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Comment by AlexTWithBeard | Hacker News Reader

Belief is a keyword here. Most of "spherical earthers", alas, also believe what they are told. They happened to pick the right set of beliefs - kudos to them - but there's no critical thinking whatsoever.

We watched some time ago a why-do-we-think-earth-is-flat movie with friends and I asked: okay, so what's wrong with their explanation? For an extra credit: how a layman like me can check what is the shape of the earth?

And suddenly it turns out that these questions are a bit harder than it seems.

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But here's the problem with this logic; I've never been presented with any credible arguments that suggest Earth is not round, and in fact have been able to use this knowledge of Earth's roundness to understand and accurately predict facts in my real life on a semi-regular basis.

* When I see my flight plan, I understand why it's curved.

* When I see clouds that a radar says should be a certain distance away, I understand what makes me able to see them.

* Where weather is moving, how it's moving, and why certain weather patterns even exist at all.

* When I see a landmass on water I can accurately guess how far away it is, and I can understand why moving even a small amount upwards lets me see so much more (and why it was such an advantage on ships to have a vantage point only 20 to 30 feet up in the air).

* How stars move in the night sky is predictable.

* Why the "north" star exists and is accurate as a directional indicator.

* Where constellations are, and how they're different depending on what latitude I'm currently observing the night sky from.

* The fact that tides exist, and how they work.

* I understand how and what satellites are, when I can see them, and I can predict when I'm going to lose signal from them given certain circumstances.

* Every other planet I've observed is round. The moon is round. The sun is round.

None of this works if Earth isn't round, and that's just off the top of my head. The point here is that so many things stop making sense if you don't accept the fact that the Earth is round, and so you're constantly failing to predict or understand a whole host of problems that, yes, do come up in my daily life.

This isn't about "happening to pick the right set of beliefs", it's about matching what you're told with what you're observing and noting a singular and profound consistency across disciplines, uniformly converging on one single, basic fact; the Earth is round.

> * When I see my flight plan, I understand why it's curved.

Just came off the plane yesterday. I remember the earth on the flight map was perfectly flat. The path was weirdly curved though...

The path is curved because that's the straight line over the surface of a sphere, and if you project that straight line over to the Earth on a flat screen, it'll look curved.

EDIT: Wait, didn't get the sarcasm, my bad. :P

Yeah lol.

Flat-earthers should be suing airlines for flying them further than needed. Make them put their money where their mouth is.

The problem is these can all be refuted without too much trouble:

1) Flight plans only look curved because you are using a map projection that assumes a spherical globe. If you use the right flat earth map you will find that flight paths follow straight lines.

2) Clouds and distances still exist in a flat earth and the clouds float above the earth. This is why they are detectable by radar.

3) Weather patterns exist on the flat earth because the sun moves across it and heats it unevenly.

4) An ant on a perfectly flat desk can also see much more from an elevated vantage point. Are you claiming desks are also spherical?

5) No one is claiming the stars nor the flat earth is stationary. Both can rotate around various axes and the resulting movement can be predicted.

6) The north star sits slightly above the plane of the flat earth and far to the north so that it appears north from any vantage point on the earth. This is just because of its distance.

7) The constellations differ depending on latitude because of the refraction of the atmosphere. This bends the light rays.

8) Tides are caused by the pull of the moon as it traverses above the flat earth and attracts the water in the ocean.

9) The other planets are also flat and only appear round because they are tidally locked with respect to the earth.

This is a good comment, because it illustrates exactly what I'm talking about w/r/t predictability. If what you've written here can help you accurately predict the answers to questions you don't know, then you might be onto something.

However, there is no predictive value in any of what you've written here, so while they may be, to some, "refutations", they are not in any way useful.

If there were actually any flat earthers who cared about reason, because of the lack of predictive value (not to mention the contradictory/unprovable nature of what you've written here), they would not remain flat earthers for very long.

I usually propose a rather simple, albeit not cheap, experiment for this.

1. Find a sufficiently large "shallow" body of water. 2. Build a number a number of masts/poles in a straight line, all with an identical height over the water.

