brought into view "from over the horizon", which would be impossible on a round Earth
Not so, "Seeing over the horizon is a common everyday experience.
Light, of any type, visible, UV, Infra-red, Laser, Radar, DOES NOT TRAVEL IN A STRAIGHT LINE.
#1. Photons follow the curvature of spacetime, which is curved by the presence of mass. The greater the mass, the greater the curvature. Stars that are actually behind the sun can be seen during total eclipses.
#2. REFRACTION: Light has different speeds in different transparent substances, always slower than in vacuum. From this differing speed, you can show that a light beam is bent at the boundary between substances with different index of refraction, which is the ratio of how much light slows down in the substance compared to vacuum. Camera lenses, eyeglasses, etc, harness this principle deliberately.
The speed of light in air is close to that in vacuum, but not exactly the same. Put another way, the index of refraction of air is almost 1, but not quite. Furthermore this index of refraction varies with the density of the air. To convince yourself of this, imagine the limiting case where you measure index of refraction of air as the pressure is gradually lowered. When it gets to 0, the index of refraction must be 1 by definition. The index of refraction of air therefore varies smoothly as a function of pressure.
Now think of the air envelope around the earth. Obviously there is a pressure gradient with altitude. When you get high enough, the atmosphere is gone and you have only the vacuum (almost) of space. In this case there isn't a sharp boundary like there is when light enters a glass lens. However, the gradient still bends light, in this case smoothly over some distance, as apposed to abruptly at the air/glass boundary in a lens. This vertical pressure gradient, and therefore index of refraction gradient, causes light to bend a little when shot horizontally thru the atmosphere.
However, there is more to it than this general effect. The atmosphere is not uniform at any one altitude. As you know, there is wind, pockets of hot and cold air, rising thermals, cold downdrafts, and lots of phenomena that are much more significant locally than the general decrease in pressure vertically. The air can have different layers at different temperatures, and the interface between layers can be much more abrupt than the general trend of decreased pressure with altitude.
Shooting a light beam with the right atmospheric conditions can exhibit much more bending than in the general average case. A mirage is a good example of this. Light from the horizon is refracted by the relatively sharp boundary at the top of a thin hot layer of air warmed by the ground. From far enough away to that the light is at a very glancing angle, you "see" sky light reflected off of what looks like the ground. This gives the visual impression of a lake, since a lake would similarly reflect sky light in normal cases even when there are no special atmospheric effects.
In the case of a mirage, light is actually bent upward. Light can just as well be bent downward using similar boundaries of layers in the atmosphere. It depends on the position of the emitter and receiver relative to the index of refraction gradients in the atmosphere which are highly variable and constantly changing.
These horizon phenomenon are difficult to impossible to see on land since land is never flat (neither is the ocean as Sea-Level varies from location to location but it's considerably flatter). At both sunrise and sunset the refraction phenomena allows the sun to remain (become) visible
after it is below the horizon.
A very rare phenomena is the 1 - 2 sec "green" flash seen just before the sun falls below the horizon. The refraction is at its maximum and beginning to separate the sun's light into its component colors as with a rainbow.