Also note that you can get software to simulate it, like POV-Ray.
This is a ray tracer program that renders 3D images.
You can do this for a flat Earth or for a round Earth, at various sizes and scales and see what it looks like.
For example, the code to look at a flat or round Earth, ignoring the atmosphere:
#version 3.7;
global_settings { assumed_gamma 1.2 }
#declare calt=228600;
#declare rad=6371000;
#declare frad=20000000;
#declare ft=1000;
#declare horang=acos(rad/(rad+calt));
#declare hypos=cos(horang)*rad-rad;
#declare hzpos=sin(horang)*rad;
camera {
perspective
location <0, calt,0>
angle 60
right x*image_width/image_height
look_at <0,hypos,hzpos>
//look_at <0,0,frad>
//look_at <0,calt,10>
}
light_source {< 0, rad*10,0> color rgb <1, 1, 1>
}
light_source {< 0, 0, rad*10> color rgb <1, 1, 1>
}
light_source {< 0, 0, -rad*10> color rgb <1, 1, 1>
}
declare rearth=sphere { <0,-rad,0>, rad
texture {
pigment { color rgb <0, 0, 1> }
}
}
declare fearth=cylinder { <0,-ft,0>, <0,0,0> frad
texture {
pigment { color rgb <0, 0, 1> }
}
}
object {rearth}
//object {fearth}
The first line is version information and stuff about gamma (how colours combine, and is just at the top of the file).
The next 4 declare the altitude of the camera and the radius of round Earth, then the radius of flat Earth and the disc thickness. These can be in whatever units you like, as long as you keep them consistent, e.g. you can have both in feet, miles, inches, m, mm, km, etc, but you can't have the radius in km or miles and the height in feet.
The next work out the angle to the horizon and the position of the horizon.
Then you have the section that defines the camera.
The perspective indicates it is a simple perspective camera rather than one with a wide angle lens or fish eye lens.
The second point determines its location (which works out to be the altitude above ground).
Then the FOV, in this case 60 degrees.
Then a scaling factor to determine the size of the image.
Then the final 3 lines are a choice of where you want the camera pointed.
The one currently set is the position of the horizon for a round Earth. However you can comment that out (add a // to the front), and uncomment the line below (remove the //) to make it look to the flat Earth horizon, or the one below that to make it look out level.
The next 3 are white light sources above, in front and behind (the round) Earth to make it fully illuminated.
Then there are 2 objects to represent Earth, a sphere of radius rad (specified above), centred at rad distance below the origin (ground level), and it is blue. I haven't bothered with any landmasses or dealing with the slight oblateness. The other is a cylinder to represent the flat disc Earth, which has its top surface level with 0,0,0.
Then the last 2 lines are calls to the above. You can have either commented out. Whichever one isn't will be shown in the render, rearth for a round earth, fearth for a flat earth.
This is just an approximation due to the atmosphere distorting things, but I think it is a fairly good one.
Also note the flat Earth is centred at the pole. I haven't bothered translating it at all to put you somewhere else like over the US.
And you can see the edge of the flat Earth.