No worries, at least we can agree - this is within the grasp of most people to figure out!

Most flat earthers believe that the earth is also 6000 years old. I'm not that concerned with their psychology to be honest, I'm more concerned with honest thinking peoples approaches to how they'd demonstrate it.

If you watch for a few minutes, you can see the curvature of the earth perfectly clearly. — Srap Tasmaner

I watched that second video and cannot see anything like that.

Here's a write-up on why the curvature of the earth is not visible to the naked eye when looking at, say, the horizon of the sea at a beach.

https://flatearth.ws/standing-on-a-beach

There IS a slight curve, but it's proportionally so tiny that you can't discern it. The apex of the curve is only 0.01% higher -- I don't think anybody can honestly say "that curve is clear".

It's there, but you'd genuinely have to have super-human vision to perceive it.

If you went in not knowing about refraction, you would think you've just proven that the earth is flat. — flannel jesus

No, you would have proven that the world is less round then the experts listed. Again, proven off of incomplete information. Testing refraction is the same. Look up how the experts proved refraction, then test yourself.

There is also one other thing you're forgetting. Flat Earthers would also need to prove the Earth is flat. If they are discounting experts due to a lack of physical evidence, they have to discount their own claims due to a lack of physical evidence as well. Deciding, "I don't trust the scientists, but its flat because I say so," is just ignorance masquerading as skepticism.

Flat Earthers would also need to prove the Earth is flat — Philosophim

Flat earthers don't often like to prove anything. They don't even tend to make clear claims. I saw a flat earther recently claim they can triangulate the distance of the sun (because no way is it over 90million miles away) -- but one thing I've never seen a flat earther do is *actually triangulate the distance of the sun*. That's because actually making specific claims takes bravery and integrity. You're putting yourself at risk of being wrong.

I watched that second video and cannot see anything like that. — flannel jesus

It does work better as a companion to the first video.

The basic idea is that he has his camera mounted on a jib so that he can raise and lower it. At maximum height, you can see the far shore of the lake, which is like 7 km away, I think; when you lower it closer to the water, the far shore disappears. It disappears because it is now below the horizon. That's the math he explains in the second video.

I don't know what else to tell you.

"I can see more of chicago than I geometrically should if you were right". They're actually right about that. — flannel jesus

I've been at the sand cliffs on the eastern short of lake Michigan. Interesting place. Houses fall down it now and then, inevitably. You can stand at the edge of it and the wind is enough to turn your eyelids inside-out, but step back 3 meters and you can set up a table and play cards.
Yes, refraction lets you see further than geometry suggests, but nowhere near as far as the flat Earth would suggest. You should see Chicago from my cliff instead of a very visible sky against water.

I personally can barely make out the curvature looking out over the Hudson river, from a vantage right at sea level, about 160 km inland. There are places you can look a long way, enough that the boat bottoms disappear behind the water, if not the horizon which is the hills in the distance. Maybe you can write that off as refraction in the other direction. Takes a good clear day to do this. This all sounds like a similar observation at the lake in the video, but without any actual instruments.

If you watch for a few minutes, you can see the curvature of the earth perfectly clearly. — Srap Tasmaner

Seemingly with the benefit of drawing straight lines onto the image. I have no such benefit when gazing at the Hudson.

So again, the flat Earthers are either being willfully ignorant, or refusing to understand the entire justification of the argument for why the Earth is round when observed from X distance away. — Philosophim

I would say 'willfully misleading'. I seriously doubt that flat earthers actually believe their own schtick. The whole point to buck the consensus. One of their advertisements urged you to join the flat-earth society. "We have members from all across the globe".

And if we're not relying on expert opinions, we might have to prove refraction too. I'm not sure how that proof would go. — flannel jesus

And that's the general question, having many of the same issues as solipsism: How can any external information be trusted? How much science could one demonstrate (not prove) if one had knowledge of the goal, but one still had to start from scratch? You probably could demonstrate Newtonian physics without too much reliance on prior expert work. The moon landing real? Not a chance, especially with all the doctored photos they published. But just because they're faking the photos doesn't mean they weren't there. The footage still looks better than the best stuff hollywood puts out today, and they didn't have AI to deep fake it back then.

