the path of astronomy takes a different path. In antiquity
cosmological study was driven by teleological explanations. One of the major achievements of
Copernicus consists in the shift from teleological explanation to mechanical explanation.
Therefore he can be regarded as a precursor to Modern Science. The shift became possible
mainly because his perspective itself was totally different from that of Greek astronomers.
Initially, the startling contents were confined to his inner circle only and he was reluctant to
publish the work for a very long time though he was convinced of usefulness of his theory for
two reasons; first, fear of reprisal because the Church was still too strong and second,
unwillingness to share his thought with those who were mathematically untrained. In his preface
to the De Revolutionibus, but addressed to the Pope Paul III, he confesses to his apparent
Pythagorean orientation which was behind his dilemma to publish or not to publish the work. He
admits that he was hesitant to ‘give light to these my commentaries written to prove the Earth’s
motion or whether, on the other hand, it were better to follow the example of the Pythagoreans
and others who were wont to impart their philosophic mysteries only to intimates and
friends……’. The exposition of an astronomical problem in a typical mathematical language
which began with Ptolemy’s Almagest matures in Copernicus. For this reason, but for the first
part the whole work is completely mathematical which requires sound knowledge of
mathematics. As a result, those who are not mathematically trained adequately cannot grasp the
essence of his work. Consequently, he argued that ‘Mathematics are for mathematicians’ which
Kuhn regards as an ‘essential incongruity’. Copernicus’ awareness of this serious limitation on
the part of readers prevented him from publishing his work.

Copernicus is well known for rejecting geocentric theory and opting for heliocentric theory.
However, to stop at this stage alone amounts to making a mole out of a mountain. This critical
aspect unfolds itself at a later stage. Further, in spite of the fact that he disagreed with not only
Aristotle but also Ptolemy on many important issues Copernicus remains more Aristotelian than
Ptolemaic. Cohen argues that Copernicus followed Ptolemy’s Almagest in ordering the chapters
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and choosing the sequence and that he admired Ptolemy. Cohen’s statement is misleading though
the first part of his assertion may be true. The structure of these two works may be similar but
there is clear opposition between these two works. What is noteworthy is that Copernicus did not
make any new observations. All his arguments are based on observational data recorded by his
predecessors which means that the heliocentric theory is the outcome of rigorous logical
approach applied to observational data. Against this backdrop, the contribution of Copernicus
must be evaluated.

1.2 GEOCENTRIC THEORY VS. HELIOCENTRIC THEORY

Apart from Pythagoreans like Philolous Ecphantus etc., according to Kuhn, there are some
Neoplatonists who gave Copernicus food for thought. Kuhn’s opinion has to be treated with
caution. Marsilio Ficino an Italian Neoplatonist and Nicholas who lived in the 15th century are
the immediate precursors of Copernicanism. Kuhn quotes from one Neoplatonist without making
clear the source of the following passage: ‘In the middle of all sits Sun enthroned……... ruling
his children the planets……’. Though the language is more mystical and less philosophical and
lends itself to equivocation easily, for Copernicus, if Kuhn is to be believed, the message was
more than clear. But this supposed latest source is, evidently, dubious. Kuhn himself admits that
Ficino knew nothing of astronomy. A poetic or a mystical narration may at best help to develop
an insight. But it is far from saying that such a narration is the source of an epoch-making
astronomical theory. At best, Nicholas might be a reliable source because he inferred the motion
of the earth from many-world theory which clearly opposed Aristotle’s one-world theory.
Platonic two-world theory, which found place in Aristotle’s cosmology, in the form of sublunary
and superlunary worlds, should not be confused with one-world in astronomical sense. Even if
this possibility is allowed, it will not be of much help because none of them thought on the lines
of applying this hypothesis to solve any astronomical problem, in particular planetary motion.
The uniqueness of Copernicus must be discerned here. This is one part. The apparent influence
of Pythagoreans will be discussed later. But there were, surely, extra-astronomical factors which
guided Copernicus’ thought. In his preface addressed to the Pope III, Copernicus refers to one
such factor. There was urgent need to restructure calendar which heavily depended upon the
exact calculation of the length of the year. Another factor which weighed heavily in favour of the
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suncentric argument was expeditions, resulting in the discovery of hitherto unknown parts of
land, which demanded that the world map had to be rewritten. Again, the target is Ptolemy.
Ptolemy’s geographical knowledge is no less than his knowledge of astronomy. Charles Singer
suggests that in all likelihood Ptolemy had access to the map prepared by Vipsanius Agrippa of
Rome which became outdated due to several successful expeditions. Further, navigation, which
had by then become almost a way of life, entirely depended upon several astronomical factors.
These are the genuine extra-astronomical factors which propelled the change.

