Section II, Chapter One: Copernicus, Heliocentrism, and New Modes of Criticism
Copernicus improved upon the prevailing theory even though data could not yet adjudicate between the two. He realized that (in)consistency with data was but one of many criticisms that one may apply.
Note—this is a draft of Chapter Four of a book I’m writing. The book is going to cover humanity’s deepest ideas from philosophy, physics, epistemology, and economics. Each chapter is meant to be short and digestible. Most of the chapters will explain just one or two ideas. The first few chapters, though, set the stage with some history, too.
‘Chapter []’ indicates a future chapter that I’ve not yet written.
Section II: Classical Mechanics
Section II, Chapter One: Copernicus, Heliocentrism, and New Modes of Criticism
For fourteen centuries, Ptolemy’s (100 C.E. - 170 C.E.) geocentric model of the solar system dominated the Western mind. As we’ve seen, the Ancient Greeks had been observing, calculating, predicting, and explaining the stars above since the Presocratics took to the stage in the 6th century B.C.
Aristotle himself had constructed a model of the universe in which the Earth was surrounded by concentric spheres, each of which revolved around our planet and carried otherwise unmoving astronomical objects along for the ride. In his model, planets and stars alike were affixed to their respective spheres. The changes we observed in the night sky from night to night were due solely to the motion of these great spheres.
The model was effective for many purposes. For instance, the fact that stars shifted their position more slowly than planets did from night to night was explained by the fact that stars were affixed to a larger sphere further from the earth than the planets were.
The model’s geometrical beauty appeased many a Greek philosopher (see Chapter 3), and its conformity to our intuitions—clearly, after all, it was the starry objects that were moving across the sky, and surely we’d feel the Earth’s movement if it were doing the same–left little room for criticism.
But pre-Ptolemaic models weren’t perfect. Ptolemy aimed to resolve their failure to take planets’ retrograde motion into account—while most celestial objects traveled across the sky in one direction over the course of a year, planets seemed to occasionally slow their roll across the heavens, stop, and reverse course. The elegant, perfectly circular motion of the nigh-divine spheres had been, to use Thomas Henry Huxley’s words, slain by an ugly fact.
Ptolemy conjectured that the apparent retrograde motion of some planets was an illusion caused by the fact that they simultaneously orbited in circular motion (a so-called ‘epicycle’) while revolving around the Earth in a yet larger path. He retained the concept of spheres, but now each celestial object was affixed to its own sphere. In this way, the idiosyncratic motion of any skybound object could be explained—add however many epicycles you need to match the data.
And historically, that is precisely what astronomers did. As they continued to accumulate astronomical data across the centuries, they’d find more gaps between observation and the predictions of the Ptolemaic model. Although Ptolemy sought to improve upon Aristotle’s construct, it was Aristotle’s idea that theory must be tempered by data that ultimately did Ptolemy in, albeit indirectly. Astronomers were not so quick to throw Ptolemy out-–they invoked ever more nests of epicycles within epicycles in order to explain their growing set of observations.
Copernicus (1473 C.E. - 1543 C.E.) was dissatisfied by the Ptolemaic model, not because of any particular clash with data, but because of its sheer inelegance and ad hoc fudges. A devout Christian, he was confident that God had created a beautiful, comprehensible universe, one that could surely be understood through compact explanations. As did many scientists of the late Middle Ages and early Enlightenment era, he regularly blended scientific, philosophical, and theological ideas: “The Universe has been wrought for us by a supremely good and orderly Creator,” he once said. “To know the mighty works of God, to comprehend His wisdom and majesty and power: to appreciate…the wonderful workings of His laws, surely all this must be a pleasing and acceptable mode of worship to the Most High…”
Nor did Copernicus make many novel observations, if any, before positing his heliocentric model of the universe. That his theory was aesthetically preferable to Ptolemy’s was enough for him: As Leonard Mlodniow says in The Upright Thinkers, Copernicus thought “it easier to believe this than to confuse the issue by assuming a vast number of Spheres, which those who keep Earth at the center must do.”
During his lifetime, there was no data nor proposed crucial experiment (see Chapter []) that could distinguish between Copernicus’ worldview and that of Ptolemy. As I’d mentioned earlier, Ptolemy’s calculations worked for most purposes. But, perhaps because his pursuit of scientific knowledge amounted to an effort to know God, Copernicus was dissatisfied with purely operational schemes—he wanted to know what the world was really like.
Granted, calculations proved simpler in Copernicus’ model, but he was well aware that he was swimming upstream against the tides of historical momentum, common sense, and a lack of surefire evidence in favor of his model against Ptolemy’s. It is unclear whether he was so discreet about sharing his ideas for fear of charges of heresy from the Church, or if he simply worried about public ridicule over offering such a ‘nonsensical’ hypothesis. Either way, over thirty years separated the scientist’s first jottings about the theory and the publication of his book on the matter, On the Revolutions of the Heavenly Spheres.
When his book came out in 1543, neither the Church nor the broader public came for Copernicus’ head—people didn’t pay it much attention at all. It would take the efforts of yet another scientific revolutionary several decades later to promulgate and improve upon the heliocentric model—and, ironically, suffer the fate that Copernicus may have tried to avoid.
In offering an alternative to Ptolemy’s model, Copernicus took many crucial steps towards the culmination of the Scientific Revolution in Isaac Newton’s Principia Mathematica (see Chapter []) and towards the widespread adoption of a scientific worldview more generally. Firstly, and most obviously, Copernicus offered a model of the universe that contained fewer errors than the prevailing framework of Ptolemy. But Copernicus’ own picture was corrected only a few decades later by Kepler and others—for example, Copernicus mistakenly thought the movement of the planets around the sun was perfectly circular. But while not all of the details of his model survived very long, Copernicus provided several epistemological tools that his successors took full advantage of.
Copernicus rejected that the world must conform to our intuitions and denied that Aristotle’s word was the final authority. As we’ll see in Chapter Five, Galileo made use of both insights to carry Copernicus’ torch.
And finally, Copernicus improved upon the prevailing theory even though data could not yet adjudicate between the two. He realized that (in)consistency with data was but one of many criticisms that one may apply to a theory. In his particular case, he recognized that a good theory should not have ad hoc fudges—instead, it should be compact, elegant, and non-arbitrary (see Chapter []). Copernicus took a first step towards this new mode of criticism that has served scientists ever since—far longer than Copernicus’ model of the universe had.
Thanks to Moritz Wallawitsch and Dennis Hackethal for early feedback.