I think the major problem with all this is that people aren't questioning or are critical of scientific facts because they've measured anything. Their beliefs are rooted in the laziness of never looking for actual answers and facts themselves. — Christoffer

There you go! It seems that a great deal of people with crazy personal ideas that are claimed to be their actual beliefs, seem to justify them via avoidance of actual evidence. Humans are not by nature rational, but they're probably the best species at rationalization. Answer first. Weak justification if one actually feels the need. Ignore anything contradicting.

Most flat earthers believe that the earth is also 6000 years old. — flannel jesus

Fantastic example of rationalization as opposed to rational. Most of the churches have abandoned this assertion by now, but per last-tuesdayism, it cannot be falsified by empirical evidence.

Well none of those words relate to visibly seeing the horizon curving. You could tell me, "yes, I understand that you can't visibly see the horizon curving". That would be cool.

Flat earthers don't often like to prove anything. I'm not that concerned with their psychology to be honest, I'm more concerned with honest thinking peoples approaches to how they'd demonstrate it. — flannel jesus

For the actually honest person without psychological issues that wishes to prove the world is round, there are a host of resources to do so. The people who insist the Earth is flat are mostly deluded people who elevate what they want to believe over truth and facts. Most people do this to a certain extent, but are eventually willing to bend once the evidence clearly shows their belief is wrong. These people are so deep into this that they are unwilling to bend to any evidence that is contrary to their beliefs.

Its mental illness at that point. You cannot reason with mental illness, nor can you reason with someone who refuses to enter into reasonable discourse.

There are places you can look a long way, enough that the boat bottoms disappear behind the water, if not the horizon which is the hills in the distance. Maybe you can write that off as refraction in the other direction — noAxioms

They actually do write it off that way, funnily enough.

I'm pretty sure it's not visibly curved. — flannel jesus

How can you be sure? The curvature might be too small to notice, say, if you only see a narrow piece of the horizon, but I'm pretty sure it's curved, also visibly if you'd look closer.

Compare it with seeing the complex tangle of shapes, textures and colours of a birch tree. You might not pay attention to all of them, yet they are there, visible, and they are the object of your visual experience.

How can you be sure? The curvature might be too small to notice, say, if you only see a narrow piece of the horizon, but I'm pretty sure it's curved, also visibly if you'd look closer. — jkop

It's been calculated to be imperceptibly small. If your view of the horizon from a beach front is x "pixels" wide, so to speak, the curvature of the horizon is 0.01% of x - as in, the number of pixels the apex of the curve is above the lowest point of the curve is x times 0.0001. That's 1 pixel of rise for every 10,000 pixels of width of an image. I linked the article calculating it on the previous part of the page.

I do not believe you can actually perceive it. I know I can't - I go to the beach pretty often, I see the horizon a couple times a month, and there's no apparent curve from a vantage point of 6-8ft above sea level.

edit. link here: https://flatearth.ws/standing-on-a-beach

On a large body of water or the ocean located next to a tall treeless hill, embark perpendicular to the shoreline after placing large brightly colored flags every 10 feet up the hill. As you get further from shore, the flags will sink out of sight. The first will disappear at around 3 to 4 miles.

Yeah this is a good one. I saw an image with something like telephone poles curving down over the horizon.

this is the image I'm thinking of

https://www.google.com/search?q=telephone+poles+over+sea+horizon+curve&client=firefox-b-1-d&sca_esv=16501dcb6f6c1178&udm=2&biw=1280&bih=559&sxsrf=ADLYWIIcaNJt_9jG3BosNMEDH2Fk0C-DlQ%3A1736530546173&ei=clqBZ52lCsaehbIPsf2l0Qc&ved=0ahUKEwidpuKy2OuKAxVGT0EAHbF-KXoQ4dUDCBA&uact=5&oq=telephone+poles+over+sea+horizon+curve&gs_lp=EgNpbWciJnRlbGVwaG9uZSBwb2xlcyBvdmVyIHNlYSBob3Jpem9uIGN1cnZlSJIKUI0DWLoJcAF4AJABAJgBQKAB5QKqAQE2uAEDyAEA-AEBmAIAoAIAmAMAiAYBkgcAoAeOAg&sclient=img#vhid=RmI4MnBl40TscM&vssid=mosaic

