We all know the answer will be "bendy light" of some kind, don't we? Consulting
the Wiki, which quotes Rowbotham's Earth not a Globe for much of the linked page, but adds another quote (shown below) as a Summary; a quote for which I can find no online corroboration. Everything I find on the web either copied it exactly as found here without attribution, or copied it and cited the TFES wiki as the source)
Emphasis added by me:
At these times it appears close to the horizon where the density of the air differs greatly. The air near the ground is denser than the layer of air just above it, and the layer of air above that is less dense still, and so on upwards until the Earth's atmosphere peters out at some 400 km. Now consider what happens when the Sun is setting. When the Sun is at the horizon, light from the top of the disc is going through the air at a different angle than that from the lower part. So the rays are bent by different amounts before they reach the observer's eye. The result is that the bottom part of the Sun's disc appears to be lifted up. In consequence the Sun's disc appears slightly compressed."
- Samuel Birley Rowbotham
The FE side places great importance on the fact (and it IS a fact, I'll agree with them in this one small way) that light is refracted by the atmosphere. But they try to force this bendy light to do two opposing things at the same time:
A. Either as Rowbotham says (and we round earthers agree) thicker air refracts light rays to make objects "
appear to be lifted up",
B. Or they cause objects to
appear lower than they are; so low, in fact, as to appear to actually go below the horizon.
It's one or the other, you can't have it both ways. And we can disprove option B with an
observation (you remember "
observation" right? Supposedly the Zetetic's foundational activity?).
Observe the apparent shape of the sun's disc at sunset. Consider that portion of the sun's disc which appears nearest the horizon (call this "the bottom" for visual descriptive purposes). Do you
observe the bottom appear to move down toward the horizon faster than the middle and top? No, you do not; if this happened then you would
observe the sun appearing to stretch out into an ellipse with the major axis vertical and the minor axis parallel to the horizon. (In fact, this should have been happening all day, ever since the sun passed your local zenith. It might only be dramatic enough to be
observed when the sun reaches the farthest point from which it is visible to you.) Instead we
observe the exact opposite effect, the sun compressing top to bottom.