Now when standing at the shore and looking at the masts I predicts one of two things.

a. you can see all masts in their entirety with a telescope, or b. you can see less and less of the masts the further away they are.

There are actual places with power lines that satisfy this requirement. For b. you can use high school math to reason about the curvature needed to explain the results. There is no such sound logic (to my knowledge) explaining this for a flat surface.

You can also watch any sailing vessel on its way towards (or away from) you.

A wise man in ancient time did this by comparing shadows in different places. Can't remember his name though. But he was quite close with his calculations. It doesn't need to be expensive. Just wait for noon. Then hop in a car/bus/train/whatever and do the same at the same time next day somewhere else. It helps if the shadow-casting thing is the same height.

That was Eratosthenes of Cyrene:

The simplified method works by considering two cities along the same meridian and measuring both the distance between them and the difference in angles of the shadows cast by the sun on a vertical rod (a gnomon) in each city at noon on the summer solstice. The two cities used were Alexandria and Syene (modern Aswan), and the distance between the cities was measured by professional bematists.[16] A geometric calculation reveals that the circumference of the Earth is the distance between the two cities divided by the difference in shadow angles expressed as a fraction of one turn.

https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_Ea...

Here's Philip Morrison showing how to do a similar experiment, from the 1980s TV series "The Ring of Truth". https://youtu.be/yRY2SkMTafc?t=567

They found a road that was almost due north and south (US 183 from Bassett, NE to 370 miles south in Coldwater, KS). They measured the elevation of Antares at its zenith one night, then again the next. From that they get the change in latitude. Combined with odometer data and some estimates for wiggles, and end with a result about 5% larger than the best-known solution.

Their measuring instrument was a Ryder van dubbed "The Van of Eratosthenes."

As an aside, later in the same episode is when I learned about GPS. https://youtu.be/yRY2SkMTafc?t=2968 . "This is the future of mapping."

What I don't like about this argument is that it assumes the sun is very far away, which I've heard flat-earthers claim it isn't.

You need to be careful with this though. I don't fully understand the reasons, but you have to consider bending of the light. See the wiki: https://en.m.wikipedia.org/wiki/Bedford_Level_experiment

I haven't done the experiment, but this brings it back to trusting scientists (light bends just above the water, but doesn't 4m above the water). I would need to do additional experiments to understand the refraction.

Yes, I believe it was Feynman that said something along those lines: Anyone with an imagination can come up with an idea about how the world is. The extreme difficulty is in ensuring it is consistent with everything that has been seen before.

Flat earthers care about firsthand observation more than reason. Reason is simply defeated by relying on false facts aka “believing”. This is why that crazy rocket dude ended up dead in the Nevada Desert last year, he was literally trying to see for himself what the shape of the Earth is.

I -believe- the earth is round. But the intellectual honesty I’ve taken from The flat earthers tells me I have delegated my source of truth on that topic to others whom I am trusting to relay the truth. Maybe one day I’ll get into orbit and see for myself. With my own eyes. But that is beside the point of what I learned from them. I learned any time I outsource what I think is true to third parties, there is a risk of me believing a lie. Because people lie sometimes. And sometimes they draw conclusions based on assumptions and conjecture. My thought process is non-binary or more honestly, tri-state. I have true, false, and I have not observed that to be either true or false.

But I haven't delegated my source of truth! I have done first hand observation!

1. I regularly do video conferences with friends and colleagues on other continents; I can see if its light or dark in their windows. I have personally verified that some of my friends and colleagues and their places exist.

2. I have seen many ships disappear behind the horizon, and I have seen mountains disappear behind the horizon from ships; I have verified the accuracy of Vesselfinder (by comparing transits through lines in Vesselfinder with what I can see) and checked that it is at the expected distance. (I was curious about the delay.)

3. I have watched the ISS (once even trailed by the ATV) and it was right where HeavensAbove said it would be. And 90 minutes later I did it again. I have personally verified the basic math behind circular orbits.

4. I have experienced changing radio band propagation conditions consistent with a day/night cycle that is different in different places. Such as grayline DX, which is good propagation along the day/night line. Which implies the day/night line is real. And based on the reports I received I know it matches the map. I have built some transmitters and receivers myself.

5. I have flown between continents, been jetlagged and changed my clock's time (or maybe changing my wristwatches time is changing the sun and causing the jetlag?). I have watched the entire flight through windows, so I know I traveled very far.

6. I have seen the motion behind Foucault's pendulum over enough time (I've been to the museum long enough to verify that the pendulum plane rotates as expected, and I saw nobody messing with it in any obvious way.) We also did the same experiment in school. In school, we did the math for the Coriolis force, and talked about how it comes up as a significant factor in unexpected places (e.g. railway). Someone would talk.

7. I've observed the direction of the toilet swirl, and whirlpools in lakes.

8. I've experienced tide levels and swum with/against tide currents (sometimes even in the same place). I have also seen the moon in the sky and I think if it would be inconsistent someone would've noticed by now.

9. I've seen the shape of day/light boundary on the moon. Now that's the moon, but that one's clearly spherical, which is one data point.

10. I had a trusted fried, but he's got a PhD in astrophysics now, so he must be in on it and we cannot be friends anymore. /s

And I'm just some nerd, I haven't done too many experiments. I'm sure I could still extend the list, but of course there's no point since neither of us believe in the flat earth "theory".

There's no intellectual honesty in coming up with fake experiments like flat earthers do (if they even go to the trouble).

Before 2020, I thought flat earth like conspiracies were harmless fun (and I still think that a significant number of flat earthers are just having fun), and maybe they are. But after spending several hundred hours "discussing" many conspiracy theories (people I know and care about started believing very outlandish ones, like chemtrails and 5G causing covid) the last two years, and first hand witnessing intellectual incompentence (complete math illiteracy down to calculating a percentage, complete rejection of the concept of mathematical proofs, zero concept of scale e.g. of organizations and communities, complete refusal to read original research, insisting on "reading between the lines" to interpret texts claims made in texts into the opposite) I have come to see conspiracy theories and fake or un-intellectuality (is that a word?) as very dangerous (the "can cause death" type of danger).

If you care to actually look into it, you'll find that flat-earthers have accounted for most/all of these. I'm not a flat-earther, but it's worth looking into before stating things that "obviously can't be accounted for with a flat earth.

1: There are time zones in the flat earth model. The earth is a flat disc with antarctica (or is it the north pole?) at the center, and the sun moves around it in a circle, so the places opposite the sun are in nighttime

2: Believe it or not, flat-earthers believe light only travels so far, which is why things disappear "over the horizon"

4: I don't actually understand this one, but I suspect they'd handwave and talk about the same point in (1)

5: Same as (1)

I don't think flat-earthers have accounted for Foucalt's pendulum however.

"If you care to actually look into it, you'll find that flat-earthers have accounted for most/all of these. I'm not a flat-earther, but it's worth looking into before stating things that "obviously can't be accounted for with a flat earth."

That does not match my experience. I did looked into it and I found no consistent model of a flat earth. Lots and lots of words and sometimes even with consistent logic at times - but build upon completely false assumptions and flawed logic and therefore worthless. (you can proof anything, if your basic assumptions are made up)

"I don't think flat-earthers have accounted for Foucalt's pendulum however. "

And some of them tried and believe they did, but only when ignoring other facts and used twisted logic.

Like some parent poster had said - it is not about proof, but way more about ideology.

Otherwise they could very easily have a real proof, by sending up a weatherballoon, or organise a boat trip to the end of the world. But since they always refuse to do so (I challenged quite some with that concept) or even really think about it - they likely suspect inside that they are wrong, but rather not confirm for real, because then they would have to abolish a lot of their other ideology like chemtrails etc.

> 7. I've observed the direction of the toilet swirl, and whirlpools in lakes.

this is not an indication of a round earth. the coriolis effect on this is negligible.

4) An ant on a perfectly flat desk can also see much more from an elevated vantage point. Are you claiming desks are also spherical?

Not true. From any non-zero height above an infinite flat desk you can see the entire desk.

How do you you know that light travels in straight lines? Maybe it travels in curved lines! In fact, that's how mirages form.

(A good response to this, of course, is to ask for a model for how much the light curves. A vague description is not a predictive, scientifically-testable model, unless it's a very wrong description.)

Now you are shifting the goal posts. From the difference between a sphere and a plane assuming light travels in straight lines to questioning whether light travels in straight lines to begin with.

How do you you know that light travels in straight lines? Maybe it travels in curved lines!

You turn on a light source and look at how the light spreads. Light does not - besides a tiny bit of diffraction - bend into the shadow area behind obstacles.

In fact, that's how mirages form.

You obviously have to perform your experiments under control conditions, so pick a time and place with negligible variation in index of refraction of the atmosphere. Or measure and account for it.

> Now you are shifting the goal posts.

I don't know that I am; you were the one to bring up the implicit “light travels in straight lines” assumption with your claim about ants on a flat table. Obviously if I think the earth is flat, I don't believe that.

> Light does not - besides a tiny bit of diffraction - bend into the shadow area behind obstacles.

It depends how much distance you give the light. Shadows are noticeably blurry if your hand is close to the light source.

> You obviously have to perform your experiments under control conditions, so pick a time and place with negligible variation in index of refraction of the atmosphere.

You're never going to get a situation where the air temperature is consistent enough for this; it varies by height (as does pressure). And, again, unless you can show me that “index of refraction” is a thing that applies to air, I'm not going to believe something as ludicrous as Snell's Law. (After all, isn't the speed of light a constant? The whole derivation rests on the assumption that it varies depending on the medium!)

I don't know that I am; you were the one to bring up the implicit “light travels in straight lines” assumption with your claim about ants on a flat table.

I would argue that it was implied before I first commented. Without any assumption about the way light moves, you can not answer visibility questions on spheres and desks. The first comment talked about a spherical earth and certainly assumed light behaves as commonly accepted, the response did not question this either. Only after it has been pointed out that the response was not valid, the behavior of light entered the discussion.

It depends how much distance you give the light. Shadows are noticeably blurry if your hand is close to the light source.

Use a point light source or account for the size and shape of your light source.

Make up your mind. Does light move in straight lines or doesn't it? Does a medium influence light traveling through it or doesn't it? If, how? You questioned light moving in straight line, it is beyond me why you would now also question light moving not in straight lines. Or is it just the mechanism that you are questioning? And nobody claims the speed of light is constant, the speed of light in vacuum is constant.

And in order to cut this cat and mouse game short, just present your complete theory of how the world works upfront. There is no point in such a discussion if you keep changing what you believe or say every time an issue is pointed out.

> Without any assumption about the way light moves, you can not answer visibility questions on spheres and desks.

Yeah you can: light moves the way it actually moves in real life: the way you can see with your eyes.

> Make up your mind. Does light move in straight lines or doesn't it?

I was assuming what you said to be true, and showing that it wasn't sufficient to rule out what I was saying.

---

In reality, I understand how light behaves (not quite as simple as “straight lines”, but that's a good enough approximation to explain mirrors). I know that you do, too. You aren't demonstrating that, though; you're just name-dropping facts without showing why they have to be true, why that's the only way the universe can work because if it didn't work that way, observations could be made that cannot actually be made.

I have not got a complete model of “flat earth cosmology” to present you, because I believe such a thing doesn't exist. I did have a consistent, incomplete model in mind when I was writing those comments, though: one where light curves upwards in a manner that causes the surface of a plane to appear spherical. (However, objects on the horizon would be a different size to on a real sphere, which could be used to falsify my model by comparing it with real-world observation.) I wasn't moving the goalposts.

Most desks have enough dust in certain areas or objects on them that you cannot see all the way across without an elevated vantage point.

What about this one:

A flat shape is impossible because the gravitational field would be vastly different on a planar body than on a spherical one, as I go further from the center of mass the further I go towards the edge.

And how gravity works can easily be demonstrated in a lab setting.

Flat-earthers already "changed" the physics of light propagation. I guess they would have no troubles bulls*tting their way out of gravity as well because none of them has actually the math/physics/etc basics to prove themselves wrong (what's a lab anyway, a brainwashing tool of the lying elite?).

Can it actually be demonstrated in a lab that gravity non-planar? One interesting explanation I've seen by the flat earthers is that gravity does not exist, instead the flat earth is simply under constant acceleration at 9.8m/s^2.

Yes. Very precise measurements of gravity on Earth have shown that, although it always points to the Earth's center, the force of Earth's gravity varies by latitude a tiny bit[1] (because Earth is a rotating oblate spheroid) and by elevation (because gravity varies by distance between center of masses). Neither of these can be accounted for on an accelerating disk Earth.

[1] https://www.britannica.com/science/gravity-physics/Accelerat...

Day length in Helsinki, Finland today is 6 hours, 53 minutes. It is now noon.

Day length in Melbourne, Australia today is 14 hours, 25 minutes. The sun just set half an hour ago.

If you know someone who lives in those places, you can call them for confirmation (unless they're part of the conspiracy).

How do you explain the observations with flat earth?

The explanations I've seen are akin to a lamp above shining onto a desk. The lamp moves across the plane of the desk and moves closer to the desk as nears Finland and further away as it moves toward Australia.

The problem is - explanations for each of these demand different flat earth models. Often several different flat earth model for several cases of one of these points.

For example to make 10 different flight paths over the pole make sense on flat earth you need 10 different flat earth maps contradicting each other. The only way to make all the flight paths make sense is round Earth.

Same with weather, day-night and seasons - you can get some of them with different flat earth models, but these models contradict each other. To get all the daily and yearly patterns at the same time you need round Earth.

As for tides I'd like to see even one good flat earth model where moon is visible in the correct place on the sky at the same time looking from equator, north hemisphere and south hemisphere. Most flat earth models put moon 180 degree away from "center of flat earth" opposite of the sun, which is obviously wrong (cause you can see moon and sun at once in many places).

And there's such problems with each point. But that's not a problem, because flatearthers don't want to have a consistent model of reality, they just want to think The System is out there trying to deceive them. So it's fine having 100 different contradicting models that explain one thing each.

> 9) The other planets are also flat and only appear round because they are tidally locked with respect to the earth.

Much harder to argue for this these days now that amateur telescopes/decent camera optics have become reasonably cheap and cameras too. Jupiter has a rotation period of only ten hours, so one can easily see it rotate within one night. The sun takes a bit longer since it has a rotation period of 25-38d (depends on the latitude which is quite interesting) and if there are no sun spots it's kind of hard to tell.

Laymen can use Eratosthenes of Cyrene's method from the mid 200's BCE.

https://www.khanacademy.org/humanities/big-history-project/s...

For those who don't know it, in brief: Put a vertical pole in flat ground at noon and measure the length of the shadow. The next day, drive an hour or two directly north or south and do the same measurement at noon again. With a bit of trigonometry, you can estimate the earth's circumference - or for our purposes, at least prove to yourself that the earth is not flat, as the shadows will be different lengths with the sun in nearly the same position both times.

Of course, there is a separate discussion about whether we should consider light trigonometry within the bounds of what modern laymen should be expected to handle.

As the article also states, the ancients observed that as ships passed below the horizon, the hulls disappeared before the masts - this is also within reach of most laymen.

>For those who don't know it, in brief: Put a vertical pole in flat ground at noon and measure the length of the shadow. The next day, drive an hour or two directly north or south and do the same measurement at noon again. With a bit of trigonometry, you can estimate the earth's circumference - or for our purposes, at least prove to yourself that the earth is not flat, as the shadows will be different lengths with the sun in nearly the same position both times.

Sure, but a flat earther will just argue that the sun is much closer than scientists claim, which explains the difference in shadow length. And again, what's a layman to do?

The best experiment to prove the earth is round that I've thought up involves the Coriolis Effect and a rifle fired at different points both north and south of the equator. Unfortunately that requires international travel with firearms, making it legally difficult to try.

Personally, I like multiple Foucault pendulums as this demonstrates rotation and curvature. I think the part that is missing in everyone's explanation is how to prove it is a sphere and not a hemisphere. Obviously, going to the "edge" would immediately shows this is false. But I think the only way to do this without moving too much is to record lunar eclipses and do the math to figure out that you would eventually see straight edges in an eclipse if the earth was not completely spherical.

Interesting idea! The sun would have to be about 6'000 km away I think. The full moon / new moon / solar eclipse proves that the moon is much closer than the sun. The moon would have to be dangerously close. Then, we know that the moon causes tides, and we can measure them as well on different cities. Being so close, the sun would have to cause tides as well, unless it is very light. It would have to take some time to calculate all that...

The only thing that is missing here is that the different lengths don't matter by themselves. The explanation could be that the rays of the sun are not parallel. Of course, this would put the sun closer than the moon but we are assuming we cannot use extraterrestrial knowledge. To complete the logic, you need to measure in multiple locations and note that the shadows do not change as expected if there was no curvature of the earth.

Totally true. More in general, it is a relatively simple mathematical fact that a sphere without a point is homeomorphic (even conformally!) to a plane. Therefore, it is impossible to prove that the Earth is spherical unless you either do measurements and run numbers (like you propose) or visit every single location of it (including all the seas and both poles; particularly the south pole, which for some reason flat earthers tend to assume it is missing). The only way to do the latter, as far as I can tell, is to be in a sufficiently polar orbit for enough time (or at least have a satellite in orbit), and few people are able to do that (directly, i.e., without trusting somebody else).

It is interesting that is not uncommon for flat earthers to be more "scientific" that many spherical earthers in recognizing that many spherical Earth arguments are not sufficient by themselves. Of course the spherical earthers happen to be right, while the flat earthers then proceed to invent completely implausible theories to support their beliefs, but they deserve some recognition for the first stage.

If you end your sample at N=1, sure, it might seem like “spherical earthers” just happened to pick the right set of beliefs by chance. But if you also consider things like 5G microchips, and chemtrails, and the moon landing, somehow the “spherical earthers” consistently pick the right beliefs.

Sure, there’s no critical thinking for a lot of it, in the same way that I don’t think critically about all the physics and engineering before I step on an airplane. The entire point of living in a society is that you don’t have to do everything yourself — you can outsource a lot of that critical thinking to other people who (are paid to) care more than you. You can just go with the flow and actually have a pretty good track record for believing the right things.

I don't know about flat-earth specifically, but lots of people only believe a few conspiracy theories. There are lot's of "spherical earthers" who believe the moon landing was faked for example. A very large fraction of the US believes in at least one of the major conspiracy theories associated with QAnon (several of which predate QAnon), but a much smaller fraction is actually involved with QAnon.

When you say, "layman like me can check what is the shape of earth?" Like reviewing satellite footage? I mean this seems ridiculously easy, unless you mean like can you prove it from your front yard.

This is slightly beyond what an average layman can do, but it's not terribly hard or expensive to receive images of our globe from NOAA weather satellites [1]. Generally the most costly part is a ~$120 USD antenna. It's a super fun and rewarding project if this is the kind of thing that interests you.

[1] https://en.wikipedia.org/wiki/Geostationary_Operational_Envi...

The problem is that then you aren't verifying it yourself, and a flat earther can claim you're just using one of the propaganda sources created by the government.

You can also use the ESA earth observation program, which provides free downloads of earth observation data every ~5 days. Look for Sentinel-2. It still won't prove that earth is round though :).

Yeah I think they mean the latter. Satellite footage could be doctored after all, unless you're launched the satellite.

It's not really that hard to estimate the circumference of the earth and prove that it has to be spherical. But most methods I know of require quite a bit of driving, or friends that are far away and can take a measurement at exactly the same time.

I'm not a flat earther but I do want to do my own measurement of the earth's circumference one day for fun.

> and prove that it has to be spherical

To be precise, the earth is not spherical. It's unfortunately not even an ellipsoid: I live well above the sea level yet my home is like 20 meters below the surface of the WGS84 reference ellipsoid.

A flat earther would tell you the "satellite" photos are faked, and could provide you with faked photos of their own.

The question is how you distinguish the two: always go mainstream? Always counter the old-fashioned ways? Trust the best credentialed person? Distrust credentials achieved during brainwashing, go with independent self-taught thinkers? Independently verify every claim, replicating experiments from first principles?

But they can always say anything is faked. They can say that my eyes distort what I see and that math can’t be trusted either.