I do not believe you can actually perceive it. I know I can't - I go to the beach pretty often, I see the horizon a couple times a month, and there's no apparent curve from a vantage point of 6-8ft above sea level. — flannel jesus

Granted there's an angle of view from which a visibly curved object may look straight. Its straight look arises from the angle of view, unlike its curved look which arises from its true shape. That's why it's called curved.

Like "scientific" skeptics about perception, also "unscientific" flat-earthers fail to distinguish between what an object may look like and its true visible shape.

Like "scientific" skeptics about perception, also "unscientific" flat-earthers fail to distinguish between what an object may look like and its true visible shape. — jkop

Kind of like what you did when you claimed you could just see the curve

Kind of like what you did when you claimed you could just see the curve — flannel jesus

No, my claim is that the curve is visible, and that its straight or flat looks are features of some angles of view. We could limit our visual field to a vanishing point on the horizon, which is a feature of central projection, not the observed object. The vanishing point is not a visible object, nor are the straight or flat looks of the earth.

Listening for the flat earthers declaring where the edge of the flat surface is located, yet hearing silence.

I'm trying to think of a Moorean answer to the OP's question but I can't think of any. That doesn't necessarily mean that there aren't any. Moorean answers, that is.

Choose a clear cool day and take a strong pair of binoculars or a small telescope to the headlands to watch a ship sail over the horizon. You will not see the ship shrink to nothing, but sink until at last the superstructure disappears.

Observe a number of lunar eclipses. The shadow of the Earth on the moon is always curved, both at the start and the end of the eclipse.

Consider how many other of your beliefs would have to be false if the world were indeed flat. And how many navigators would have to be in on the conspiracy.

But most especially, why not look up the answer, why doubt the consensus view, why think that your own experiences should have a primacy that is beyond doubt? Think about the attitude that folk take into a discussion such as this - are they looking to disprove their existing view, or just to confirm it? What, for them, counts as a disproof?

Becasue no evidence ever forces you to a particular position. There are always auxiliary hypotheses that you can employ to prevent your pet doctrine from being falsified. For some, the cup really contains the blood of Christ, despite all the evidence to the contrary. At some stage you, and only you, must decide what to believe, and that is about you, not about the way the world is.

Choose a clear cool day and take a strong pair of binoculars or a small telescope to the headlands to watch a ship sail over the horizon. You will not see the ship shrink to nothing, but sink until at last the superstructure disappears.

Observe a number of lunar eclipses. The shadow of the Earth on the moon is always curved, both at the start and the end of the eclipse.

Consider how many other of your beliefs would have to be false if the world were indeed flat. And how many navigators would have to be in on the conspiracy.

But most especially, why not look up the answer, why doubt the consensus view, why think that your own experiences should have a primacy that is beyond doubt? Think about the attitude that folk take into a discussion such as this - are they looking to disprove their existing view, or just to confirm it? What, for them, counts as a disproof?

Becasue no evidence ever forces you to a particular position. There are always auxiliary hypotheses that you can employ to prevent your pet doctrine from being falsified. For some, the cup really contains the blood of Christ, despite all the evidence to the contrary. At some stage you, and only you, must decide what to believe, and that is about you, not about the way the world is. — Banno

Ok, this could be a Moorean answer in some sense. And Moorean answers tend to be correct. Perhaps not due to the right reasons, but Moorean arguments tend to have true conclusions. Just gotta make sure that the premises are also true, and that the reasoning is deductive.

