This happens because as an object (like the sun) becomes more distant it drops in apparent height meaning the light that emanates from said object makes shallower and shallower angles relative to the ground until none of the light is able to be seen from the object at all. This is all pretty simple stuff that logically follows from the basic FE explanation for sunrise/sunset as described in the Wiki.
It
does follow from the FE sunrise explanation, but unfortunately that explanation is wrong. Tom tried to use this explanation in a previous thread about how clouds can be seen lit from below and how shadows are cast upwards by a sun apparently thousands of miles above the surface of the flat earth. This is the image we were talking about at the time
Tom said that it was perspective which caused this effect, the sun appeared lower in the sky because it was far away horizontally, it looked like it was below the mountain ergo the shadows were cast upwards. But that isn't how shadows work. The angle of a shadow does not depend on your perspective, it depends on the physical relationship between the light source and the object which the shadow is cast of. So a row of lamps may appear like this because of perspective:
But in reality each lamp is the same height. So if I look at the row of lamps and place my hand high enough with respect to my eyes that the lamp appears to be lower than my hand then the shadow my hand casts from the furthest lamp will still be in a downward direction because the light is physically above the level of my hand:
I did a quick experiment to show this. I used a lamp as the light source and a Rubik's cube as the object. You can see the top of the cube is illuminated because the light source is physically above the level of the cube:
If I change my perspective so the light source
appears to be lower than the cube, the lower side of the cube is still in shadow:
The only way that shadows can be cast by the sun upwards, or objects shorter than Everest can cast shadows on to Everest if light travels in straight lines is that the sun must be
physically lower than the level of the object it is casting the shadow of. Which is is with a globe earth, as the earth rotates the sun goes physically below the horizon.
The only other possibility I can think of is that light doesn't travel in straight lines, this is where EA wins as a flat earth theory. The sun would by physically higher than Everest and the mountains beside it but the light would be bending in such a way that the sun would appear to be below it and the light would be bending in such a way that shadows could be cast upwards. I've yet to see any experimental evidence that EA exists, but it would better as an explanation that perspective.