[Bobby Shafto]

I've seen this touched on before, but many globe earth proponents and globe earth skeptics alike expect to be able to see curvature along the lateral horizon. But that's a mistake. On a sphere, the surface curves AWAY in all directions from a vantage point. Attempting to prove earth's curvature by finding curve along the horizon is as incorrect as trying to disprove the "rotundity" of earth by demonstrating that there is no curve along the horizon.

Samuel Rowbotham makes the same mistake in his Experiment 7 of "Earth Not a Globe" http://www.sacred-texts.com/earth/za/za12.htm

[Bobby Shafto]

Fascinating simulation:

http://walter.bislins.ch/bloge/index.asp?page=Flat-Earth%3A+Finding+the+curvature+of+the+Earth

[edby]

Re your OP, reminds me of a quote attributed to Wittgenstein

Meeting a friend in the corridor, Ludwig Wittgenstein (1889-1951) said: "Tell me, why do people always say that it was natural for men to assume that the sun went around the earth rather than the earth was rotating?"
 His friend said: "Well, obviously, because it just looks as if the sun is going around the earth."
 To which the philosopher replied: "Well, what would it look like if it had looked as if the earth were rotating?"

Likewise, what would it look like if the Earth looked spherical? Surely, the way it looks now.

[Bobby Shafto]

Likewise, what would it look like if the Earth looked spherical? Surely, the way it looks now.

In the same way, curvature along the lateral would appear if the Earth is a flat circle.

Here are two outputs from that sim I posted. One is using a FE model. The other, a globe model. I removed grid lines to take away surface shape clue. Which is which?



If one is a flat earth, why is it showing curve?

[Max_Almond]

If one is a flat earth, why is it showing curve?

Because the observer is incredibly high above the disk, and with a very wide-angled view?

There's a thread here about whether it's possible to see the 'lateral curve':

www.metabunk.org/a-side-view-of-the-curvature-of-the-earth-at-lake-pontchartrain.t9268

We never really reached a conclusion.

It is possible, however, to capture the 'curve of the horizon', as Rowbotham said we should. He just needed a bit more altitude, that's all. And some ability to compress an image of what he is seeing:

www.metabunk.org/measuring-the-curvature-of-the-horizon-with-a-level.t7832

[Manny141]

This is what so many people don’t care to look at. Why are people so ignorant to see the basis behind the flat earth theory? We need to spread this information all around the globe. Though this information has already spread across the horizon because of the internet we need to expand it. If people see these facts, I trust you that we can convert many.

[BigGuyWhoKills]

A YouTuber once phrased this in a way I hadn't thought of before.  She asked what do FE'ers expect to see from the lateral curve.  As they continue to turn, do they expect the horizon to eventually appear vertical?  Here is my ELI5 version:

Imagine standing on one of those 12' diameter jumbo beach balls.  You can clearly see the curvature of the beach ball in front of you.  A small area directly in front of you is horizontal, but curves down as your eye moves away from center.  So we turn 90° to follow the curve.  What do you see?  The exact same curve, still horizontal in front of you, and still curving down and away from that center.

Now imagine if we made that jumbo beach ball the size of a football stadium.  You can still see the curve, but that curvature is significantly reduced.  You turn 90, and see the same horizontal section that gradually curves down and away.  Now imagine if the jumbo beach ball was the circumference of France.  At this size, you would not discern any curvature.  So why do some FE'ers think a 6' tall person would see the curvature of a ball that is 25,000 miles in circumference?

This proves we can not turn to see more of the curvature.  But you can clearly see the "vertical horizon" if you hop off that 12' beach ball, and step back a ways (about 1 radius away).  How would a FE'er do that for Earth?

[HorstFue]

The horizon is a circle around the person. The person, at least his eyes is not at the center of the circle but somewhat above. So the person is looking at that circle with a tilt or dip. The projection of that circle would only be a straight line, if the person would be at the exact center of the circle. If the observer is above that circle, you get the projection of an tilted circle, an ellipse.
So in the field of view - it's essential not to change the view or orientation of the observer - the ends of the horizon would appear a tiny bit lower than the middle, curved down.
I estimate this additional dip at the sides would be around half the dip in the middle.

I now calculate with the nautical formula, as this can easily be converted to arc minutes.
One arc minute is one nautical mile.
The formula for the distance of horizon in nautical miles (nm) is: 2.1 * sqrt(hight of observer in meters).
With above the result is also the dip in arc minutes.

So why do some FE'ers think a 6' tall person would see the curvature of a ball that is 25,000 miles in circumference?

I would say Yes and No.

• No, if the person is placed at zero level.
   2 meters hight would give a dip angle of only 3 minutes (3 nm), the additional dip at the sides would be 1.5 arc minutes. With an eye resolution of max. 1 arc minutes, this is barely visible in any case
• Yes, if observer is lets say 100 meters above zero level, with very good atmospheric viewing conditions AND observer has an additional reference line.
  The distance to the horizon and so the dip would be 21nm or 21 arc minutes.
  The additional dip at the sides about 10 arc minutes. Might be visible with a reference line, but never without a reference.

For the experiment from above, top of thread:
Rowbotham even got the right numbers, but overestimated them, painted some grossly disproportionate "Paper CGI" and did not apply the necessary accuracy:
For 10 statute miles he got an additional dip of 66 feet.
that's 16090m versus 20m
atan(20/16090) = 0.07° or 4.2 arc minutes - barely visible, unless you apply high accuracy.

[Bobby Shafto]

I lifted this from a video by someone using an infrared filter to extol the possible evidence of earth's flatness. As with the other videos I've found of the videographer using IR to show evidence of a flat earth, I feel they do the opposite.

In this case, I was surprised to think I was seeing lateral curvature. I've been skeptical that curve along the transverse/horizontal was detectable from typical airliner altitudes due to the "fuzziness" of the horizon at the land/atmosphere boundary. But here, with an IR filter to cut down on the atmospheric haze, the boundary is clearer. Not totally clear, but clearer.

When I overlay a straight line from left to right at the boundary, the earth does "bulge" in the middle.

According to the author, this was at an altitude of 33,000' flying eastward near Las Vegas, NV. The prominent ridge is Mt Charleston, and by comparing slant perspective using Google Earth, I estimate this was a 45-50° angle of view with a horizon distance of 225 miles or so. At that dip, the distance from side to side is around 175 miles.


Looking at a lateral horizon of 175 from an elevation of 33,000 or the earth (radius 3959 miles), the curve should look about like this:



Look again at the IR image. On the far right, there is an elevated band of clouds with a dark band between it and the earth/near earth weather. That upper white band peters out  toward the left side. If I overlay this image with the curve graphic, scale and align the images together, the arc of the curve graphic lines up with that lower earth/atmosphere boundary from edge to edge. It's just the right amount of curve.

If that curve is being caused by the window or by the camera lens, it's a remarkable coincidence.

[Bobby Shafto]

- from JTolens, video with IR filter from 33,000 ft.