The Theory of Heliocentrism

About 250 to 300 years before the birth of Jesus Christ, a man named Aristarchus looked up at the sky and dared to think differently from anyone else about what he was seeing there. He posited that maybe — just maybe — the Earth rotated around the Sun instead of the other way around. What he proposed sounded as crazy to his contemporaries as the convoluted, counter-intuitive theories of the Flat Earth Society sound to us today. And yet Aristarchus was unlike the Flat Earthers in one hugely important respect: he was right.

Our information about this loneliest of voices in the wilderness is sketchy. Aristarchus was living and studying in Alexandria, Egypt, when he dropped his bombshell. It is reported that he came to that young and exciting city, the emerging new intellectual capital of the Mediterranean world, after first studying in Athens, whose reputation Alexandria was fast superseding. But he was born on the Greek island of Samos, yet another haven for thinkers, being the place where Pythagoras, the man who had proven more or less definitively that the Earth was round, had been born two and a half centuries earlier.

We don’t have the actual treatise that has made Aristarchus retroactively famous. The only complete example of his writing that we do have is a rather less impressive work, in which he attempts to deduce the sizes of the Earth, Sun, and Moon in relation to one another by observing them under both normal and eclipse conditions, then applying trigonometry to the angles and ratios he has collected. He comes close to being correct about the size of the Moon, positing it to be about 35 percent as large as the Earth; the real figure, we know now, is 29 percent. But he is less successful calculating the size of the Sun. He concludes it to be 6.7 times as large as the Earth, rather than the real figure of 109 times.

Still, as wildly inaccurate as this ratio was, it did point to the bare fact that the Sun was much bigger than the Earth, and this may in turn have been the wellspring of Aristarchus’s greatest insight. For ancient thinkers of the Pythagorean school — people like him, in other words — were obsessed with harmony and proportion. A Sun that was many times larger than the Earth, yet nevertheless rotated around it, failed that test.

Aristarchus’s revolutionary theory of heliocentrism — meaning a universe with the Sun at its center, as opposed to a geocentric one that has the Earth in that position — has come down to us only because another, even more famous Alexandrian thinker chose to comment upon it in a treatise of his own. When he wasn’t engaged in practical pursuits like inventing the compound pulley or the screw pump, the mathematician and engineer Archimedes loved to pose bizarre problems for himself, then do his best to work them out using whatever intellectual tools he had to hand. We have a number of the treatises that resulted. One of them deals with the question of how many grains of sand would fit into the universe. Needless to say, this is an impossible question for Archimedes to even begin to answer accurately, given the current state of cosmology. He eventually concludes that the universe contains 10⁶³ grains of sand, a number that makes Aristarchus’s estimate of the size of the Sun seem like the nearest of misses in comparison.

Nevertheless, Archimedes didn’t undertake his thought experiment in vain. Far from it. For it caused him to expand the bounds of the existing Greek number system and invent exponential notation from whole cloth in order to be able to write the enormous numbers that even a hopelessly inaccurate answer like his own must entail; all of this gives it a claim to being among the most important works Archimedes ever produced. In the midst of it, he also wrote in passing that “the ‘universe’ is the name given by most astronomers to the [heavenly] sphere, the center of which is the Earth. This is the common account. But Aristarchus brought out a book consisting of certain hypotheses… His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun in the circumference of a circle, the Sun lying in the middle of the orbit.”

This, then, is our primary evidence that a man named Aristarchus, living and writing in Alexandria at around the same time as the far better documented Archimedes, proposed that the Earth travels around the Sun. But there are other mentions of him and his idea to be found here and there, some of which seem to ominously foreshadow the story that is to come in these pages by hinting at controversy and condemnation from the more conservative elements of society. Writing around 350 to 400 years later, for example, the Roman essayist and biographer Plutarch claimed that some people in Alexandria “thought to lay an action for impiety against Aristarchus.” We do not know whether he actually did come to suffer any consequences for his heterodoxy at the hands of the religious authorities or anyone else.

But, you might be wondering, what did Aristarchus cite by way of proof for his extraordinary claim? Sadly, lacking the full text in which he made his proposal, we just don’t know for sure. In particular, we don’t know how he addressed the fact that was taken as the most incontrovertible of all evidence for a geocentric universe, should any proof of that reality truly be required: the fact that the stars above did not appear to move across the sky over the course of a night, as they ought to do if the Earth was moving in relation to them rather than hanging fixed in space.

