1) The horizon always appears perfectly flat 360 degrees around the observer regardless of altitude. All amateur balloon, rocket, plane and drone footage show a completely flat horizon over 20+ miles high. Only NASA and other government “space agencies” show curvature in their fake CGI photos/videos.
I of course shared a picture from the international Space Station of the Mediterranean Sea. The International Space Station is a modern marvel that orbits the earth at 250 miles above the oceans. The flat Earthers like to claim that it is a fake, but they do make pretty convincing photos, and an amateur astronomer can see it with a telescope. If the photos of Earth from Earth are fake, I would ask where are the pictures of a flat Earth? I have yet to see one photo. Flat Earth sites have only drawings and paintings.
Even with overwhelming photographic evidence, there is the concern that the horizon does not appear to curve. The truth is the Earth is an enormous ball with a circumference that is 24,000+ miles. To drive around the Earth in a car going 100 miles an hour would take 10 days along any of the great circles (even traveling North/South). Because of the large nature size of Earth, the curve is almost imperceptible.
In order to perceive the curved horizon, two conditions are required. (1) The horizon must be unobscured with sufficient field of view. No trees or mountains in the way and more field of view than out of a small port window. (2) The observer most be sufficiently high enough to see enough of Earth that a curve would be expected.
For an understanding of this, imagine an near infinite spherical planet. In order to be so large, the curve of the planet would have to approach a straight line. To demonstrate this in another way look at the following circles and notice the sharper curve in the smaller circles:
Each of the black lines is a circle–even the largest blue one in the farthest background. This image was built in PowerPoint using circles of the following diameters: 0.25″, 0.5″, 1″, 2″, 3″, 4″, 5″, 6.5″, 7.4″, 50″, and 200″. It is difficult to see the curve in the 200″ circle, because it has less curve than the smaller circles. Now imagine a circle with a 2.5 million times bigger than that largest circle (diameter of 8,000 miles). It would be very flat. In fact it would have so little curve, that at small scales of about 300 miles you could use the horizon to approximate a straight line.
The higher elevation an observer is, the more of a sphere he will see. This is why am observer sees further away as he started out the window of an airplane climbing to altitude.
From Mount Everest (5.5 miles of elevation), an observer can only see 300 miles away. Below is a scale drawing of a spherical planet with diameter 7,900 miles. The small circle on the right is the 300 mile viewing radius an observer would see if Mount Everest were surrounded by an ocean (in fact it is surrounded by other mountains that obstruct the horizon). Even with an unobstructed horizon, when zoomed in, this is very flat looking::
From an airplane flying at the highest altitude used by a Concorde (no longer in service), an observer would only just begin to see the curved horizon (without clouds in the way and as much view as possible pressed against the window glass). This is the scale of a plane flying 7.7 miles elevation with a 350 mile viewing radius:
From the International Space Station, astronauts (and cosmonauts) can see in any direction 2000 miles from their 250 mile high perch. It is difficult not to notice the curvature from this height:
The photos flat Earthers really like are the ones cropped in some way that show how flat the Earth is from the International Space Station. Unfortunately, cropping out part of the view will just show what would be seen at a lower elevation:
Or these 3 photos of flat earth from the ISS:
It isn’t difficult to do this with a globe either. This is a photo of a globe taken with my cell phone with the camera as close to the globe as I could get it:
Where is the curve in the horizon? It is everywhere.