Each of these two theories implies a significant conclusion; what is central to the universe is also
static. Therefore Bernard Cohen aptly describes them as geostatic and heliostatic theories
respectively. It is surely profitable to contrast Copernican system with Ptolemaic system, his
immediate predecessor.

Mercury Sun
Venus
Stellar sphere

Mars

Earth

Saturn Jupiter

Fig.1a Copernican model of planetary system

Mercury

Moon Venus 4 Earth

Stellar sphere
Fig.1b Ptolemaic system

From these two figures it is clear that the positions of the earth and the sun are not just
transposed but there is slight reshuffling. The table 1 shows the relative positions of these two
from the respective centres in Ptolemaic and Copernican systems.

Table 1

Ptolemaic System Copernican System

1 The moon Mercury

2 Mercury Venus

3 Venus The earth (with its moon)

4 The sun Mars

5 Mars ___

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It is easy to notice that there is a mismatch between these two systems because the moon is not
assigned a separate orbit. Secondly, the table makes it clear that there is at least partial relocation
of all bodies mentioned above. However, the positions of Jupiter and Saturn are common to both
the systems. In regarding the sun as the centre of the universe Copernican system is encountering
the first anomaly. Copernicus continues, surprisingly, to regard the sun as a planet despite the
fact that in his system the sun is static. He ought to have changed the definition of planet itself or
he ought not to have regarded the sun as a planet which he did not. The shift raises certain
significant questions which need to be probed. Before we do so we must consider certain
postulates which constitute the framework of his system.

1.3 POSTULATES OF COPERNICAN SYSTEM

Copernicus’ investigations are based upon seven postulates which figured in his earlier work,
‘The Commentariolus’. They are as follows:

1. There is not a single centre of all the celestial orbs or spheres.

2. The centre of the earth is the centre of gravity and lunar sphere, but not of the universe.
3. All the spheres encircle the sun, which is in the middle of them all, so that the centre of the
universe is near the sun.
4. The ratio of the earth’s distance from the sun to the height of the firmament is so much
smaller than the ratio of the earth’s radius to its distance from the sun that the distance
between the earth and the sun is imperceptible in comparison with the loftiness of the
firmament.
5. Whatever motion appears in the firmament is due, not to it, but to the earth. Accordingly, the
earth together with the circumjacent elements perform a complete rotation on its fixed poles
in a daily motion, while the firmament and highest heaven abide unchanged.
6. What appears to us as motions of the sun are due, not to its motion, but to the motion of the
earth and our sphere, with which we revolve about the sun as (we would with) any other
planet. The earth has, then, more than one motion.

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7. What appears in the planets as (the alternation of) retrograde and direct motion is due, not to
their motion, but to the earth’s. The motion of the earth alone, therefore, suffices (to explain)
so many apparent irregularities in the heaven.
(Source: The Tests Of Time: Lisa M. Dolling, Arthur F. Gianelli and Glenn N.Statile, Ed.
Princeton University Press, New Jersey, 2003)

It is profitable to consider these postulates in some detail and surely some or in need of
clarification considering their significance.

Consider first and third postulates together. First postulate states that there is no one centre of all
the celestial orbs which means that there must be more than one. It is possible that every orb has
its own centre. If so, in the strict sense of the term the sun cannot be regarded as the centre of the
universe. This is what the third postulate asserts. All the spheres may encircle the sun. But it does
not imply that the sun must be the centre of all orbs. If it is not the centre of all orbs, then it
logically follows that it cannot be the centre of the universe. Therefore Copernicus’ theory is not,
strictly speaking, heliocentric. It only approximates to the same. This conclusion leads to two
interesting consequences. In the first place, Copernicus is contradicting himself when he asserted
in the preface to the De Revolutionibus that the use of eccentrics and epicycles by others (by
‘others’ Copernicus means Ptolemy and his followers) did not yield the desired results. What
Copernicus has done is not different from Ptolemy and his predecessor Appolonius (3rd century
B. C.) who defined eccentric as a displaced circle with reference to the centre of the earth. We
only have to substitute the sun for the earth. The mathematical structure of the circle remains
unaffected. In the second place, if the sun is not the exact centre of the universe, then the orbs
cannot be the exact circles. Thereby Copernicus has unwittingly deviated from Aristotelian
tradition. Now consider this deviation in conjunction with the third postulate. It takes Copernicus
to one step short of Kepler who replaced circle by ellipse.