Once again, though, Archimedes does give us a strong hint of the tack Aristarchus may have taken to address this problem. He notes in his own treatise on the size of the universe that, the farther away a moving object is from an observer, the slower and less pronounced its movements seem to be to that observer. Imagine, for example, that you’re standing right next to one long straightaway of an oval race track. The cars will seem to be moving at a blistering pace when they pass directly in front of you, yet much slower as you watch them driving down the opposite straightaway, even though they’re really moving at approximately the same speed the whole time. Further, you might have to turn your head from extreme left to extreme right to watch a single car’s progress up your side of the track; the distance covered by cars on the opposite straightaway will appear shorter and require little to no head movement to take in, even though that distance is exactly the same. These visual displacements, which are so ingrained in us that we never even think about them as we go about our daily lives, are known to astronomers and other students of optics as the phenomenon of parallax.

Importantly, parallax is a symmetrical phenomenon. That is to say, it applies from the perspective of both the moving and the stationary object — or, for that matter, if neither object is moving, or both. Indeed, often it can be hard to tell who or what is actually moving and who or what isn’t. (If this was not the case, the whole question of whether the Sun goes around the Earth or vice versa would never have come up, and therefore this book would never have come to exist.)

There will come a point when a faraway object that is enormous enough to be visible at all from such a distance will appear to the naked eye to simply sit still in space, regardless of how fast it and/or we may actually be moving. Archimedes grasped this in his commentary on Aristarchus’s treatise:  it could be that the stars are just so far away that we are unable to see them move, even though we ourselves are moving in relation to them. His speculation has since been proven correct. Modern astronomical technology has been able to confirm for us that the phenomenon of stellar parallax does in fact exist; it’s just that the apparent shifts in the positions of the stars overheard are far, far too small to be observed with the naked eye, thanks to the inconceivably vast gulf of space that lies between us and them.

Did Archimedes borrow this brilliant insight from Aristarchus, or did he come up with it on his own? It is impossible to know as long as we lack the full text of Aristarchus’s original argument.

Be that as it may, we would be living in a markedly different world today if Aristarchus’s bold new theory of heliocentrism had taken hold during his lifetime. But it did not. His idea was apparently regarded by the intellectual elite around him as a curiosity at best, a wild “what-if” to play with, just in the way that Archimedes does in his own fanciful treatise.

It is not immediately clear why this should have been the case. Certainly a Sun-centered cosmos seemed less overtly at odds with the pagan beliefs of the ancient Mediterranean than it later would with some of the core tenets of Christianity. After all, virtually all pagan cultures had a strong tradition of Sun worship, combined in many cases with a less inherently exalted view of the Earth’s place in the universe than the one espoused by the Book of Genesis. Meanwhile Aristarchus’s core idea, if it had been allowed to flower and to be elaborated upon, would have neatly eliminated inconsistencies and complications that began to spring up apace as astronomers sought to square the geocentric model with their empirical observations of the heavens. By the time of Plutarch, these had forced them to adopt a gnarly model of the universe that had the five visible planets in the sky making orbits within orbits within orbits, the precise opposite of the harmonious simplicity that ancient philosophy so fervently desired to find in nature.

Nonetheless, paradigms of thought are hard habits to break, especially in the context of ancient natural philosophy, which was less dedicated to pure empiricism than our modern science. Aristarchus’s great insight remained for centuries no more than an anecdote to be passed around at the banquets where natural philosophers gathered: “Have you heard about the crazy thing this guy came up with?” Of course the Sun rotated around the Earth. The alternative that had never been taken all that seriously anyway passed out of the Western cultural memory altogether after the fall of the Roman Empire.

What follows, then, is the story of how Europe finally came to reckon with the truth glimpsed by Aristarchus and with its implications for humanity’s role in the cosmos, almost two millennia after that man had lived and died. It is the story of a handful of innovative minds scattered around the continent who figured out the truth independently of their ancient forefather, and of how they navigated a culture that was determined to believe otherwise, that was in some cases fully prepared to shoot unwanted messengers like them. It is a story of science, yes, but it is also, and perhaps even more importantly, a story of human beings and the human psyche, of how we began to learn to accept that, on the cosmic scale of things, our little planet just isn’t all that. Like many an adolescent drama, it was a tough thing to be in the middle of, but it might just be that we are better off in the long run for making it through to the other side.

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(A full listing of print and online sources used will follow the final article in this series.)