# Can you detect the curvature of the Earth with a taut line 3 miles long? [No]

You know this is why I love you guys at MB. Around here its considered "complicated" to take standard 100lb test line which is nearly neutrally buoyant ,as it is, and place a length of it into a body of water while stretching it out
No, that in itself not a complicated idea. But playing out three miles of line with no tangles, anchoring it to a suitable point a fixed distance below the surface, applying tension to it without breaking it, and, let's not forget, finding and keeping track of the near-invisible fishing line a foot or more below the surface of the water, one and a half miles from the nearest point of reference?

That doesn't sound simple to me. If it does to you, then by all means do it. I would love to see the results, and I mean that quite seriously. I am not trying to be needlessly negative, it's just that I can think of some much easier ways to demonstrate curvature.

Although in theory it sounds like an easy experiment, as others wrote, in practice it would be rather difficult. At three miles, the height of the Earth curvature in the middle is some 45 centimeters (around a foot and a half). Now you would need a perfectly flat water surface with no waves and no currents. But not only that. You would have to exclude that no garbage, fishing lines or nets, dirt, sediments, water plants, fish/plankton, or gas bubbles impact the buoyancy of your line over all the length. You would have to exclude any boat traffic in the proximity. You would have to account for the influence of the tide, atmospheric pressure difference, wind, water temperature, depth, sun exposure, evaporation rate, and vertical currents - all of that can have quite important impact on irregularities of the flatness of water body surface (look up for example the term Atmospehric tide in WikiPedia). And even if you eliminated or accounted for all of those named effects, and measured the expected dip of ~45cm in the middle, the Flat Earth supporters would still come with some easy explanation - like for example that an Illuminati diver pulled your line to the bottom, so you would have to have cameras all over the 3 miles to prove the opposite.
You know, I'm fairly certain that, for the most part this would shut up the flat earthers (at least the ones not willfully ignorant) for the better part of a century. Make it 5 or 10 miles and 10,000 lb line with an army of "tempory duck relocators", slowly adding tension to the line over the course of a week with minutely measurements. .. At some point the flat earth argument gets brushed off with "hey, now you dont need a reservation on the ISS to see the curve. What you do is grab your tackle box..." Something repeatable, on a smaller scale perhaps, by anyone with the inclination would fix most of the problem.

No, that in itself not a complicated idea. But playing out three miles of line with no tangles, anchoring it to a suitable point a fixed distance below the surface, applying tension to it without breaking it, and, let's not forget, finding and keeping track of the near-invisible fishing line a foot or more below the surface of the water, one and a half miles from the nearest point of reference?

That doesn't sound simple to me. If it does to you, then by all means do it. I would love to see the results, and I mean that quite seriously. I am not trying to be needlessly negative, it's just that I can think of some much easier ways to demonstrate curvature.
Well Chip lake (Alberta) is half frozen here atm. It's 28 square miles. Next summer will be ideal. now to see if a manufacturer of monofilament will donate to the cause

The length of the line won't change anything on the impact of all those effects I listed above - the longer line, the bigger effect they'll have, and vice versa. Hence executing the test will be still quite a tough task, and probably much less conclusive than a plain sight on disappearing masts of boats, or photos from the ISS, and plenty of other available clear evidence showing the nonsense of the flatearthers' claims.

Before you start developing your idea, you should propose the test on the Flatearthers forum - I bet they'll come up with arguments showing you clearly that any attempts to show the the truth are futile anyway.

The length of the line won't change anything on the impact of all those effects I listed above - the longer line, the bigger effect they'll have, and vice versa. Hence executing the test will be still quite a tough task, and probably much less conclusive than a plain sight on disappearing masts of boats, or photos from the ISS, and plenty of other available clear evidence showing the nonsense of the flatearthers' claims.

Before you start developing your idea, you should propose the test on the Flatearthers forum - I bet they'll come up with arguments showing you clearly that any attempts to show the the truth are futile anyway.
Well to be honest they are of the opinion that it is the round earthers who will find the flaws in the evidence. Question for you though... have you ever watched a boat sail off over the horizon with your naked eye and then pulled out binoculars to restore it, watching it sail over the horizon again?... One time I brought my 8 inch refractor and got to see it a third time. It definitely cast some doubt in my mind on the shape of the world.

Question for you though... have you ever watched a boat sail off over the horizon with your naked eye and then pulled out binoculars to restore it, watching it sail over the horizon again?... One time I brought my 8 inch refractor and got to see it a third time. It definitely cast some doubt in my mind on the shape of the world.
It's a neat trick, but it is still a trick. If the observer stays at a high ground, say 20 metres above the sea level, the horizon will be about 16 km (10 miles) away. At this distance, depending on lighting conditions, the boat may "vanish into thin air" well before it sails over the horizon. Unlike vacuum, air is not absolutely transparent, and 10 km of it between the observer and the boat probably will scatter more light to the naked eye than comes from the boat itself. Binoculars will help to cut off some of the scattered light, enabling the observer to see the boat again, but a more powerful optics may be needed to see the boat actually sailing over the horizon.

One time I brought my 8 inch refractor and got to see it a third time. It definitely cast some doubt in my mind on the shape of the world.
Not sure what doubts you have (perhaps doubts about the quality of your sight in comparison with binoculars), but once a boat disappears below the horizon, even the best telescope won't help you to see it again (unless it comes back, or there are really big waves). I witness it daily. What you speak about, is not disappearing below the horizon, but disappearing because of limitation of our eyesight, or because of bad atmospheric visibility (as Trailspotter correctly pointed out).

Have a look at the following video that demonstrates what I mean. If the Earth was flat, the dipping of the boat and the mast below the horizon would mean every boat approaching the horizon sinks (or falls over the edge of the Earth :-D ). That would not explain the opposite motion - boats appearing from below the horizon when they approach the observer.

Moderator Note - deirdre
The topic of this thread is 'can you detect curvature with a rope'. Any more off topic comments will be removed

It wont be. However in a round earth it will, in all cases move further from the surface as any tension is applied. The "straighter" we attempt to pull it the more the curve of the earth should be shown at the mid point.
And if I were a flat earther, I would just brush it off by saying that the "curve" you measure is because the line is sagging a lot. If you have no way of measuring the curve of the line itself there's no way to use it as a reliable reference point.

And if I were a flat earther, I would just brush it off by saying that the "curve" you measure is because the line is sagging a lot. If you have no way of measuring the curve of the line itself there's no way to use it as a reliable reference point.
True they can say anything they want. Even though neutrally buoyant is neutrally buoyant...however the line, measured as an average over, time will show a relative "dip" at the center point in direct proportion to an increase in tension from any source other then a mechanical upward thrust at the mid point. Any slack in the line should cause a distortion and lateral movement in the direction of the strongest current or force. I digress... the point here though, is to be able to say "Hey toss you're tackle box in the tin boat and look"... this is the only language that true flat earthers understand.

We tell you it toss it away, because there are millions of other direct or indirect clues of evidence clearly showing the Earth is round, which are way less complicated, and much more reliable than your proposal. Still, they are ignored by the FE folk, which means there is no chance you could persuade some of them with your line test.

You still ignore the earlier listed effects - the buoyancy and the depth of the line can be influenced instantly or within minutes by microbubbles of gas, by sediments and dirt, plankton or plants, by objects floating in the water, by animals or humans, by waves, sunlight, evaporation rate, wind, by vertical and circular currents, and who knows how many more effects. How do you want to eliminate them all, so that your experiment can be considered reliable?

Last edited:
True they can say anything they want. Even though neutrally buoyant is neutrally buoyant...however the line, measured as an average over, time will show a relative "dip" at the center point in direct proportion to an increase in tension from any source other then a mechanical upward thrust at the mid point. Any slack in the line should cause a distortion and lateral movement in the direction of the strongest current or force. I digress... the point here though, is to be able to say "Hey toss you're tackle box in the tin boat and look"... this is the only language that true flat earthers understand.
Is it truly neutrally buoyant, or is it just close enough to neutrally buoyant that your average fisherman isn't going to notice the difference?

The problem with using this as a sort of "layman's test" is that I doubt many people are going to have miles of fishing line and fully stagnant water free for the testing. If "grab your tackle box" is an easy and convincing way to measure curvature, "grab your binoculars" should be even easier and just as convincing.

I've been thinking about an idea for measuring the curve without relying on long light paths since they suffer from some refraction and introduce a realm of unknown in the minds of some people.

For example I measure the predicted dip with my theodolite on distant mountains. But I have to rely on terrestrial refraction to correct my reading, and how do I prove to a flat earther that it's not just a fudge factor or that the curve just appears that way due to some aspect of optics instead of actual curve? (Rhetorically I ask. This post is about something else.)

I'm thinking of taking a couple KM of neutral density (but slightly floating and super strong) string strung super tight at the surface of a calm freshwater lake.

In theory, the string would drift up when slack, but then sink down about 3 inches in the center when super tight - thus proving that the water is curved without having to rely on long light path.

A doubter could not argue that the string was sagging due to it's own weight because by letting the string go slack, all could see that it slowly floated up to the surface. But when tightened, it would sink down. And the depth could be easily measured at any number of points along the string, and a curve could be plotted.

The string I have in mind is supposed to come in 1000 meter rolls for \$28 on amazon. It's supposed to be Polyethylene and have a 100lb test. I'm thinking a red roll and a green roll for 2km, tied in the middle, so there's no question about where the center point is.

I realize that the slight buoyancy would bend the results toward flat earth but that could possibly be eliminated if there was very little buoyancy and the string was really tight.

It's kind of a long shot, but I'd love to hear what others think!

I'm thinking of taking a couple KM of neutral density (but slightly floating and super strong) string strung super tight at the surface of a calm freshwater lake.

You posted this in a new thread, but I've moved it here as there are some very similar ideas in this thread.

You posted this in a new thread, but I've moved it here as there are some very similar ideas in this thread.

Regarding this idea of a very slightly buoyant string under the water's surface:

I agree that even if this works to show a curved earth, it won't convince most flat earthers. Then of course, we all know that nothing in the world we might present to them will convince them. Even if we flew them to space they could just say it was a virtual reality machine that simulated freefall and never left earth. But my goal is not to change all flat earthers, but to find proofs that work for me and work for other truth seekers.

I agree that this is a difficult exercise, but it is also fascinating because it doesn't rely on long light paths. And we all know long light paths *always* add complexity of their own whether it's refraction or mirage: A method which is simple to explain and has very little room for outside undetectable influences - such a test is worth being tricky to perform.

(And I am by no means diminishing long-path optical tests. When I sighted to the top of Mt Baker from 75 miles away, I calculated the height within 1.07 feet - which obviously was pure chance because my theodolite isn't that accurate, and neither was my figure for observer location. I would expect an error of at least +/- 50 feet I'm sure.)

Anyway, I see this concept as being rather unique in it's plain directness and it's lack of reliance on the unknown.

It's one of very few demonstrations where you could literally paddle up in a canoe, reach out and touch the curve.

The only valid objection is that the line was sagging, and that could be easily dispelled by letting the line go slack and see it slowly float up.

The line I'm eyeing is this: https://www.amazon.com/gp/product/B075TTJ995/?tag=cowboyprogra-20
It comes in different colors and in a 1000m roll of 100lb test. I don't know if it's heaver or lighter than water though.

I have a little lake not too far from me which can be glassy smooth, so that's where I'm headed with this.

Using the formula for power line sag, upside down, to estimate the rise of the line in the middle, I get the following:
Sag=WL^2/8/T where W = Weight of line in N/m, L = Length between support points in meters, and T = Tension in Newtons

The specs say the diameter of the line is about .55mm. There is probably a little air between the fiber braid, and I'm going to assume a square cross section of .5mm for ease of calculation. So that's .5mm^2 or .25 square mm. One meter then has a volume of 1000mm*.25mm^2 = 250 cubic mm. There is a million cubic mm in a liter, so that is 250/1000000 L or .000250kg if the density is the same as water. Now multiply by 10 to convert kg to Newtons so .0025N/m for weight in air. But it's supposed to be a little less dense than water, so lets say at 90% water density its floating force is 10% of its weight in air, so 0.1*0.0025N/m = .00025N/m = W
Finally, Rise = (.00025N/m)*(2000m)^2/8/250N = .5m

If you can get the line density within 1% of water then you can get the float to be similar to earth curvature, but I'd say you need to get within about .2% of water density to give a clear demonstration. And then errors might be hard to avoid.

The good news about this sag formula though is that it looks like a good way to substitute for or verify the straightness of a 2m straight edge. Using the line above and estimating its density the same as water gives .0025N/m for W, so:
Sag = .0025N/m*2m^2/8/250N = .005mm or about 1/10,000 of an inch.

That's excellent. Is it really that good or did I mess up that calculation? One could wrap a 2m builders level along both the top and bottom with string to keep the load on the level in straight compression and to allow comparison of the sag above and below the level. Spacers of accurate thickness at each end could preserve a gap between the string and the level surface. Then for full verification of straightness, the gap between the string and level could be compared with a magnifying glass and/or feeler gauge when the level is horizontal and when hanging vertically to eliminate sag. We could have issues with the string being visible to the camera. Painting the string black with a marker could make it more visible if one doesn't have a black string. The chemicals in a permanent marker might weaken the fibers though. A short teeter toter at one end of the level could equalize the tension in the top and bottom string to prevent level bending.

Replies
0
Views
3K
Replies
26
Views
4K
Replies
45
Views
7K
Replies
31
Views
6K
Replies
134
Views
27K