The second postulate is, evidently, Aristotelian. While explaining rectilinear motion Aristotle
said that the natural motion of heavy bodies is towards the centre of the earth. The concept of
gravity, obviously, was unknown to Aristotle. This is one part of the story. Copernicus thought
that the sun is too far from the moon to regard it as the centre of the lunar sphere. Copernicus did

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not go that far to regard the moon as a satellite. In fact the very concept of satellite was unknown
in Copernican age. The combination of these two led to a serious anomaly of which Copernicus
was most likely unaware. Can any two planets (here the earth and the moon) have a common
orbit? When a scientist is not aware of problem, naturally, he does not think of any answer.
Suppose that the earth and the moon have a common orbit. Under what conditions can the earth
be the centre of lunar sphere when they have a common orb? The earth could have been the
centre of lunar sphere had the path of moon been an epicycle with the earth as the centre. This
possibility must be ruled out because the orb of the earth and the moon is the same though
Copernicus hinted at this possibility and this he did in rather a half-heated manner. He says that
‘the lunar sphere, however, revolves around the centre of the earth and moves with it like an
epicycle’. This shows that Copernicus is not sure whether the path of the moon is an epicycle or
not. Evidently, Copernicus’ postulate does not make sense. What is noteworthy is that in the
history of astronomy this inherent problem did not render the concept of the earthcentric lunar
sphere useless, but played a pivotal role. It acted as a fulcrum for Newton who propounded the
theory that the earth exercised gravitational force on the moon because from the functional point
there is no difference between Aristotle’s concept of the centre of the earth and Newton’s
concept of gravity which later covered the whole universe. Both are concerned with the direction
of the motion.

Perhaps these were not the consequences which Copernicus could foresee, consequences of his
own theory. Kuhn aptly remarks that ‘The significance of the De Revolutionibus lies, then, less
in what it says than in what it caused others to say’. Further, he continued to remark that the De
Revolutionibus has a dual nature….. the tradition from which it derived and…..the tradition
which derives from it’. These postulates perfectly justify Kuhn’s remark.

The fourth postulate is somewhat closer to Ptolemy’s fourth theorem which asserts that the
magnitude of the earth is imperceptible. Copernicus slightly alters the theorem. Here the earth’s
distance from the sun in comparison with the height of the firmament becomes imperceptible.
Therefore if Ptolemy and Copernicus are taken together, then in two important respects the earth

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becomes atomic; magnitude and distance from the sun. This change, in one sense, undermines
the significance of the earth. While the first three postulates take Copernicus much closer to the
Aristotelian tradition, the fourth postulate takes him away from the same.

The fifth postulate affirms, on the one hand, the motion of the earth and on the other, denies the
motion of firmament. While the former may not be a novel idea surely the latter is. This
postulate, in effect, turns Aristotelian cosmology completely upside down. Aristotle and Ptolemy
regarded the stellar sphere as Primum Mobile, the Prime Mover which is the source of motion of
all planets down to the moon while the earth alone remained outside the influence of Primum
Mobile. However, in Copernicus this Primum Mobile becomes stationary. If there is any
Revolutionary idea in Copernican doctrine, it is the idea of stationary stellar sphere.

Nicole Oresme anticipated the sixth postulate which in turn laid the foundation for relativity of
motion. Let us start with the narration of Oresme’s account of motion which, in reality, is a
criticism of Aristotle’s theory. In his own words local motion can be perceived only when one
body alters its position relative to another. We shall call one body ‘x’ and another ‘y’. Aristotle
says that ‘x’ (the earth) is stationary and ‘y’ is mobile. Oresme only suggested that it could be the
other way round as well. Not that it was so as a matter of fact. He admits that neither reason
(logic) nor faith (scriptural authority) can disprove the rotation of the earth. Nor can it prove the
same. According to him it was only a logical possibility. Now imagine the antithesis of Aristotle.
What if the earth moves and stellar sphere is stationary? The result remains the same. This
suggestion made by Oresme must have provoked Copernicus to frame sixth postulate which is
nothing but near repetition of Oresme. Again, the difference between Oresme and Copernicus
consists in the application of mathematics by the latter in order to prove the mobility of the earth.
Therefore Copernicus concluded that the earth must be having more than one motion. While
diurnal motion causes day and night the apparent motion of the sun is caused by real motion of
the earth. And in addition to these two motions, Copernicus lists third type of motion which he
calls ‘declination’ referring to the rotation of the poles. Copernicus was convinced that the third
type of motion was due to the fact that the axis of the earth’s rotation is not parallel to the earth’s

