If we contrast ancient with modern scientific theories we find
that the chief distinguishing characteristic of the former is that
they employ principles drawn from other branches of knowledge or
speculation. It would be, perhaps, rash to say that modern science,
in all its branches, is yet completely autonomous; sometimes, for
instance, it seems to make assumptions which are the result of an
uncritical philosophy, but even the grossest of these examples,
compared with many celebrated early scientific theories, shows how
great is the purification that has been effected. The chief error of
the old speculators consisted in imagining that the world is a more
obvious unity than we have now any reason to suppose. Hence they were
always willing to argue by “analogy,” comparing terms between which
we cannot now find the slightest resemblance. The method was not only
illegitimate, but sometimes led to quite unnecessary complexities of
explanation. The Ptolemaic system of astronomy, for instance, conceived
as the theory that the heavenly bodies revolve round the earth, was
a perfectly reasonable and satisfactory theory. It was capable of
explaining all the observed planetary motions, except a few minute
irregularities requiring precise measurements for their detection. Its
proper development required, of course, complete docility in face of
the facts. But in its actual development it was forced to accommodate
itself to quite other considerations. It had to take into account the
venerable principle that, the celestial bodies being obviously sublime,
incorrupt and perfect, their orbits must be perfect and described with
uniform velocities. The only possible perfect orbit was as obviously
a circle. Hence the Ptolemaic theory was loaded with the task of
explaining the observed heavenly motions on two grounds: first, that
the earth was stationary and at the centre of the system, and second,
that the planetary orbits were circular and described with unvarying
velocities. Alternative hypotheses were not only stupid but impious.
The task thus set to the early astronomers was one of considerable
difficulty.
The observed path of a planet, say Mars, or Jupiter, or Saturn, is
by no means simple. If its motion amongst the stars be watched from
night to night it is seen to be moving sometimes from east to west
and sometimes from west to east. Further, in changing its direction
of motion it does not retrace its path amongst the stars. Its actual
observed path exhibits irregular loops, and, more rarely, a twisted
line. It was at once obvious that a circular orbit, traversed with
uniform velocity, would not suffice to explain these appearances.
Nevertheless, the principle must be preserved. The astronomers
overcame this difficulty by a device that strikes one as being almost
disingenuous. They imagined a small circle whose centre traversed the
circumference of the big circle with a constant velocity and round
whose own circumference the planet moved with a constant velocity. By
assigning suitable velocities to these two motions the crude features
of the planet’s actual observed motion could be represented--it would
sometimes be retrograde and sometimes direct. This is ingenious, but it
is questionable whether it preserves the principle. The planet’s motion
is obtained by circular motions, it is true, but it is not itself a
circular motion with reference to the earth as centre. The astronomers
have entered on a slippery path. We view them with the same suspicion
with which we watch a Broad Churchman expounding the Thirty-Nine
Articles. But they had to go further. The theoretical and the observed
motions did not fit well enough. On the little circle it was necessary
to imagine a still smaller circle, and to place the planet on its
circumference. After all, this interpretation of “circular motion” once
admitted, there was no reason why it should not be followed up. But
progress in this direction soon came to a halt. It became evident that
this method would not, by itself, reconcile observation and theory.
The principle had to be strained again, and this time in an almost
indefensible manner. It was declared that the big circle was eccentric
with respect to the earth and that the little circles were eccentric
with respect to their supposed former centres. This assertion must have
been a great strain on the faith of the orthodox believer. He may well
have wondered whether, by this time, the pure doctrine of his fathers
had not been subtly undermined. Circular motion was still preserved, in
a way, it is true, but with so many circles, and their centres all over
the place--this must have appeared something very different from what
he supposed the principle to mean.
The same difficulty was felt by simple minds in modern times, when the
correct explanations of statements in Genesis were worked out by the
theologians. And just as the simple story of the Creation in Genesis
became transformed into an extremely obscure and ambiguous anticipation
of the discoveries of Geology, so the interpretation of circular motion
advanced from complexity to complexity. Immutable principles must
exist, of course--it is part of the glory of man that he should have
been able to discover so many of them--but they sometimes seem more
trouble than they are worth. The old astronomers found that yet again a
more liberal interpretation must be given to the principle of circular
motion. This time it was found that the circles do not all lie in one
plane. Each circle has its own plane, which may be inclined at any
angle to the others. By this time the theorists, whom we might call the
“commentators,” had forged a very powerful method. Circles could be
multiplied; their centres could be placed anywhere; their planes could
be inclined at any angle. The rich content of the principle of circular
motion was now fully revealed. With all these variables to play with a
very close correspondence between theory and observation was effected.
The rise of the “higher criticism” of this system leads to the history
of modern astronomy. It is to be noted, however, that the first higher
critic, like the first higher critics in other departments, was not
wholly emancipated from his early teaching. Copernicus effected the
immense revolution of placing the sun in the centre of the system, but
he did not abandon circular motion. So he had to retain parts of the
epicyclic apparatus. The revolution was first completely effected by
Kepler, but even he conducted his early researches as a semi-believer,
a kind of very Broad Churchman. He made nineteen successive attempts
to explain the motions of Mars by the arrangements of eccentric
and epicyclic motions, and only then did he frankly throw the great
principle of circular motion overboard, and state that the actual paths
of the planets were ellipses. And so, in a few years, a great immutable
principle, a whole system of beliefs, the industry and thought of
generations went for nothing, and now exist merely as an occasional
cold reference in a treatise on Astronomy to the Ptolemaic system as a
“monument of misplaced ingenuity.”