Does anyone disagree with any of the following?
1. The horizon appears increasingly below eye level the higher we rise in elevation
2. This angle can be measured fairly easily with a variety of instruments
2. The amount the angle increases is consistent with being on a ball of around 7,900 miles in diameter
3. A horizon occurs as a result of curvature
4. There is no mechanism to cause a horizon on a flat plane
5. Even if there was, the actual measured dip of the horizon (or anything) isn't in line with what it should be on a flat plane
6. The measurable dip to the horizon (or another distant object) is an excellent proof of a spherical earth
The first #2 is rather ambiguous. I've found it difficult to measure the angle of horizon dip with any sort of precision without a precision instrument (which I don't have access to). Coarse angle measurements -- which is all you need to tell if the angle increases with elevation -- is easy enough with non-precision and home-made leveling instruments.
The second #2 relates to the first one, but with the added caveat that (as I've been gradually learning from my own experimentation) the atmosphere plays a very large role in influencing deviations from what would be consistent with a sphere of a particular diameter. When the atmosphere bends light to more closely follow the curve of the earth, measurements will appear consistent with an earth of greater diameter. And conversely, if refraction is weaker, the atmosphere will bend light less making the curvature of the earth more pronounced, as if the diameter was smaller.
Calculations of curvature that don't account for the affect of an atmosphere layered over a terrestrial sphere will never meet consistency requirements.
I don't think you'll get agreement from flat earth advocates on 3, 4 or 5 but I won't speak for them.
However, the reasons I've seen for why there is a perceived horizon on a flat plane suggest there should be consensus on #6, although I would reword to say that "measurable dip to the horizon (or another distant object) is
a distinguishing feature between a concave earth and a flat earth.Horizon dip doesn't prove spherical-icity, er, spherical-ness...doesn't prove the earth is a globe. Horizon dip would occur on a cylinder earth too. You need dip in all directions from all locations on the surface to conclude a sphere. And phrasing it as a conditional (
if this then convex, if that then flat) makes it a more neutral statement rather than an argument for one or the other.
#6, by the way, is the key premise of this topic, started way back when. If there isn't agreement between flat and globe earth proponents on that, then much posted on this topic has been pointless. But if there is a mechanism that could explain the apparent dip in the horizon with increased elevation on a flat earth, I'd love to invite it.
Note: Electromagnetic Accelerator Theory offers an explanation, but it is contradictory to other flat earth arguments so it can't (or shouldn't) be offered in isolation. In my opinion, the EAT-based flat earth model is a substantially different model and not conflated with what I'll call the more orthodox arguments in defense of a flat earth.