OK - so let's just re-couch the original statement of the problem - with full scientific rigor - and without the snarkiness - here's my shot at that:
Who here has seen the sun set? (The "sun" being defined as "the star about which we orbit" for RET and "the hot glowing ball that moves in some manner above the Earth" for FET).
Along with every other object in the sky (except planets, asteroids, comets, birds, aircraft, helicopters, frizbees...), it moves across the sky at an (approximately) constant angular velocity and then appears to cross the horizon and disappear below it, whether you are on the sea, atop a skyscraper, or in a valley. (But not in the ISS because it's orbital motion causes retrograde sunsets and other complicating factors...fortunately the FE'ers don't believe in the ISS - so we can nix this complication!)
How can this be? If it moved around in a plane that's approximately parallel to the plane of the flat earth, and if light travelled in straight lines then we should almost always be able to see it!
The explanations for this in FET sometimes talk about "perspective" or "refraction" as if those words completely explained the nature of light that would be necessary for the bottom half of an object to be blocked by something that isn't in a direct line between the object and observer.
It is (I believe) an indisputable fact that in FET, the rays of light from the sun graze the horizon - even though the straight line from sun to our eyes would pass a considerable distance above it. That is the only way that the sun can appear as a semi-circle resting on the horizon when it is in fact still a considerable height above the flat earth.
(There is an additional problem here. If the rays of the sun are distorted this way - then so are the rays of light from the moon, stars and planets - which also appear to rise and set over the horizon.)
However, for this to happen, then one of two things must be true:
1) Some kind of "refraction" is happening through the atmosphere near sunrise and sunset...or...
2) The "laws of perspective" are wrong. But there really isn't such a thing as "laws of perspective" - the only physical law involved here is that light travels in straight lines through any uniform medium...since we're agreed that in a flat earth setting we still see sunsets - then light cannot be travelling in straight lines...and the only known mechanisms for that are:
a) Relativistic space distortion due to immense gravitational fields (inapplicable in FET because either gravity does not exist - or the gravity of the flat earth is far too small to distort light...AND because gravitational distortion would bend light in the wrong direction to create sunsets).
b) Refraction...which is the other theory here.
So even if you claim that some weirdly "different" perspective is the cause - it still has to be caused by SOMETHING - and since relativistic bending is off the table, we're back to refraction. So whether you claim "perspective" or "refraction" - the end result is "refraction".
Let's consider refraction then. Refraction can only happen when light passes through different materials - or through one material at different densities.
Which means that the atmosphere has to bend the light of the sun, when it's near sunset. We could certainly say that the air is both warmer and denser close to the ground compared to at high altitudes - so this would have to be the "smoking gun".
Trouble is - we know how air bends light - we can easily measure it with the simplest of equipment - and what we know for 100% sure is that warmer/denser air would bend the light AWAY from the horizon line - not towards it. So if this theory was correct, the sun would actually get higher in the sky the further away it was.
Even if by some means, reverse-refraction was the cause - it would cause the sun to travel in a weird shaped arc as it approached the region of maximum distortion because refraction has to happen in the north-south direction as well as in the east-west. If this theory were correct, then the sun would approach the horizon in an every steepening J-shaped curve.
If you deny that problem by somehow claiming that "the perspective effect preferentially favors the east-west distortion over north-south" - then the Sun and Moon would become elliptical (well, more "egg-shaped" actually) as it approached the horizon...and they clearly don't.
Pleas that sunlight is somehow special fail because the moon, stars, planets, comets and asteroids also rise and set over the horizon...and because you get the same results with refraction of sunlight as for any other light when you do this in the laboratory.
Hence neither refraction NOT perspective can properly explain the sunset phenomenon without producing other artifact that we don't see.
I politely request clarification from the FE community for these problems with their world-view.