[flabdablet]

Could somebody more familiar with Flat Earth theory than I am please explain to me what tools I would need to use in order to prove beyond all doubt that a line I'm following, by walking or driving or flying an aeroplane or sailing a boat along it, is truly and completely straight and does not deviate up or down or left or right from that straightness even by a little bit? Would prefer simple tools that NASA could not hack, if any exist.

If I'm finally going to get clear on what shape the world is, this seems to me to be where I'd need to start.

Thanks in advance.

[DSC]

Laser.

[Ratboy]

Laser.

Go to one of those mesas in Arizona or some other similar location.  Take three fence posts and this surveyor's laser.  Pound your first post.  Walk a ways and put in the second post using your laser to get the top in line.  Then go further and put in the third using the laser so that all three tops are in a nice straight line.  Now that you are done, I forgot to mention that you should do this so you can look down below the top of the mesa into the lowlands.  And now that I think of it, use a level to make the posts in line and the tops perfectly horizontal.  Oh ya, make the line run east and west more or less. Now at sunrise (or sunset depending on your posts) look along the posts to see where the sun is.  You know where the sun is directly overhead at that time (check local weather sites to find out if you do not know).  Then figure out how far away that is.  Let us say it is 6000 miles.  If the sun is 3000 miles up, the angle should be 26 degrees up.  Why is the sun lower than your sight line?  Done. 

[flabdablet]

Lasers are not going to be convincing enough. I've seen round-earth defenders say they're subject to atmospheric refraction effects near the surface, and flat-earth defenders say that perspective messes them up. Looking for a straight-line reference that everybody will agree makes a straight line that I could in principle walk and/or fly along and doesn't ever deviate from perfect straightness by less than ten parts per million under any conditions. Not really interested in talking about sun angles and whatnot until that's sorted out, so I'll leave those points to others.

[Dr David Thork]

Could somebody more familiar with Flat Earth theory than I am please explain to me what tools I would need to use in order to prove beyond all doubt that a line I'm following, by walking or driving or flying an aeroplane or sailing a boat along it, is truly and completely straight and does not deviate up or down or left or right from that straightness even by a little bit? Would prefer simple tools that NASA could not hack, if any exist.

If I'm finally going to get clear on what shape the world is, this seems to me to be where I'd need to start.

Thanks in advance.

A perfect gyroscope is the only thing you can use to see your position in space changing. But you need one that hasn't been wickedly tampered with to suggest it compensates for precession (the round earth get out).

[Ratboy]

Could somebody more familiar with Flat Earth theory than I am please explain to me what tools I would need to use in order to prove beyond all doubt that a line I'm following, by walking or driving or flying an aeroplane or sailing a boat along it, is truly and completely straight and does not deviate up or down or left or right from that straightness even by a little bit? Would prefer simple tools that NASA could not hack, if any exist.

If I'm finally going to get clear on what shape the world is, this seems to me to be where I'd need to start.

Thanks in advance.

Before lasers, surveyors used tight chains to get things straight.  The entire western prairies of North America are divided into square mile plots of land.  In many places the roads are spaced every mile going east and west and every two miles going north and south.  Going at right angles, the roads no longer point north. So the "correction line" roads going north and south have to keep shifting over so that there are fewer north running roads the farther north you go.  The same thing happens when you go south in the southern hemisphere.  You know you are going north because you have the north star directly in front of you.  It would be pretty hard to convince a farmer that he is being fooled into thinking he owns more or less land that he actually has.  No lasers.  Pretty hard to pull off a hoax that tricks every farmer in the west.  Hope this satisfies you.