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orb around the sun but is inclined to form 230. This particular postulate has extraordinary
significance because it laid the foundation for relativity of motion which acquired phenomenal
importance in the history of physics.

The last postulate attempts to explain the retrograde motion of the planets. Copernicus was
convinced that none of his predecessors, Ptolemy included, succeeded in satisfactorily explaining
the retrograde motion of the planets. He thought that if the earth is regarded as mobile instead of
being regarded as static, then it is possible to resolve more effectively the retrograde motion of
the planets. Therefore Copernicus preferred new theory to old theory. In the history of astronomy
we discover the first ever instance of accepting a theory on the grounds of usefulness, a doctrine
which came to be known as Instrumentalism at a later stage. Is there any philosophical
significance in preferring one theory to another or is it merely a random choice devoid of any
merit? This aspect of Copernicus demands a closer attention to which we shall return later.

After considering these postulates Copernicus makes a very significant statement which runs as
follows. ‘Accordingly, lest anybody suppose that, with the Pythagoreans, I have asserted the
earth’s motion gratuitously, he will find strong evidence here too in my exposition of the circles’.
The upshot of the previous statement is this; though Pythagoreans in antiquity asserted that the
earth is neither the centre of the universe nor static they did not attempt to prove either
deductively nor did they resort to observation to substantiate their assertion. Given the reputation
which Pythagoras enjoyed and his obsession with esoteric religion, it is most unlikely that
Copernicus would have acknowledged that any Pythagorean could be regarded as his precursor.
What counts is, according to him, the basis for making such a proposal. The competition is
between poetic imagination or mythical speculation and rigorous mathematical analysis. In this
regard mathematics holds the key which seals the fate of Pythagoreanism.

1.4 SUMMARY OF BOOK ONE

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It is profitable to contrast Copernicus with his predecessors. However, before we contrast them
let us consider in brief Book One of the De Revolutionibus which consists of twelve sections.
First section deals with the shape of the universe. It says that the universe is spherical in shape.
There are three possible reasons. First one is that sphere is the most perfect shape. Evidently,
Copernicus inherited this possible reason from ancient cosmology. Second possible reason is that
it is all-inclusive. Third possible reason is that all planets and stars are spherical in shape.
Extension certainly matters. However, third reason is significant. If the content is pressed by the
container, then, obviously, the content takes the shape of the latter. Therefore Copernicus is
arguing from effect to cause. Third reason is very much analogous to water taking the shape of
its container. What is the consequence of the determinate shape of the universe? If we admit that
the universe has determinate shape, then we have to admit that the universe is finite. So
Copernicus is still in Aristotelian mould. In the second section Copernicus proves the shape of
the earth on empirical grounds. He admits that its shape cannot be perceived directly. He
considers what one experiences when he travels northward and compares it with what he
experiences when he travels southward. The results are different. In the former case the northern
vertex of the axis of diurnal motion also moves overhead and therefore the northern stars do not
appear to set. In contrast, the southern stars do not seem to rise any more. The result is reversed
when the journey is southward. Second reason which he considers is geometrical. It says that the
inclinations of the poles have the same ratio with places at equal distances from the poles of the
earth everywhere which is possible only if the shape is spherical. Remaining reasons are more or
less similar. Hence there is no need to consider them separately. In the third section also the
shape of the earth comes up which includes Aristotle’s version. Fourth section deals with the
motion of the celestial bodies. An important aspect of this section is that in this section
Copernicus maintains that the motion of celestial bodies is regular. Therefore for the first time in
the history of astronomy the supposed irregular motion of the planets is discarded. In the fifth
section there is the first reference to the ancient Greek doctrine which regarded the earth as a
planet a doctrine which never gained social acceptability. Copernicus draws support from
Philolaus who is supposed to have said that it is a planet because it also moves in a circle and
wanders in some other movements. Obviously, the latter applies to its revolution around the sun.
The sixth section describes the immensity of the magnitude of the heavens in comparison with
the earth. This is a problematic section because Copernicus is not very clear of what he says. Nor

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is the reason for regarding the heavens as immensely great convincing. The reason which he has
advanced is that the boundary circles can bisect the whole celestial sphere if and only if the
magnitude of the earth in comparison with the heavens and its distance from the centre are
negligible. He does not elaborate further. Instead, he passes on to second evidence where he
assumes the horizon as a circle and later he regards the same circle as ecliptic. He does not
explicitly say so. It is very clear because in his analysis given straight line is the diameter of both
the horizon and the ecliptic. But the problem is compounded when he says ‘in this way the
horizon always bisects the circle’. The same circle cannot bisect itself. Surely, it cannot be a
sound reason at all. Seventh and eighth sections respectively deal with the reasons for ancient
philosophers’ belief in static earth and the inadequacy of those reasons. The most important
reason is that immobility was regarded as divine which means that they ought to have regarded
the earth itself as divine. But the real situation is different. Secondly, Copernicus maintains that it
is absurd to ascribe movement to the container rather than to the content. It is absurd because it is
counter-intuitive. There is no need to consider remaining sections which deal with the order of
the orbits of planets and three types of movement of the earth which have already been
mentioned. This is the essence of Book One written for men ignorant of mathematics. Perhaps
the attempt misfired. The hazy nature of some sections, particularly, the sixth section might have
earned him wrath of his detractors.

1.5 SOLUTION TO RETROGRADE MOTION

Copernicus claims, rather, rightly that the replacement of earth by the sun satisfactorily explains
the problem of retrograde motion. This is an instance of trial-and-error method, may be the first
instance recorded in the history of science. It is useful to begin with an example to know how he
solved the problem. Assume that two trains A and B are stationary waiting to move in opposite
directions. Also, assume that one of them moves. Then there are two possibilities.
a) A moves but we think that B is moving.
b) B moves but we think that A is moving.
In both the cases visual impressions are same. The difference consists only in our thought.
Therefore no matter what we think, the visual impressions do not change. Let A stand for the
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earth and B stand for the sun. The rest can be explained easily. Philosophers in antiquity believed
(b) whereas Copernicus believed (a). Both of them recorded same observational data. Shift from
(a) to (b) does not make any change in observational data. Therefore Copernicus thought that this
shift must be permissible provided other factors are favourable. Suppose that the earth does not
have one motion, but more than one. The earth has three types of motion and all three are
simultaneous. The result is that the inhabitants of the earth experience complex visual
impressions. The supposed irregular motion of the planets is one such impression. Splitting these
impressions, Copernicus said that the apparent diurnal motion of the sun and other planets is
caused by the rotation of the earth on its own axis. On the other hand, its revolution along with
other planets around the sun explained the retrograde motion of the planets. The principle is very
simple; smaller the distance between the sun and the planet, smaller the orbit and greater the
distance, bigger the orbit. Further, as the distance increases the orbital velocity decreases.
Consequently, the positions of the planets vary from time to time. Obviously, when two trains
move in the same direction with different velocities the slower train appears to recede. In reality
it is not so. The same explanation holds good for planetary motion. This explanation reveals an
important element. Copernicus did not require any new set of observations. He achieved his aim
with the same set of data. Revolution consists in demonstrating that the retrograde motion is only
apparent which can be resolved by making the earth mobile. It is not just change in the centre
which matters.

Check Your Progress I

Note: a) Use the space provided for your answer

b) Check your answers with those provided at the end of the unit

1. Name the Neo-Platonists who influenced Copernicus.

……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
……………………………………………………………………………………………
…………………………………………………………………

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1.6 TELEOLOGY VS. MECHANICAL PHILOSOPHY

