This was my entry in
this year's ACX book review contest. Since it's not one of the finalists, I'll repost it here. Apologies for the uncharacteristic use of fancy Unicode characters -- I originally wrote this in a Google Doc as per the contest rules and I don't care to re-edit it. :P (Also, yeah, the "book review" format is why there's the "I recommend this book" section at the end as I try to wrap up... not something I'd normally include!)
The Globe: How the Earth Became Round, by James Hannam
We all know that the world is round; people have known this for thousands of years. Why, even the ancient Greeks knew the world was round; Eratosthenes famously computed its circumference. But – naïvely – it looks flat. Presumably, people didn’t always know the world was round, and at some point they figured it out. Who figured it out? How did the idea spread? Who knew, when? “The Globe: How the Earth Became Round” gives us an answer to this question, and it’s a disorienting one.
The Greeks didn’t always know the world was round; they had to figure it out. But they did figure it out. So, given how important astronomy was to so many ancient civilizations, you might reasonably guess that other advanced civilizations of the time – the Persians, the Indians – figured it out too.
James Hannam tells us that no, they didn’t. Sure, they found out – but they all learned it, directly or indirectly, from the Greeks. According to Hannam, the idea that the world is round was discovered precisely
once. (OK, maybe not by one
person, but only by one civilization.)
In the case of, say, Persia, this perhaps isn’t too surprising; they had a lot of contact with Greece, so it makes sense that they’d learn it by transmission before they ever solved the problem themselves. But what about China? They had plenty of time to figure it out for themselves before Greek learning on the matter was brought to them. Well, two millennia later, when the Europeans showed up to bring the news… even their best astronomers still had no idea.
The world is round?
Now, there’s three things we might mean when we say “the world is round” – the Earth is circular; the Earth is spherical; or the universe is spherical. (Or the universe is circular, but that doesn’t seem to have been ever much believed; or, I suppose, that the universe is a 3-sphere, but that way of thinking wasn’t available to the ancients. These days we’d say that the universe is overall flat!)
Obviously, it’s the middle one of these that we care about here, because it’s the one that’s true, and the one that took work to determine. The idea that the Earth is circular was commonplace among cultures that believed the Earth to be flat – and so was the idea that the universe is spherical. Sorting all this out in the sources can be a bit tricky – these can all be described as “the world is round”, and not only in English, so statements are frequently ambiguous!
But it is possible to sort out a history with enough reading, and this book attempts to present that history; a history of who thought it was round when, and of many of the ways in which people thought it was flat.
The ancient Greeks were the ones who first figured out the globe. There’s not really one discoverer among the Greeks, but Hannam gives the credit to Aristotle for properly putting the pieces together; saying not only that the Earth is round but also how we can know this, and, importantly, stating explicitly the idea that on a round Earth, “down” is not a fixed direction in space, but rather towards the Earth’s center, thereby explaining why the oceans don’t drain off and such (a point that would confuse flat-earthers throughout the ages).
From Greece the idea of the Globe spread to Rome, to India, to Persia; to the Christians, the Muslims, the Jews; to Europe; and eventually to China. The book has chapters (sometimes more than one) on all of these, as well as some others I left out.
The book talks about opposition to the idea of the globe as well as support for it. Like, prior to the ninth century, Jews were largely skeptical of the idea of a round Earth because of the idea’s Greek origin. And yes, the book does discuss modern flat-earthism at the end.
But if you want a full rundown, go read the book. I’m not here to recap how the idea from ancient Greece outward; I’m here to discuss what in that history is particularly surprising, and the implications.
The Chinese case gets worse
So – until the Jesuits started showing up in the 1500s, hoping to trade superior knowledge of astronomy for permission to preach freely, Chinese astronomers were not aware that the earth was round.
Now, I say “Chinese astronomers”, but I should be clearer about what I mean by that. Because back in the late 13th century, when Khublai Khan conquered China, he set up a second astronomy bureau in his capital, one that practiced Islamic astronomy, as needed for computing the Islamic calendar. The Muslims, of course, had long known that the Earth is round; they’d learned it from the Greeks, and also from the Indians who’d learned it from the Greeks.
Which means, yes, that there were people in China who knew that the Earth was round. But the Chinese astronomers and the Islamic astronomers – both set up in the Chinese capital – apparently didn’t talk to each other, because 300 years later the Chinese astronomers still held to a flat Earth!
An unsettling counterfactual
All this raises the question: what if the ancient Greeks
hadn’t figured it out?
