Eagerly awaiting the results, even though i know what they are likely to be.
Even though it's been overcast, it's a ceiling and I had a sharp contrast horizon today, so I took a cheap plastic aquarium to just try out proof of concept.
Not good. The tank really needs to be good and square (not rounded corners) and the sight through the transparent layer (glass, plastic) very clear. I measured and drew in index lines at every 1" horizontal on the tank itself, but when I tried to draw in perspective lines in the captured images, I got two different apparent focal points (and even those merged to a small area of uncertainty and not a precise fix).
This is just for fun, mainly, so I'm hesitant to go out and spend over $20 on a rectangular glass fish tank just for this. Maybe I can find one at a swap meet or garage sale this weekend for under $10.
One thing that did become apparent was water tension wouldn't give me a clean level line. Maybe it was the plastic. Or maybe I need to add something like antifreeze or dish soap. I just used plain bottled water with some food dye, but in places, it would "reach" up along the edge and ruin the straight edge. Also, trying to sight exactly level along the surface of the tank liquid is quite difficult. You really need to move off axis to line up the liquid level along the back edge with the side, and create a straight line with those two. But then you lose the perspective lines of the tank, which is the whole point of using the tank instead of the hydrostatic water level with the tubing.
I thought of adding two little floats with cross hairs on the level water and then use those like a gun sight. But the simplest way to me seems to not even worry about sighting level with the water and just let the perspective lines tell you where the vanishing point of "eye-level" is. I just don't know if skeptics will buy that, even though perspective and vanishing point is their argument for the apparent horizon.
(Of note, it's almost impossible to do this where I want to do it without drawing attention. When asked, I just explained I was recording observations for an optics/perspective demonstration and didn't mention anything about it being related to the globe/flat earth debate. I may be open minded, but I'm still in the closet about how much attention I've been giving the subject lately.)