"Don't rely on any experts, scientists, NASA photographs -- prove yourself that the earth is round," what do you do? — flannel jesus

Find a large area of flat terrain (here in Australia that is not difficult.) Point to a feature on the horizon of said area. Drive to that feature and observe it is no longer on the horizon. More or less the same argument as in @Srap Tasmaner's video.

An aside. Eratosthenes of Cyrene (c. 276 BCE–c. 194 BCE) was an ancient Greek philosopher who calculated the circumference of the Earth with remarkable accuracy using differences in shadow lengths. Here's a brief outline of his method:

Observing Shadows: Eratosthenes knew that in Syene (modern-day Aswan, Egypt), the Sun cast no shadow at noon on the summer solstice, as it was directly overhead (evidenced by sunlight reaching the bottom of a well). However, in Alexandria, some 800 kilometers north, the Sun did cast a shadow at the same time.

Measuring the Angle: By measuring the angle of the shadow in Alexandria, Eratosthenes determined it was approximately 7.2 degrees, or 1/50th of a full circle.

Calculating the Earth's Circumference: Eratosthenes reasoned that if the Earth were a sphere, the arc between Syene and Alexandria corresponded to 1/50th of the Earth's total circumference. Knowing the distance between the two cities (measured through caravan travel), he multiplied this distance by 50 to estimate the full circumference.

Result: His calculation, about 40,000 kilometers, was astonishingly close to the modern measurement of the Earth's circumference.

Find a large area of flat terrain (here in Australia that is not difficult.) Point to a feature on the horizon of said area. Drive to that feature and observe it is no longer on the horizon. — Wayfarer

Did you leave something out here?

Find a tower on a large salt plain. Drive away from it and you will notice that the bottom of the tower disappears below the horizon. Did it just go below a slight rise, previously unseen? That's the reason for using a body of water, which could not have a "hill" in it.

That's the reason for using a body of water, which could not have a "hill" in it. — Banno

Why are you using scare quotes? Honest question, genuinely curious. Like, just say it: a body of water cannot have a hill in it. It has waves. And a hill is not a wave.

I read the quote marks to refer to the problem of barely perceptible changes of grade. Salt flats are even enough to permit driving over at very high speeds. But that is not as reliable a measure of continuity as a fluid that seeks its own level.

It is true that waves involve local variations but average out in the limits of the visible. These observations are best made while not experiencing a tsunami.

I read the quote marks to refer to the problem of barely perceptible changes of grade. — Paine

Sure, so did I. That wasn't my point though.

So, if you were challenged, someone said "Don't rely on any experts, scientists, NASA photographs -- prove yourself that the earth is round," what do you do? Don't look up the answer, try to come up with one yourself. — flannel jesus

Go to the ocean shoreline on a clear sunny day and look at how outgoing ships simply "sink" into the horizon and incoming ships emerged from the horizon. If the Earth would be flat, the ships would just get tinier and tinier.



And this is why there's a very common "submarine sighting" on the ferries going from Helsinki to Tallinn. Observing people notice the "submarine" emerging, then slowly going past the ferry and later submerging again. Well, it's not a submarine, it's a well known rock that simply looks like a tower of a submarine.



Or if you have a friend, put your head on the ground (perhaps at the shore) and watch the sun go down while your friend is behind you somewhere higher, perhaps on the fifth floor of a building. Talk to each by phone and yell "now!" when the last glimpse of the sun's circle has dissappeared to the horizon. The difference is notable. Now I've done both of these "experiments" and have seen how large ships drop into the horizon as well as seen the difference between the sighting of the sunset.

There is a difference of about three minutes between the first sunset and the last sunset. For Islamic ritual purposes, the building is divided into three zones. In Ramadan, people in the highest floors have to break their fast about 2 minutes later than people on the lowest levels.

Yeah totally.

I came across this response to that type of experiment the other day: https://www.reddit.com/r/globeskepticism/comments/1dej3ox/perspective_not_curvature/

Of course, the difference is, when you zoom in on the reddit scenario, the bottom comes back - when you zoom in on the ship in the horizon, it doesn't.

Of course, if you haven't been turned on, you wouldn't know, so no blame: