Two Mythological Arguments
There are two popular, but
totally mythological, arguments given for why Copernicus adopted a sun-centered
cosmology. In the first place, we know
that the procedures for predicting planetary positions given in the Almagest
were not very accurate, and Copernicus himself was aware that in 1503 Saturn
was ahead of the tables by a degree and a half, whereas Mars lagged by two
degrees. Could Copernicus have embarked
on a new cosmology to improve this state of affairs? In fact, Copernicus never mentions that the calculations were
inaccurate, and for good reason.
Because of a close geometric equivalence between the two systems, the
predictions come out essentially the same way, and merely converting to a
heliocentric framework does not by itself buy greater accuracy.
An even more popular wrong
explanation is the notion that during the Middle Ages astronomers added a
increasing number of small circles to the Ptolemaic system in order to make it
more accurate, and by the end of the 15th century, the whole scheme was ready
to collapse under its own weight. This
idea has become so firmly embedded in modern lore that hardly a year passes
without an author in the Astrophysical
Journal or the Physical Review
remarking that “perhaps my theory has too many epicycles in it.” Historically, this is complete nonsense.
So why, then, did Copernicus
come to espouse such a radical cosmology?
What Copernicus saw in his mind’s eye were two beautiful aesthetic results from the heliocentric
layout. Since antiquity the most
puzzling aspect of celestial motions had been the fact that, while planets
normally drift eastward against the background of stars, periodically they come
to a standstill, and then move westward for several weeks. Mars, for example, goes retrograde about
every two years, becoming especially bright, as it is even now in April of
1999. The Copernican system offers a
completely natural explanation. What was
apparent even to Ptolemy, but as a completely unexplained detail, is that the
retrogression always occurs when the planet is opposite the sun in the
sky. For example, nowadays with Mars in
retrograde, it rises at about the time the sun sets, in opposition to the sun. In the Copernican, sun-centered system, it
becomes completely obvious why this is so.
Mars will appear to retrograde when the faster-moving earth is bypassing
it, and this can happen only when the earth is closest to Mars and the sun is
then necessarily opposite to Mars in the sky.
Now when Ptolemy’s circles
for the planets are all scaled and superimposed to create a compact,
sun-centered system, it automatically happens that Mercury, the fastest planet,
comes out with the smallest orbit, closest to the sun, and Saturn, the slowest
planet, falls into place with the largest orbit, with the others falling
rhythmically in between. As Copernicus
explains in the central cosmological chapter in his book, “In no other way do
we find such a wonderful commensurability and sure, harmonious connection
between the movement and size of an orbit.”
It’s too beautiful to be wrong, surely a theory “pleasing to the mind.”
Contributed by: Dr. Owen Gingerich
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