[HorstFue]

As said before, laser or other light rays are suspect to refraction in normal atmosphere.
The only really straight line I can think of is a light ray in vacuum. So you have to establish a vacuum around the light ray.
Take a long pipe, evacuate it, and send the light ray through it. But this could be a tedious job to make such a pipe 'straight'. It will bend, alone by it's own weight. One such thing I came across can be found at Stanford.
https://en.wikipedia.org/wiki/SLAC

[Ratboy]

Not sure why we need a laser in a vacuum.  We did not specify perfect, only to within 10 parts per million.
If you walk around Colorado, you would have to deviate 15 feet from the border going north/south or 20 feet from the border going east west to be out by 10 parts per million considering the length of the borders.  A surveyor would be fired if they put a stake 15 feet from where it is supposed to be.
So walking around Colorado, you find the east and west borders are 280 miles long, which they should be considering they are the 25th and 32nd meridians, between the 37th and 41st parallels (they are called parallels because they are parallel, unlike meridians).  When you walk along the north border you find it is 370 miles long following the 41st parallel. The southern border is 390 miles long.  If you follow the surveyed border it should keep you within the 10 parts per million as mentioned above.

This is easier than building a vacuum tube long enough to do something with and I am not even sure what you would do with a tube 100 miles long. 

[JohnAdams1145]

Unfortunately, the Earth is so large and the curvature so small that I doubt your method would be able to ascertain its flatness given the uncertainties involved. You could try using high-energy light like gamma or x-rays, which are not easily bent by the atmosphere, but that also means that you can't really control where they go with lenses. If you follow an x-ray, it'll be mostly straight.

[ShowmetheProof]

I agree with John Adams, our second president back from the dead!  It wouldn't be very easy.  The only way I can think of off the top of my head is sending a small craft to space and making sure it has cameras that haven't been tampered with.

[flabdablet]

When you walk along the north border you find it is 370 miles long following the 41st parallel. The southern border is 390 miles long.  If you follow the surveyed border it should keep you within the 10 parts per million as mentioned above.

Taking the surveyed border as a straight line wouldn't be the right thing on a globe, which is kind of the point of this entire line of inquiry.

If I had some means of generating a completely straight guideline that everybody - flat Earth theorists and globe theorists alike - could agree did generate lines that really are objectively straight, then all that would be required to settle the matter would be to set up a straight line that grazes both the peak of some mountain and the peak of another that projects above the horizon when viewed from the first, then send teams to follow the line in both directions.

If the Earth is flat, then either one of those teams would find a place where the line runs into the ground, or both would end up at the edge of the Earth.

If the Earth is a globe, then both of those teams would end up attempting to fly into space.

I would be interested in critiques of this thought experiment - especially on the definitiveness of its predicted outcomes - from a flat Earth theorist's point of view.

[Ratboy]

Flat earthers have to believe in straight lines since the whole long and tedious beginning of the Earth is Not a Globe by Rowbotham is all about how his lines were straight. If we have to throw out his experiments because we do not believe him and his measurements, why are we even here?  The whole point of the argument is that his measurements were straight. If we do not trust Rowbotham being able to look at a straight line, then we have to doubt his book and there is no basis for a flat earth.  If the glove does not fit, we must acquit.

[Macarios]

For equinox go to Equator.
There's place in Brazil caled Sao Gabriel de Cachoeira.
In the morning Sun is above Macapa, Barsil. From Sao Gabriel de Cachoeira you see Sun at azimuth of 90 degrees, which is directly to the East.
In the afternoon Sun is above Quito, Ecuador. From Sao Gabriel de Cachoeira you see Sun at azimuth of 270 degrees, which is directly to the West.
Macapa is at Equator, Qito is at Equator, and line the Sun travels during the equinox day is straight.

So, if you fly between Macapa, Brasil and Quito, Ecuador via Sao Gabriel de Cachoeira, you will fly in straight line.

Same goes for flight between Kampala, Uganda and Libreville, Gabon via Bosio, Congo, and Mbandaka, Congo.

Sun path above those places for equinox is straight line above heads of all those people, and they know it.

