196 Richard J. Hall
KUHN AND THE COPERNICAN REVOLUTION
S. KUHN states that Copernicus' system is neither simpler nor more
accurate (observationally) than the Ptolemaic system (Kuhn, 1962,* p. 169;
1962,* p. 75). If this is so, and if simplicity and observational accuracy are the
only rational grounds for preferring one scientific theory to another, then those
people who preferred Copernicus' system to Ptolemy's must have done so from
non-rational motivations. Kuhn agrees with this and suggests one such possible
motivation in the following passages.
as Copernicus himself recognized, the real appeal of sun-centered astronomy was aesthetic rather than pragmatic. To astronomers the initial choice
between Copernicus' system and Ptolemy's could only be a matter of taste,
and matters of taste are the most difficult of all to define or debate. (Kuhn,
I962,1 p. 171)
There m u s t . . . be a basis, though it need be neither rational nor ultimately
correct, for faith in the particular candidate [theory] chosen. Something
must make at least a few scientists feel that the new proposal is on the right
track, and sometimes it is only personal and inarticulate aesthetic considerations that can do that. Men have been converted by them at times when
most of the articulable technical arguments pointed the other way. When
first introduced, neither Copernicus' astromical theory nor De Broglie's
theory of matter had many other significant grounds of appeal.' (Kuhn,
1962,* p. 157)
I do not wish to deny the efficacy of such arguably non-rational factors as
aesthetic appeal in causing people to prefer the Copernican system. However,
it can be shown that, contrary to what Kuhn says, the Copernican system is
also less arbitrary than the Ptolemaic in several clear-cut ways, so that, other
things being equal, the Copernican system was preferable on undisputedly
rational grounds. (Of course, other things presumably weren't equal since
Ptolemy's system fits in somewhat better with the general Aristotelian framework, for which there was not yet an adequate alternative.) I shall mention three
planetary phenomena for which the explanation on Copernicus' view is clearly
simpler than on Ptolemy's.
1 THE MAXIMUM ELONGATION of Venus and Mercury. The inferior planets,
Mercury and Venus, share an observational characteristic which differentiates
them from the superior planets. It is that they are always seen relatively close
to the sun. Mercury's maximum elongation varies from 180 to 280 and Venus'
is about 470. In the Ptolemaic system with the earth at the centre and the planets
and sun going around it, this can only be accounted for by postulating a certain
relationship between the motions of the inferior planets and that of the sun.
In particular, it is assumed that the centres of the epicycle of Venus and Mercury
are always in line with the sun. In the Ptolemaic system this is simply an arbitrary assumption required by the facts but not explainable by any deeper principles
of the theory. In the Copernican system, that there is a maximum elongation
for Venus and Mercury follows immediately from the fact that Venus and Mercury are inside the Earth, and this ordering of the planets follows in turn from
a deep seated principle common to both Ptolemy and Copernicus, namely the
longer a planet's period of revolution, the greater its distance from the centre.
THOMAS
Kuhn and the Copernican Revolution 197
2 RETROGRADE MOTION and opposition. The superior planets only retrogress
when they are in the opposite part of the heavens from the sun. In the Ptolemaic
system retrograde motion is determined by the planet's motion along the epicycle while position in the heavens is primarily determined by motion along
the deferrent. An arbitrary relationship, therefore, must be postulated between
these motions and also between these motions and that of the sun. In Copernicus' theory a superior planet appears to retrogress when the Earth overtakes
it on the inside. The planet, therefore, must be on the outside, that is, away from
the sun. Thus again a relationship which is arbitrary for Ptolemy—necessitated
by the facts but not a consequence of anything else in the theory—is a natural
consequence of the basic Copernican viewpoint.
3 FREQUENCY OF RETROGRESSIONS. Of the superior planets, Saturn retrogresses
most frequently, Jupiter next, and Mars least frequently. With the inferior
planets, Mercury retrogresses more frequently than Venus. In the Ptolemaic
system the frequency of a planet's retrogression is determined primarily by the
frequency of its revolution around its epicycle. However, that Saturn should
revolve in its epicycle more rapidly than Jupiter or Mars is not implied by any
fundamental principles of the theory; rather it is again an arbitrary fact dictated
by the observations. For Copernicus, retrogression of a superior planet occurs
when the Earth overtakes that planet and retrogression of an inferior planet
occurs when that planet ovetakes the Earth. Since the Earth overtakes Saturn
most frequently, Jupiter next, and Mars least often; and since Mercury overtakes the Earth more frequently than Venus, the facts about frequency of retrogression are immediately accounted for.
Several further instances quite analogous to those above could be pointed
out and indeed were pointed out by Copernicus. But perhaps the three mentioned
will be sufficient to demonstrate my point that many features of the Ptolemaic
system are quite arbitrary from the standpoint of theory. In so far as Copernicus'
system has fewer of these arbitrary features, it seems to me obviously preferable
on rational grounds.
Kuhn is well aware of the planetary phenomena I have mentioned and seems
to recognise that the Copernican system is in some sense superior in the way it
accounts for them. But he suggests that this superiority is not rational. He calls
it aesthetic, a 'personal and inarticulate aesthetic consideration', a 'matter of
taste'; and he seems to infer from this that it wasn't rational (see the second
passage quoted above). But Kuhn makes no attempt to show either that these
considerations are 'merely' aesthetic or that aesthetic considerations are not
rational. In any case, they are obviously neither personal nor inarticulable, and
I see no grounds to dismiss them as non-rational. This example does not,
therefore, as Kuhn supposes, lend rational support to Kuhn's hypothesis of the
non-rationality of scientific change.
RICHARD J. HALL
Michigan State University
East Lansing
REFERENCES
KUHN, T. (1962) ' The Copernican Revolution. New York.
KUHN, T. (1962) * The Structure of Scientific Revolutions. Chicago.