Do you want a serious answer?

The "flat Earth" belief nowadays is such a source for ridicule because the shape of the planet is proven by a dozen of scientific means and unequivocally documented, also, these sources can be easily accessed by everyone and education is widely available. The only way you can believe in a "flat Earth" is by actively denying facts.

In the world of TES, this would not be the case. There is no photographic proof of Nirn's shape. The average person who is not a scholar has no way to check it. Let alone the average beet farmer or camel herder probably doesn't even care because they have more pressing things to think about. Realistically, most people in a medieval society were uneducated - it's already weird that in Tamriel, (almost?) everyone seems to be able to read. So in this world, not knowing the shape of the planet wouldn't be about denying science, but just about a lack of knowledge not even that special. What would a remote Bosmer village know about astronomy? Or an Ashlander tribe somewhere in the wilderness? An eccentric Altmer scholar defending the idea of flat Nirn - that might cause laughter among his colleagues. But otherwise, it doesn't really have the same effect.

Elsonso wrote: »

Wait. Nirn isn't flat? We don't fall off the edge?

Nah, but if you want to fall off something, you may climb this instead:
https://en.uesp.net/wiki/Oblivion:Orrery

Photographic proof!

I don't know about "FlatNirers" but it looks like q-anon has found it's way to Tamriel...

Nirn is indeed flat and bordered by an ice wall. Several years ago I broke out of the map in Direfrost Keep in search of Direfrost 2. I traveled across the ice wall and found the edge of Nirn. I lost my footing and fell off the edge. Fell for several minutes. Couldnt port anywhere because I was in movement. I thought that I was doomed to fall through the icey air for the rest of eternity, a fate worse than oblivion. My vision went dark and I thought it was the end, but alas the gods had mercy on me and placed me back in Mournhold- the last place I had been before my foolish adventure.

It's a common misconception that the Earth was believed to be flat until circumnavigated for the first time in the early modern period, but in reality, the Earth has been known to be a sphere since ancient times by various cultures and the flat Earth is a modern conspiracy theory. You don't actually need to go to space to be able to confirm the shape of the planet, there's empirical evidence you can observe from the ground. Ancient Greek scientist Eratosthenes performed an experiment in the third century BC to measure the Earth's circumference by measuring shadows in different locations at the same time of day. His results were accurate to values calculated by modern scientific methods.

With Tamriel's technology level, prominent scholarly community and access to magic, this kind of understanding would absolutely be within reach, and we do in fact see plenty of in-game and in-lore representation of Nirn as a sphere from various races:

• Altmer, Dwemer (1, 2, 3, 4) Ayleid, Daedric, and Clockwork (1, 2) orreries that depict Nirn as spherical
• A globe seen in Arena's intro, another one seen in Mages Guilds in Daggerfall, and another one in a Dark Brotherhood DLC promotional artwork from ESO
• Words such as 'planet', 'globe' and 'sphere' are frequently used to refer to Nirn by NPCs and in lore texts.
• Observatories appear in various cultures (1, 2), indicating the active study of astronomy and cosmology.
• Navigational/timekeeping instruments such as astrolabes, sundials and sextants are common as treasure items, implying most sailors navigate with a mathematical understanding of the world and sky.

To the average citizen, the shape of the world probably wouldn't matter that much because their concerns are going to be with the local world around them. But people wouldn't think you're crazy for talking about a round planet, it seems to be pretty common knowledge.

Cosmo_Nova wrote: »

You don't actually need to go to space to be able to confirm the shape of the planet

Of course not. But an aerial photograph is certainly easier to conceive (and to accept as the truth) for the average person, no matter their level of education, than a scientific theory based on physics or difficult computations.

And especially when we're talking about a medieval (fantasy) society where most people would realistically be peasants: Would they be able to understand a scientific treatise? Would they believe a scholar that can't give them any visual proof? And the average person would certainly not have access to one of Tamriel's orreries either.

Cosmo_Nova wrote: »

the flat Earth is a modern conspiracy theory.

Modernity (in terms of the era), yes. But it's not a recent internet phenomenon; it already showed up at the beginning of the 19th century, by some people who took religious writings very literally (sadly, I can't go into more detail, as discussions about religions are against the TOS of this forum).

Flat nirnists are unfortunately a thing. There are at least three in a guild I am in where the focus is on lore. Two I believe push the theory simply for fun. Though others have referenced "Disc world" as an example of a flat world being used in a fantasy setting. This normally gets followed up with original creators quotes being referenced as well as what their inspirations may have been back in the day.
I view this as no different than analyzing an artists work. Though simply as someone may have been inspired or motivated by something does not mean that is incorporated and (as far as I am aware) there is no interviews documenting such.
I do plan to follow this topic though, if nothing else for future arguments against the Flat Nirnists in the future.