It’s easy to say that if they hadn’t learned it from the Greeks first, India and Persia would have figured it out themselves. Or the Muslims, later. They all had perfectly capable astronomers, after all. But… so did China. And they didn’t.
What about Europe? In the very worst case, Europe would have to have figured it out by the time of Kepler at the latest, surely? I mean, one might say that the idea of a round Earth is a prerequisite to the ideas of Copernicus, Brahe, and Kepler – and I imagine not having it would have slowed things down some. But given that the scientific community of Europe (and not just a few isolated figures) were discovering heliocentrism and related ideas at this time, this to me suggests that there was a general capability there, that certainly they would have figured out the globe as part of the astronomical revolution regardless.
But whether they actually would have is impossible to know. And of course – the astronomical revolution is still a pretty late time to figure all this out! That’s almost as late as the idea spreading to China. Would they have figured it out earlier? One would think so, but again, the example of China suggests perhaps not.
Perhaps Europeans would have figured it out from all the long-distance sailing they were doing? That’s before the Copernican revolution, but only by about a century or so. (And even if sailors had known, how long would the idea have taken to spread to astronomers?) But other people, including the Chinese, were already doing long-distance sailing by then, including the use of stellar navigation, and Zheng He’s voyages recorded plenty of evidence for the globe, and yet China didn’t figure it out; was European long-distance sailing different enough that it would have mattered?
I’ve gotten pretty used to the idea of multiple discovery being the norm, of ideas coming up when their time is ripe. New things are figured out because the groundwork has been laid, the measuring instruments have been improved, new evidence has been coming in, and so forth. The astronomical revolution in Europe is a fine example – invent the telescope and the rest seems inevitable.
The idea I’ve most often seen as an example of the opposite is general relativity; people often say that if it hadn't been for Einstein, it wouldn’t have been figured out for another decade or longer. (Which is kind of odd, actually, given that Einstein wasn't the only one at the time developing a geometric theory of gravity. I would have thought that the idea of a geometrical theory of gravity, rather than the specific field equation, would have been the hard part, but I suppose not?)
I'm used to considering the idea of the globe as so basic that of course it must have been multiply discovered; after all, the Greeks knew it! And indeed, within Greece – among that particular milieu – it does seem to have been something of a gradual (if not multiple) discovery. Certainly it’s nowhere near as non-obvious as General Relativity! But perhaps we should regard it to be pretty high on the scale of non-obviousness, perhaps approaching that of heliocentrism? That doesn’t sound right to me. And yet, the lack of an Aristotle delayed the Chinese discovery of the globe not by a mere decade, but rather two millenia. What was so special about ancient Greece, that the time was ripe there but not elsewhere?
More ancient Greek speculation
It really does seem that ancient Greece had more speculation than other places about non-flat shapes for the Earth. Everywhere had cosmological speculation, but in most places all of this elaborated on the idea of a flat Earth rather than replacing it.
For instance, the very first person to suggest the idea of a spherical Earth may have been the Greek philosopher Philolaus; his surviving work doesn’t explicitly state this but seems to suggest it. The notable thing here is that Philolaus is not a geocentrist; he has the Earth, and the other planets, orbiting the “universal fire” or “universal hearth”. So… he’s a heliocentrist two millenia early? Nope! The Sun is distinct from the universal fire, and also orbits it. Huh.
Meanwhile, about a hundred years before Philolaus, a Greek philosopher named Archelaus correctly reasoned that the Earth can’t be flat due to the difference in sunrise and sunset times as one travels East and West… but his conclusion was that the Earth was shaped like a concave bowl, which gets backwards how those times should vary. Seems he had the basic idea, but failed to think it through! Still, ultimately it was ancient Greece that was thinking about this and ancient Greece that got it.
Where does the Sun go at night?
There is, as I mentioned earlier, a fair amount of discussion of just what sorts of flat-earth cosmologies various ancient peoples believed in. For instance, lots of civilizations thought that the Sun goes underneath the Earth at night, but others said, how can it go
under the Earth, it’s solid underneath there! The Talmud, for instance, says that at night the Sun travels from West to East not underneath the Earth, but rather around its northern edge. I would suggest reading the book for more of this nature.
But once again it’s the case of China that’s eyebrow-raising. Let’s consider – one of the classical pieces of evidence that the Earth is round is that as you travel North and South, you see different stars; some stars become hidden behind the Earth, which is only possible because of its curvature.
