For most of human history, the Earth was seen as the stationary center of the universe, with the sun, planets, and starry firmament circling around it at various speeds. The ancient Greeks quickly managed to work out that the Earth was spherical but struggled to explain the motions of the heavens.
Eventually Eudoxus, a student of Plato, worked out a mathematical model involving concentric spheres, which the planets rode on around the Earth in perfect circles, with the outermost sphere being the firmament. Aristotle took this model and posited as a physical one, with the spheres being crystalline orbs.
Several centuries later, Ptolemy worked out a fairly rigorous model based on these views that was mostly predictive of astronomical observations. But while the Ptolemaic system worked, it was widely regarded as problematic, positing a lot of ugly conceptions, such as epicycles, to explain what was happening.
Early on, there were people who pointed out that changing some basic assumptions might simplify the model. Aristarchus of Samos came up with a heliocentric model way back in the 3rd century BC, with the sun at the center and everything, including the Earth, revolving around it. But heliocentrism didn’t seem to garner a substantial following among ancient astronomers.
When Copernicus developed his heliocentric model, he was careful to cite Aristarchus and other sources, to make sure that his readers knew there was ancient precedent for the idea, that it wasn’t something completely novel. At his point in history, in the early 16th century, the completely new remained suspect, even after the discovery of the new world.
Copernicus was a mathematician and his model was rigorous, but it wasn’t without its own problems. Copernicus retained the spheres and perfect circles, so he had to add his own kludgy anomalies, although these were seen as less severe than the ones in the Ptolemaic model.
In many ways, Copernicus’ model was no better than the Ptolemaic one at making predictions. His primary justification for it was that the Ptolemaic system was a “monster”, and that his own model was, “more pleasing to the mind.”
Copernicus, fearing the ridicule his theory might provoke, held off publishing it until the end of his life, although he had circulated rough outlines of it earlier. After publication, there wasn’t much ridicule. There actually wasn’t much reaction at all. Astronomers found his mathematics more elegant than Ptolemy’s and were happy to use his system, but most regarded his physics as a convenient fiction.
Throughout the 1500s, it’s estimated that there may only have been about a dozen astronomers in Europe who were convinced Copernicans. Aside from the reasons noted above, heliocentrism was seen as simply absurd, too much of a departure from common sense. And that was aside from the issue that it may have contradicted scripture.
And there remained many unanswered questions. If the Earth moved, why didn’t everyone feel the movement? Why weren’t birds in flight or clouds affected? Ancient philosophy held that the element earth fell toward the center of the universe. But if the Earth wasn’t at the center, then what caused things to fall toward it? And if the Earth changed position throughout the year, why weren’t the stars seen to shift in relative position, to exhibit parallax? Not having detectable parallax would mean they were incomprehensibly far away.
There would be weakening of the Ptolemaic system in the later part of the century. Tycho Brahe, making more rigorous and precise naked eye observations than anyone had ever made before, discovered novas, indicating that the heavens could change, and observed comets that seemed to cross the location of the supposed spheres, implying the spheres didn’t really exist. But these were issues for Copernicus’ models just as much as they were for Ptolemy’s.
Open minded astronomers continued to note Copernicus’ theory as an interesting, if somewhat bizarre speculation, but only a few adopted it. The result is that in 1600, 57 years after its publication, Copernicus’ theory seemed in danger of going down the same path as Aristarchus’ earlier proposition, of being little more than a footnote of history.
Then the telescope was invented in 1608, and Galileo took the design, improved it, used it to look at the heavens in 1609, and published his results. It was only at this point that the different predictions between the models could be tested. Galileo’s observations were much more compatible with Copericanism.
Galileo would eventually get in trouble with the church for his subsequent advocacy of heliocentrism, but as the observations accumulated, the reality became increasingly undeniable. Newton would eventually answer many of the lingering questions caused by the new model. By then, virtually all astronomers were Copernicans.
What interests me about this story is, what could people in the 1500s have done to better assess the Copernican model? In 1543, when it was published, it amounted to an alternate theory that largely made the same observable predictions as the existing one. It didn’t really make fewer assumptions than the Ptolemaic one, so parsimony (Occam’s razor) wasn’t much of a guide.
The main thing it seemed to have going for it was it’s more convenient mathematics. Everyone acknowledged its mathematical elegance early on. And many astronomers seemed willing to use those mathematics, even while not accepting the implied reality. A preface added to Copernicus’ book, although not written by Copernicus himself, even suggested that approach.
It wouldn’t be the last time in science that someone said, “Don’t worry. This is just a mathematical convenience, an accounting gimmick. It’s not like this crazy thing is true.” Max Planck used a similar line when he discovered that quantizing energy made his calculations work, which eventually turned out to be the basis for quantum physics. And I think of Chad Orzel’s recommendation that we not think of Everett’s many worlds as real, just take them as metaphor, an accounting device.
Of course, it’s important to remember the misses as well as the hits. In recent years we’ve had theories with elegant mathematics that eventually didn’t turn out to be reality. The LHC reportedly has eaten a lot of such theories. Although an argument could be made that those theories started much further from empirical motivations than the successful ones above. Admittedly, this is a subjective standard.
All of which is to say, judging the plausibility of rigorous theories is far from simple.
What do you think? Was there some standard early modern astronomers could have used to better judge Copernicus’ theory? Or do we simply have accept that our ability to assess many speculative theories is limited until actual empirical data becomes available?