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By Owen Gingerich
A page from De Revolutionibus. Reprinted from Copernicus: Yesterday and Today
Let me remind you of what Galileo said nearly a century later, when the matter was still far from settled: "I cannot admire enough those who accepted the heliocentric doctrine despite the evidence of their senses." I believe that Copernicus relied on aesthetic principles, "ideas pleasing to the mind," and that such concepts are exceedingly powerful but highly treacherous in physical reasoning. Until technology marches on to provide empirical grounding, the aesthetic ideas must be regarded as dangerously seductive, possibly sheer quicksand for the unwary. I'll describe two aesthetic principles that Copernicus endorsed, and I'll show how our modern evaluation essentially turns upside-down the initial reception of Copernicus' De revolutionibus, his life work that was finally published in the year of his death, 1543.
What Copernicus had to offer were two quite independent aesthetic ideas. One was that celestial motions should be described in terms of uniform circular motions, or combinations thereof. The unending, repeating motion in a circle was compellingly suitable for the heavenly movements, where corruption and decay were never found. There was something almost sacred about this proposal, and it appealed strongly to the sensitivities of the sixteenth century. Unfortunately this beautiful idea was wrong, dead wrong. It was not dumb - it was in fact the most intelligent way to start approximating the motions of the heavens, but in Renaissance celestial mechanics it was destined to be a dead end.
Copernicus' other aesthetic idea is quite independent of the aesthetic requirement of circular and uniform motion. It is the great idea that makes copies of the first edition of De revolutionibus nowadays estimated at auction at over half a million dollars. This was, of course, the heliocentric arrangement of the planets. But to the sixteenth-century mind, this idea was highly suspect. To begin with, it required new physics. Building a new scaffolding to replace the neatly dove-tailed Aristotelian physics would require more than a generation of inspired work. As Tycho Brahe said, "The Copernican doctrine nowhere offends the principles of mathematics" - that is, aesthetic idea number one is just fine - "but it throws the earth, a lazy, sluggish body unfit for motion into action as swift as the aethereal torches."
But it wasn't just new physics that made the new cosmology seem radical and dangerous. Tycho said that Copernicus offended both physics and the Holy Scriptures, always in that order. Biblical passages such as Psalm 103, "The Lord God laid the foundation of the earth, that it not be moved forever," seemed to call for a firmly fixed earth. Copernicus' heliocentric vision was seen as a challenge to the traditional sacred geography, and hence generated the pervasive unease touching even those who would never worry about mere physics. Because today Copernicus' heliocentrism, his second aesthetic idea, endures, while the first - "celestial motion is uniform and circular or composed of uniform and circular parts" - has faded away into obscurity, it is easy to overlook the appeal of uniform circular motion in the 16th century.
Aesthetic ideas can be seductively wrong, and in the absence of empirical support it is perhaps best to take a wait-and-see attitude. That's the course the overwhelming majority of 16th-century astronomers adopted. What is unusual about the Copernican revolution is that it took so very long. This leaves the writers of modern secondary sources very uneasy. What was the matter with those people? Were they dumb or something? Or where they just blinded by superstition or religious orthodoxy?
What was lacking was observational evidence to confirm or refute these ideas. Toward the end of the 16th century the idea of an empirical test of the heliocentric idea gradually occurred to a few leading astronomers, including Tycho. He attempted to distinguish between the Ptolemaic and Copernican systems by determining the distance to Mars and he expended a major observational effort on it. He even built a new subterranean observatory to get better stability, and he redesigned the instruments originally built for the windy balconies of his Uraniborg castle to provide greater rigidity and accuracy. Yet in the end he fails to mention his Mars campaign, something that caused his biographers to long overlook this centrally motivating research.
Why did Tycho give this major effort the silent treatment? Because, unknown to him, the solar system was 20 times larger than he or anyone else imagined, and his carefully organized research agenda was doomed to failure. Had he been successful, his new technology would have provided the empirical evidence for Copernican astronomy almost three decades earlier than actually happened, and Tycho's reputation as an observer/cosmologer would shine brilliantly in the astronomical firmament. Yet from the ashes of his failed campaign there arose, like a phoenix, the evidence that Copernicus' aesthetic principle number one had to be abandoned. The magnificently precise observations of Mars were the grist for Kepler's mill, who showed that an ellipse worked better and more simply than the circles and epicyclets of Copernicus. Furthermore, it offered the prospect of serious new physics, which to Kepler made all the difference. And that physics was a heliocentric physics.
But meanwhile, the acceptance of Copernicus' second aesthetic principle, the heliocentric doctrine, was greatly hastened by an unexpected discovery, one that was critically dependent on a fresh advance of technology. In Galileo's hands, what had been a novel toy was converted into a scientific instrument. When he used the new telescope to examine Venus, he found that the planet exhibited the entire set of phases shown by the moon, guaranteeing that Venus orbited the sun, contrary to the Ptolemaic arrangement. This evidence, in the rhetorical setting of Galileo's Dialogo, essentially turned the tide in the favor of the Copernican heliocentric arrangement.
Why had it taken so long? There were comparatively few astronomers in those days, and the pace of invention was not as swift as it is now. Nevertheless, in early modern science we can see in slow motion what can happen in a decade or less today. But it distorts the story to demand that Copernicus's contemporaries should have been able to choose and endorse the great aesthetic idea that we know is right only by 20-20 hindsight. Instead, we should give some sympathy to those who withheld judgement until the evidence was in hand.
Owen Gingerich is a senior astronomer at the Smithsonian Astrophysical Observatory and Professor of Astronomy and the History of Science at Harvard University. This article is based on a talk given at the April meeting of the APS in Long Beach, California.
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