[JohnAdams1145]

For equinox go to Equator.
There's place in Brazil caled Sao Gabriel de Cachoeira.
In the morning Sun is above Macapa, Barsil. From Sao Gabriel de Cachoeira you see Sun at azimuth of 90 degrees, which is directly to the East.
In the afternoon Sun is above Quito, Ecuador. From Sao Gabriel de Cachoeira you see Sun at azimuth of 270 degrees, which is directly to the West.
Macapa is at Equator, Qito is at Equator, and line the Sun travels during the equinox day is straight.

So, if you fly between Macapa, Brasil and Quito, Ecuador via Sao Gabriel de Cachoeira, you will fly in straight line.

Same goes for flight between Kampala, Uganda and Libreville, Gabon via Bosio, Congo, and Mbandaka, Congo.

Sun path above those places for equinox is straight line above heads of all those people, and they know it.

You see, the problem is if people can fly in straight lines (really great circles), FE theory cannot work. If you can fly in a straight line, then you'll see that you can track a line of latitude; with FE this should be impossible. If you can fly in a straight line, you can measure distances which would disprove the FE theory (as has already been attempted). If you can fly in straight lines, you can measure the angles of a triangle, which disproves FE theory.

So FE theory will always point to a "distortion" or "not perfectly straight" argument to throw out these proofs.

So I propose something else: why not use a statistical argument to prove something? We should note that by symmetry the deviations will be about 50% to the "left" and 50% to the "right". I'm sure the compass experiment could then be repeated many times and we could figure out exactly what's happening.

[Macarios]

For equinox go to Equator.
There's place in Brazil caled Sao Gabriel de Cachoeira.
In the morning Sun is above Macapa, Barsil. From Sao Gabriel de Cachoeira you see Sun at azimuth of 90 degrees, which is directly to the East.
In the afternoon Sun is above Quito, Ecuador. From Sao Gabriel de Cachoeira you see Sun at azimuth of 270 degrees, which is directly to the West.
Macapa is at Equator, Qito is at Equator, and line the Sun travels during the equinox day is straight.

So, if you fly between Macapa, Brasil and Quito, Ecuador via Sao Gabriel de Cachoeira, you will fly in straight line.

Same goes for flight between Kampala, Uganda and Libreville, Gabon via Bosio, Congo, and Mbandaka, Congo.

Sun path above those places for equinox is straight line above heads of all those people, and they know it.

You see, the problem is if people can fly in straight lines (really great circles), FE theory cannot work. If you can fly in a straight line, then you'll see that you can track a line of latitude; with FE this should be impossible. If you can fly in a straight line, you can measure distances which would disprove the FE theory (as has already been attempted). If you can fly in straight lines, you can measure the angles of a triangle, which disproves FE theory.

So FE theory will always point to a "distortion" or "not perfectly straight" argument to throw out these proofs.

So I propose something else: why not use a statistical argument to prove something? We should note that by symmetry the deviations will be about 50% to the "left" and 50% to the "right". I'm sure the compass experiment could then be repeated many times and we could figure out exactly what's happening.

You don't need flight.

Globe Earth, equinox (March 21st):
Sunrise is directly to the east at azimuth of 90 degrees.
Sunset is directly to the west at azimuth of 270 degrees.
It is like that anywhere on Earth.

Flat Earth, equinox (March 21st):
Sunrise and sunset depend on observer's latitude.
If you are on Equator your sunrise is at azimuth of 45 degrees (NorthEast) and sunset at azimuth of 315 degrees (NorthWest).
If you are more to the north, sunrise azimuth is closer to East and sunset azimuths is closer to west.
If you are more to the south, sunrise and sunset azimuths are more to the north.

Draw north pole at center of paper, Ice Wall as big as possible around it, and Equator with half ot Ice Wall radius.
Then draw line of solar noon from north pole outward, and sunrise and sunset will be on equator 90 degrees left and right.
Then measure angles where sunrise and sunset will be visible as you move observer position along solar noon line.
Finally, compare with angles measured in real life on March 21st.

EDIT: Like this, and it doesn't change however you draw continents.
.