Chinese astronomers were aware of this phenomenon, but they had a different explanation. Their explanation, which they also used to explain where the Sun goes at night was – and I swear I am not making this up – that the universe had a draw distance. If you get too far from something, its light simply can’t reach you. The book doesn’t use the term “draw distance”, but that’s basically what it’s describing!
Notice, by the way, how that explanation requires the Sun and stars to be quite close! “The Gnomon of the Zhou”, from around the year 20 CE, gave the distance to the heavenly canopy as 43,000 kilometers, and the “draw distance” as 90,000 kilometers. Which doesn’t seem to make sense offhand, because 90,000 is more than twice 43,000, so that would mean the Sun would be always visible. Hm, well, “The Gnomon of the Zhou” isn’t a very consistent text in the first place, and may not be reflective of what later astronomers thought.
It’s possible that I’m portraying things uncharitably here, and that actually Chinese astronomers said that the light becomes continuously attenuated over distance, only eventually fading out entirely, but the book sure makes it sound like a draw distance. Certainly “The Gnomon of the Zhou” does, although, again, that may not be the best text to rely on.
Either way this explanation is baffling, because when the Sun sets it clearly vanishes a bit at a time, rather than uniformly fading out (with the attenuation explanation) or vanishing suddenly (with the draw distance explanation). Apparently the common people generally said that the Sun becomes hidden behind a mountain at night – a fairly common belief among flat-Earth-believing cultures (who often further identified this mountain with the mountain believed to exist at the world’s center), and to my mind a more sensible one – but Chinese astronomers said otherwise.
I wish the book had gone into more detail on the question of how well this explanation worked for explaining the vanishing stars upon North/South travel; how well does this explanation actually match the data? How big is the anomaly, and were people worried about it? Did Chinese scholars of optics have opinions on this mechanism? Unfortunately, the book answers none of these questions.
For that matter, which way is North?
“The Gnomon of the Zhou” contains more surprising assertions I wish the book had discussed further. I mentioned above that most civilizations that believed in a flat earth believed that the world was a circle; in China, however, it was believed to be a square. Now if you believe in a flat Earth this raises the question of, where is the center of the world, something many civilizations had opinions about. In China, the center was generally believed to be the capital city, although which capital city varied (earlier Dengfeng, later Kaifeng).
“The Gnomon of the Zhou”, however, has it different. It says that the center of the world is at the North pole. And that’s a statement that I really have to wonder what it means.
As mentioned, the Chinese believed that the Earth was a square; moreover, it seemed they considered it to be an “axis-aligned” square, with sides running North-South and sides running “East-West”.
If the “North pole” means the northernmost point, then already we’ve got a problem; such a square Earth has a northern edge, not a northernmost point. But even if we ignore that, the northernmost point sure as hell isn’t at the center!
I really have to wonder how whoever wrote that was interpreting the ideas of “North”, “South”, “East”, and “West”. My general understanding was that in China at that time these were understood as fixed directions in space, with the axis-aligned picture I mentioned earlier. For the North pole to be at the center, that would instead suggest a view where North is not a fixed direction, but rather “towards the center” – much as how on a round Earth, “down” is not a fixed direction but rather towards the center. But if the authors were thinking this way, it doesn’t seem to have led them to the idea of a round Earth. Which isn’t surprising; that’s quite a leap.
This point, of what “The Gnomon of the Zhou” meant by this claim, is something the book really needed to discuss more, I’d say. I don’t know how possible it is to answer this question, especially as that particular work just isn’t all that consistent in the first place; unfortunately “The Globe” never mentions the question at all.
I recommend this book
I’d recommend reading “The Globe”. I’ve covered what I consider to be the highlights, but the book discusses a lot more. It does have some notable omissions as mentioned above. Also, a bit of a warning about the prose – when it goes off its main topic, it sometimes reads like a high-schooler’s essay (although never in the endnotes, so I guess this was a deliberate stylistic choice?). This mostly goes away once you get past the first few chapters, though, and the main topic comes more into focus.
Of course really the main thing I’ve taken from this book is that unsettling counterfactual that it never explicitly asks – what if the ancient Greeks hadn’t discovered the Earth to be round? How long would it have taken India, Persia, Europe, to all get it? This question is for sure going to haunt me. I guess you don’t really need to go read the book for it to haunt you too! If you want your speculation on the matter to be a bit more informed, though, probably this is a good place to start.
