See, all the examples above are limited by the curvature of the earth, and that is why the horizon seems to be the limiting factor. But there are two additional variables in play that can allow for some truly amazing sightlines: the height of the target and the temperature of the air.

The height of the target is intuitively obvious, as we all realize we can see Calgary's skyline from way further away than 4.7 km. In fact, Calgary's skyline can be seen at least 50 km away since the height of the city's buildings allow them to "peak" above the horizon, allowing for a fairly significant increase in sightline length. The taller the target, the further behind the horizon it can be and still have its top visible.

The air temperature variable is not quite as intuitive, but it really is simply a real world example of a concept all spectacle wearers rely on daily: refraction. Refraction allows for light passing through a medium of different densities to bend and focus in predictable ways. The medium in a pair of spectacles is glass, and the glass is shaped in either a convex or concave manner to help focus light into a person's eye based on if they are nearsighted or farsighted. However, the medium the light is passing through can also be air, and if the density gradient of the air is just right, it can allow light to bend around the curvature of the earth. Cool air near the earth's surface with hotter air above is required to bend light in such a manner, which would further increase our maximum line of sight (the opposite conditions, hot air at the surface and cool air higher up will cause light to bend away from the earth, which is how mirages in the desert are formed).