RE community will accept defeat when the Earth some day really becomes flat.

Considering the observations, measurements, laws of physics, facts and figures, it is highly unlikely that it will happen in any near or far future.

But these things are not about "victory" and "defeat".

When we stand on the surface the line going along the horizon all around us looks flat because the drop is equal and too small wherever you look.

But any line going on the surface from observer towards the horizon and beyond is clearly curving downwards and after the horizon can't be seen any more.

If the Earth was flat that line would be visible for much farther than now.

Now, how high you have to be to see the horizon curved (if we exclude refraction)?

From an altitude of 1.8 m the distance to horizon is 4.8 km.

The circumference of the horizon is 30.16 km.

If we assume the viewing angle of 60 degrees, then the observed part of the horizon is an arc of approx. 5.026 km with cord of 4.8 km.

This arc has height of 643 m.

From the distance of 4800 meters and altitude of 1.8 meters that height is seen under the angle of 0.0221 degrees.

It is 89.9779 degrees away from the perpendicular view.

Cosine of 89.9779 degrees is 0.000386 and those 643 meters look like 0.248 meters (say 0.25).

The bulge of 25 centimeters surely can not be visible on the line 4800 meters long.

Take a high resolution photo of such horizon, make it 4800 pixels wide, and the curvature would have to be

**one quarter of a pixel**.

On a Flat Earth horizon would seem flat as well, but surely not just 4.8 kilometers away from observer.

So, from 1.8 meters we can't see the curve.

Can we see it from 10 000 meters?

Horizon distance is 357 km, circumference 2243 km, 60 degrees arc length is 374 km, arc width 357 km.

Arc height 47.83 km, seen under the angle of 88.4 degrees away from perpendicular view, appears like 1.33 km.

3 x 357 = 1071

3 x 1.33 = 4.

So, if we make image 1071 piels wide, the bulge would be 4 pixels.

Probably not enough to see with naked eye, but if we keep the height and compress the width, the bulge will be visible.

Like in the image below.

The left wide part is 1071 pixels wide image of a dawn from 10 000 meters. Looks pretty flat.

The right, narrow part is the same image compressed horizontally to 5% of the original width.

You can see how the middle of the horizon bulges just a bit.