[btilly]

Flat Earthers look for ways to confirm what they believe.

My favorite demonstration goes like this. Get into a plane. Sight down the spine of a magazine at the horizon then look at it. You'll see that it is tilted a few degrees. Do the same out the other window. It is tilted a few degrees the other way.

Can we work out by how much it should tilt? Why yes! If the Earth is a ball, then you, the horizon, and the center of the Earth make a right angle triangle with the right angle at the horizon, one leg being the distance from you to the horizon, the other being from the center of the Earth to the horizon, and the hypotenuse is the distance from the center of the Earth to the ground below plus your height. Furthermore if you draw the picture, the triangle from you to the horizon to a vertical line up from the horizon to horizontal back to you is a similar triangle to the first, so the angle that the horizon is below horizontal is the same as the angle at the center of the Earth of this triangle.

The distance from the center of the Earth to the ground is approximately 3950 miles. Your plane is usually 6 to 7 miles off the ground, let's say 7. By Pythagorus that puts the horizon at sqrt(3956^2 - 3950^2) which is roughly 235 miles away. Apply basic trigonometry, and it should be about 3.4 degrees below horizontal.

I've done this. You've seen that the calculation is easy. The measurement is easy. Running in circles to avoid this conclusion is a sign that you aren't interested in knowing the real answer.