Why does the horizon rise to eye level?
A common question from flat-earthers is why the horizon always appears to be at human eye level (i.e. the center of your normal vertical field of view), no matter how high you are above sea level. The short answer is that it doesn't, but before we get to the evidence let's consider a relevant question:
Why do flat-earthers think this would help their case anyway? In both the globe-earth and flat-earth models, the horizon would be expected to fall below eye level as the observer rises above sea level. A "rising horizon" does not support the flat-earth theory any more than the globe-earth. So it's unclear exactly what flat-earthers are trying to prove by claiming that the horizon rises.
It doesn't actually matter though, because the horizon does not rise. The way to test this is to use any device that can accurately measure level and take it very high. Remember that the Earth is so big that you're not going to see a noticeable drop until you get to thousands of feet in altitude, so ideally you'll be in a plane over the ocean.
Fortunately, airplanes have built-in devices to do this. The HUD (Head-Up Display) below includes two long horizontal lines that show the "eye level" position. You can clearly see that the horizon does not rise to meet this point, it falls below as the plane rises.
The standard flat-earth response is that these HUDs are fake or do not represent the true eye level. However, given that these HUDs are a standard component of airplanes and pilots use them to fly straight, it's obvious that they are real. If you're still not convinced, it would be a relatively easy experiment to design a camera with a level to take photos like this from a passenger seat in a commercial air flight.
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