Twenty to 30 miles from the coast is enough if you are in a cargo ship, which means you are already higher than sea level.

But to expose even more your silliness, you don’t need to be at sea to realise that; you can be in land seeing ships approaching and going up a mount to see a ship earlier or an island nearby.

— Maimonides, Guide for the Perplexed

“Why bother with theology, when we already have philosophy and Scripture? Truth is found in different ways. You can learn the world is round by astronomy, by mathematics, and by geography.”

— Aquinas, Summa Theologica

Have you actually done this yourself? As in, do you know what it actually looks like when you are in that position, and how far from a given coastline you must be in order to actually notice the effect with the naked eye, given various heights above sea level?

I have.

The idea that Greco-Roman era or Dark Age sailors, who used shore-hugging galleys — which, incidentally, could not go out of sight of land, not just because they were physically incapable of handling high seas but because they were incapable of carrying enough supplies for the oarsmen for any noticeable length of time, unlike Renaissance-era sailing ships, which had a much better crew to cargo space ratio — did this routinely is simple nonsense. The Odyssey, after all, is about an island nation’s king and his crew of experienced sailors — members of a seafaring people, whose familiarity with “the wine-dark sea” is a constant refrain — getting lost in the Mediterranean, of all places, for ten years. The same Mediterranean that overloaded dinghies full of Africans cross today by paddling with their hands and broken planks.

Then there’s the fact that the classical galleys simply didn’t have very tall masts — just not enough height to make the curvature effect anywhere near as easily distinguishable as from the top of an 1800s clipper, or even a 1600s galleon.

Atlantic seafaring in general was very, very limited for a long time, and very much in the same vein as the shore-hugging style that had developed in the Mediterranean. Columbus’s ships, and Henry the Navigator’s, were the start of mastering deep-ocean travel. Those craft were significantly more advanced than shore-hugging fishing boats, but even so they were rickety primitive contraptions that were hardly suited to the task at hand. The shipbuilding art improved tremendously over the next few centuries, but when the Age of Discovery started, it was absolutely still in its infancy. All the lessons that might be drawn from experience in the far reaches of the ocean were simply not generally available.

In other words, you are simply, completely, totally wrong. You are making assumptions based on things that are obvious to you about what would be obvious to people in vastly different circumstances with vastly different technology and habits, and you are wrong.

Precisely.

The flat-earth argument is so silly. Everyone that rises to the top of a ship mast or a mountain sees curvature. A mere boat that goes out to sea after a while stops seeing earth. That was known thousands of years ago.

It usually comes from people that mistake flat earth with geocentrism.

Here’s Strabo’s Geographica, written in the first century:

As the size of the earth has been demonstrated by other writers, we shall here take for granted and receive as accurate what they have advanced. We shall also assume that the earth is spheroidal, that its surface is likewise spheroidal, and above all, that bodies have a tendency towards its centre, which latter point is clear to the perception of the most average understanding. However we may show summarily that the earth is spheroidal, from the consideration that all things however distant tend to its centre, and that every body is attracted towards its centre of gravity; this is more distinctly proved from observations of the sea and sky, for here the evidence of the senses, and common observation, is alone requisite. The convexity of the sea is a further proof of this to those who have sailed; for they cannot perceive lights at a distance when placed at the same level as their eyes, but if raised on high, they at once become perceptible to vision, though at the same time further removed. So, when the eye is raised, it sees what before was utterly imperceptible.

And St. Augustine’s City of God in the fifth century claiming that the sphericity of the earth does not imply that there is inhabited land on the other side:

But as to the fable that there are Antipodes, that is to say, men on the opposite side of the earth, where the sun rises when it sets to us, men who walk with their feet opposite ours, that is on no ground credible. And, indeed, it is not affirmed that this has been learned by historical knowledge, but by scientific conjecture, on the ground that the earth is suspended within the concavity of the sky, and that it has as much room on the one side of it as on the other: hence they say that the part which is beneath must also be inhabited. But they do not remark that, although it be supposed or scientifically demonstrated that the world is of a round and spherical form, yet it does not follow that the other side of the earth is bare of water; nor even, though it be bare, does it immediately follow that it is peopled.

Often the same people asserting that the flat earth view is a myth will also assert that the Dark Ages weren’t dark. In that case, mere numbers are sufficient for disproving the idea; the collapse in terms of total population and land under cultivation between 300 AD and 800-900 AD was monumental. This doesn’t stop the claim from being made. Often highly correlated with the argument that it was a very Christian period therefore it COULDN’T be a bad time.

I’m willing to believe that certain academicians in their ivory towers discussed among themselves reasons why the earth might have one shape or another. The idea that it was “known” or even generally considered that the earth wasn’t flat, however, is not at all proven.