The foregoing discussion brings to the fore the fundamental change in the outlook of
Renaissance European, a change which Ptolemy also could not bring about for obvious reasons.
The earth-centric theory which ruled the roost from antiquity down to the end of the Middle ages
brought with it Man-centric view, a natural corollary of the earth- centric view. Teleological
explanation is the direct outcome of Man-centric view. The very idea of creation is at the root of
teleology. Again, teleological approach is evident in the development of astrology as a sort of
byproduct of astronomy. The dislocation of the earth’s position demolished all such
nonastronomical developments at one stroke. If there is no purpose anywhere in the universe,
then the universe is governed by rigid laws, which in no way reflects the needs of man. And
these laws alone are capable of satisfactorily explaining what happens and what does not happen
in the universe. Initially, we can understand mechanical philosophy of the universe as the kind of
explanation which makes the universe Man-independent. It is true that Copernicus does not
mention this dichotomy in the De Revolutionibus. Perhaps it did not occur at all to him that his
doctrine had the potential to generate this controversy. As Kuhn has remarked it provoked
revolutionary ideas in other works though the work as such did not contain those ideas on a large
scale. It laid the foundation for radical change in our world-view. This is the greatest
contribution of Copernicus to the growth of knowledge.

1.7 COPERNICAN SYSTEM VS. NON-COPERNICAN SYSTEMS

Before distinguishing himself from Pythagoreans, Copernicus asserted that only knowledge of
mathematics convinces that his system ‘excellently…. agrees with observations’. His argument
makes it very clear that astronomy is no longer speculative but essentially mathematical. Against
this background, the position of Copernicus must be evaluated. Disregarding what Copernicus
said on behalf of his doctrine one has to admit that conceptually moving earth is not a novel
concept. Therefore what constitutes Copernican Revolution lies elsewhere. Possibly, Copernicus
was aware of its relevance. This awareness may explain why, with the exclusion of the

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introductory part, the De Revolutionibus is entirely mathematical. This change in the structure
points to the need to attempt at proof and also evolve an acceptable method of proof. Therefore it
is not mere shift in the position of the earth but it is the structural change in the nature of
astronomy caused by the application of mathematics and more importantly change in the attitude
with which Copernicus approached astronomy that constitutes Revolution. The latter is more
significant because Ptolemy also is a mathematician but his approach was not found to be
satisfactory by Copernicus because he believed that Ptolemy’s analysis did not bring about the
required structural change in astronomy. A satisfactory explanation of planetary motion alone
could effect the required change which Ptolemy’s mathematics could not achieve.

We shall return to the question raised earlier. Certain historical perspective is required to tackle
this question. Andreas Osiander, a theologian, added a note in the form of introduction to the De
Revolutionibus. If Kuhn is to be believed, Osiander did it without the permission from
Copernicus and Copernicus could not reply because he did not survive for long after the
publication of the De Revolutionibus. In this preface Osiander raised certain key philosophical
issues. He says that ‘ it is the job of the astronomer to use painstaking and skilled observation in
gathering together the history of the celestial movements and then…….construct whatever
causes or hypotheses he pleases such that……those same movements can be calculated from the
principles of geometry for the past and for the future also’. Osiander is perhaps the first person to
suggest the application of inductive method to astronomy. He agrees that these hypotheses need
not necessarily be true. Nor can they be probably true. It is sufficient if any hypothesis provides a
calculus which fits into these observations. Further, he observes that a particular astronomical
phenomenon may generate two or more than two hypotheses. Considering the context in which
Osiander made these observations it becomes clear that his observations implied that plurality of
hypotheses characterize scientific approach. Is there any criterion to choose any one hypothesis
from among several hypotheses? Osiander suggested that the scientist prefers the one which is
easiest to grasp. This is what is understood today as simplicity. A simple theory is preferable to a
complex theory. If Copernican theory is simpler than Ptolemaic theory, then the former is
preferable to the latter. This view, which found support in some contemporary philosophers of
science, took its birth in Osiander’s suggestion. At this stage it is necessary to make distinction
between psychological simplicity and logical simplicity a distinction to which Norwood Hanson

