A paper I wrote for PHI 251 (
Philosophy and Classical Physics) at
Uniky. It is an analysis and comparison,
based on mid-
sixteenth-century knowledge, of
the theories of
Copernicus and
Ptolemy. Note that 1550 predates
Galileo and
Kepler; hence their
observations are
irrelevant
within the scope of this paper.
The year is
1550. Two opposing
models of
heavenly motion are
contending for the
minds of
astronomers and
philosophers.
They are both based on
circular motions of the
planets, and each
seems to provide a close
mathematical approximation to the
observed motions of the
heavens. There the
similarity ends.
One
model, put forth by
Ptolemy fourteen centuries ago, places
the
Earth at the
centre of the
universe, and explains the
motions of the planets in terms of many
superimposed circular
rotations and
revolutions. The other, by a recently deceased
Polish philosopher named
Kopernik (Latinised as
Copernicus),
turns
established scientific and
religious thought on its head
by placing the
Sun, not the Earth, at
the centre of the
universe, and claiming that
the Earth is in constant motion about
the Sun.
The Ptolemaic system is based on millennia of astronomy,
theology, and physics. It has as a direct ancestor the
Aristotelian conception of the universe, as developed by
Aristotle, Eudoxus, and others. Aristotle and his followers
believed that the motions of the heavens about the Earth could be
expressed in terms of the natural motion of the heavens---that
being a perfect circle. However, this view did not completely
correspond to reality. In particular, it did not explain
irregularities---such as retrograde motion, where the planets
for the period of a few nights reverse their normal course; and
changes in apparent speed---observed in the motions of the
planets. Ptolemy adapted Aristotle's system to explain the
actual apparent motions of the planets. He introduced a
complicated system of epicycles, eccentrics, deferents, and
circles-within-circles which gave predictions closely adhering to
the observed data.
The Copernican system, however, eliminates much of Aristotle's
system and Ptolemy's machinery. Some things remain: the planets
and stars move in circular orbits around a common point.
However, Copernicus places this point not at the centre of the
Earth, but rather just outside the sun (the point is not placed
precisely at the centre of the sun in order to account for the
apparent speeding-up and slowing-down of planetary motions). In
addition, the firmament, rather than rotating once a day,
remains fixed. In this model, only gravity and the orbit of the
moon are governed by the Earth. The Earth itself, like the
planets, orbits the sun---in this case, once a year, accounting for
the progression of the seasons and of the fixed stars. In
addition, the Earth rotates on its own axis once a day,
accounting for the daily transition from darkness to light.
Despite their many differences, the Ptolemaic and Copernican systems
do share one thing in common: they are often considered as models
only. That is, many scholars have viewed the two systems from an
instrumentalist standpoint: the `correctness' should be
determined by the accuracy of the model's predictions, rather
than their correspondence to reality. This is in opposition
to the Aristotelian tradition, which is realist---it purports to
explain the actual operation of the universe as it is. In
Ptolemy's case, the instrumentalist view is most likely taken
because the model is so complicated. The system of inscribed
circles is so terribly complicated that one has no real reason,
theological or otherwise, for believing that it corresponds to the
actuality of things.
In Copernicus's case, there are likely other reasons for taking an instrumentalist stance. Copernicus
himself almost certainly believed that his model painted
a true picture of the heavens. However, religious and
scholarly expediency demand that scholars treat his theory as
merely a mathematical model---a claim that the Earth is not at
the centre of the Universe would go against years of scientific
and theological thought. In an effort to save Copernicus's theory
from the wrath of the Church, Andreas Osiander, a Protestant
scholar, inserted an anonymous introduction into Copernicus's
Revolutions. This introduction argues for an
instrumentalist view of astronomy---that this new theory, as
well as the old ones, were simply means for calculating
mathematically the motion of the spheres.
From an instrumentalist perspective, Copernicus's model is
probably a much better one. It is based on a few basic axioms,
outlined in his Commentariolus. It requires only three
simple circular motions: the motion of the planets and the
Earth about the sun, the rotation of the Earth, and the
motion of the moon about the Earth. Ptolemy's system, in
contrast, requires each planet to have a number of circular
motions which, taken together, explain the planet's motion.
Because of its conceptual simplicity, the Copernican model lends
itself better to mathematical calculation and prediction---to
instrumentalists, the only thing that truly matters.
One ought, however, to consider the models as actual, realist,
descriptions of the workings of the Universe. The only real
problem with the Ptolemaic system here is that it involves so many
circles. Thousands of years of experience has not given us
any measurements terribly at odds with Ptolemy's predictions.
Beyond the complexity of the spheres, this theory is in accord
with all common sense and recorded experience. In addition,
most importantly, it is in accords with the Bible and the mind of
the Church and Pope.
The Copernican system, on the other hand, holds up less well.
While it simplifies celestial mechanics, it does not consider the
terrestrial at all. In particular, it does not explain the fact
that we do not feel the Earth's motion. Under this model, we
ought to feel a constant easterly wind as the earth rotates on
its axis. For that matter, the Earth must, given its size, spin
very quickly in order to account for the perceived daily motion
of the firmament. Such a rapid circular motion ought, by all
rights, throw us (and perhaps, given the speed of the rotation, its
own matter) into space like clay off a pottery wheel. Finally,
this model seriously complicates our theories of gravity---why
should objects fall to the centre of the Earth when that
centre is itself moving? Natural motion cannot be so
arbitrary. Copernicus does not even attempt to answer these
objections. Others have offered explanations, but none is very
satisfactory.
We must, in light of these facts, say that the Ptolemaic system is
almost certainly closer to the truth than that of the esteemed
Polish gentleman. We should not, however, claim that the
Ptolemaic system is perfect. Certainly we have not made
precise enough measurements to establish the absolute truth of
any theory. In all reality, the heavens move in ways our
philosophies have not yet divined. That is for future
astronomers to determine. However, we may be certain that, when
the truth is finally discovered (if, indeed, it shall be revealed
to mortals), it will resemble Ptolemy's and Aristotle's systems
rather than Copernicus's.
-- Neil Moore