The only way you could really prove it is to define a set of axioms that you some how convince them to agree to. And then get them to agree to logic. And then construct an experiment using those rules whereby if the Earth is round then they die. And see if they are willing to test it. And if they don’t then you win. And if they do it and die then you also win.

More like a geometric method used in https://news.ycombinator.com/item?id=29975544

The simplest way I could come up with while discussing the issue with my kids, was to call a grandma on the other side of the globe and ask here when did she have a sunset. It still could mean that the earth is a hemisphere, but at least it's not pancake-style flat.

Or that there is more than one sun. Once you are willing to throw all science out the window there are a lot of options.

There is a simple experiment to show curvature that was actually done by some flat earthers, and it proved curvature, but then they somehow classified it as an interesting but not conclusive result. Here's the experiment: take 3 wood planks, make a hole at the same eight in each of them. Then put them standing in the soil making sure that all have the same height (meaning that the hole is at the same height for all of them). Now use a laser to have a straight line of light (you can observe by other means that a laser beam is a straight line). If the wooden planks are far enough form each other, the laser will not e able to pass trough all 3 holes, despite them being at the same height from heart's surface. This implies curvature.

That's too prone to terrain irregularities, and I can't imagine that there's a single area of land on earth that's remotely as smooth as the surface of an sphere. Further, even if you assume perfect instrumentation, how would one account for gravity?

> I can't imagine that there's a single area of land on earth that's remotely as smooth as the surface of an sphere

Salar de Uyuni - "As a result, the variation in the surface elevation over the 10,582-square-kilometer (4,086 sq mi) area of Salar de Uyuni is less than 1 meter (3 ft 3 in) normal to the Earth's circumference".

That's less than 1 in 6371000 - pretty smooth, no?

> That's too prone to terrain irregularities

Nah, it's still doable, especially on that^ salt flat - if you have two close holes and a 3rd one 55km away, a flat earth will let you line up the laser through all three with less than 2m of adjustment. By my rough calculations, on a spherical earth, the laser will be ~250m above ground at 55km - pretty obviously proving the hypothesis incorrect, no?

That's a great example, thanks!

Sorry, I fail to see what would be the problem with gravity here. Do you mean we should consider that gravity curves the laser beam?

I think this is right, in the same way that you can't ever really "prove" anything. (Direct proof relies on firsthand experience, and human perception and memory are infamously faulty. QED. Alternately, even the strongest experimental conclusion can be chalked up to the most extreme of coincidences.)

However, to move forward in life, we must accept certain axioms so that we can be effective and productive. (Prove food is nutritious, prove I won't die in the next 5 minutes, prove my parents are benevolent actors, prove my children are actually mine). At some point, you can litigate anything you want, and the real decision is, what hills are you willing to die on. For me, that's determined strongly by impact to my life, and also partly by interest.

What I'm saying is, don't argue that the flat earthers are wrong. Argue that they are useless because what they are arguing about doesn't matter.

You can check the shape of the earth by replicating the early experiments performed by ancient scientists who discovered the earth was spherical in the first place. Detailed and very easy to follow instructions for this also appear on YouTube

For me, this was "notice that Orion is upside-down in Australia"

Draw a 6 on the ceiling, and the person opposite you in the room will see a 9. That doesn't mean the room floor isn't flat.

Orion isn't directly overhead. In your analogy I'd be drawing a 6 on the wall, and the other person would have to bend over backwards to claim it's a 9.

I mean, I guess they could, but they'd clearly be bending over backwards to make a logical fallacy.

> Orion isn't directly overhead.

Maybe not for you :)

In my example the number need not be directly overhead either. It could be drawn in the middle of a very long room where you and the other person are at opposite ends. You're both looking at it at an angle and both still seeing different numbers, although in a very skewed perspective - I don't know how flat earthers account for perspective and projection in their skydome or whatever.

Well, flat-earthers appearently believe that Australians are all actors paid by NASA to convince everyone the earth is a globe. I'm not going to waste much time listening to them.

The Orion-is-upside-down-here thing was the first time I noticed something that drove it home to me that we live on a sphere.

You do not even need to trust the established science. There are experiments done by flat earth believers which proves that earth is actually not flat at all.

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