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has referred in The Encyclopedia of Philosophy (Vol. I). On most of the occasions, what is
logically simple is not psychologically simple and vice versa. Copernican simplicity is an
example for logical simplicity while Aristotelian or Ptolemaic simplicity is an example for
psychological simplicity. Against this background, we should try to understand the aim of
Copernicus. He argued in his preface addressed to the Pope Paul III that ‘mathematicians are so
unsure of the movements of the Sun and Moon that they cannot even explain or observe the
constant length of the seasonal year’. Who are these mathematicians? Copernicus does not name
any mathematician or philosopher in his preface. But the references, which he has made, make
the point clear. Reference to concentric circles indicts Aristotle; reference to eccentrics and
epicycles refutes Ptolemy and reference to equant others. Therefore when he accused
mathematicians in general and Ptolemy in particular of creating a monster, Copernicus, in reality,
referred to these constructions only. Why was Copernicus critical of Ptolemaic astronomy?
Copernicus was not against epicycle as such, but he was against the number of epicycles which
made his system a complex system. In this context, it is interesting to consider Hanson’s
reference to numerical aspect. He begins with comparing 83 epicycles accepted by Ptolemaic
system with 17 epicycles required by Copernican system. According to him this difference is
misleading because Ptolemaic system did not consider, according to him, planets collectively.
On the contrary, the system considers all planets individually. According to his own admission,
if any one planet is considered singly no more than 4 or 5 epicycles are sufficient. The sun,
though a planet, is static. Therefore there is no epicycle associated with the sun. After its
exclusion we are still left with seven planets. If Hanson’s argument is accepted, then we still
require 35 epicycles, more than twice the number of epicycles admissible in Copernican system.
Even then the latter is simpler than Ptolemaic system. Hanson concludes that ‘the number of
epicycles in any calculation would tend to be, not less, than that required in a corresponding
Ptolemaic problem. This simple calculation proves that Hanson’s argument breaks down on this
count. However, he admits that Copernican system is simpler as a ‘system’ because it requires
only those concepts which are deductively interlocked. When the concepts are interlocked in the
manner mentioned above, the number becomes irrelevant. However, what is really significant is
that it did not occur to him that the concept of epicycle itself was a flawed one and that planetary
motion was, really explicable without making use of this concept. Next he attributes their failure
to the admissions of principles ‘something foreign and wholly irrelevant’- principles entirely

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different from original hypotheses. Philosophers of science today call such additions ad hoc
hypotheses. Copernicus did not use precisely this word, but he, evidently, meant it. Copernicus
was convinced that his explanation succeeds where his predecessors’ explanation fails. On this
ground, Copernicus argues that his theory is preferable to the previously held theory or theories.
This approach to the status of theories is known as instrumentalism.

These are the grounds on which Copernicus rejected a good part of Aristotelian and Ptolemaic
theories. This rejection paves the way for the resurrection of Heraclides and others. And
precisely for this reason Kuhn believes that the revolutionary element in Copernican system
consists not in overthrowing philosophical and speculative elements but in applying a new
system of mathematical principles which, ultimately resulted in startling revelations for which
Copernican system is known for.

Check Your Progress II

Note: a) Use the space provided for your answer

b) Check your answers with those provided at the end of the unit

1. How did Copernicus solve the problem of planetary motion?

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1.8 LET US SUM UP

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Copernicus is the first astronomer of reputation after Ptolemy. He is more Aristotelian than many
medieval philosophers. Copernicus was dissatisfied with Ptolemy and his followers. He wanted
to rewrite astronomy. His main work De Revolutionibus regards the sun and moon as planets
only. He did not give up the idea of epicycle. He only reduced the number because he thought
that Ptolemy accepted too many epicycles. In this book he advocates to switch from the earth-
centric theory to the sun-centric theory. Copernicus is credited with abandoning teleology in the
explanation of the universe.

1.9 KEY WORDS

Velocity: Velocity and speed are not identical though both of them describe the quantity of
displacement. The difference between the two is that when the direction changes velocity
changes but not speed. Secondly velocity is always calculated per second only.

1.10 FURTHER READINGS AND REFERENCES

Cohen, I.B. The Birth of a New Physics. London: Penguin Books, 1992
Dolling, Lisa M., Gianelli, Arthur F. and Statile, Glenn N., eds. The Tests of Time. New Jersey:
Princeton University Press, 2003.
Edwards, Paul, ed. The Encyclopedia of Philosophy. Vol.2, pp. 219- 222. New York: Free Press,
1967.
Kuhn, T. S. The Copernican Revolution. New York: Vintage Books, 1959
Singer, C. A Short History of Scientific Ideas to 1900. London: Oxford University Press, 1959

1.11 ANSWERS TO CHECK YOUR PROGRESS

Check Your Progress I

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1. Philolous, Ecphantus, Marsilio Ficino and Nicholas influenced Copernicus.

Check Your Progress II

1. Copernicus claims that the replacement of earth by the sun satisfactorily explains the
problem of retrograde motion. Copernicus said that the apparent diurnal motion of the sun
and other planets is caused by the rotation of the earth on its own axis. Its revolution along
with other planets around the sun explained the retrograde motion of the planets.

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