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