What are the two most distant intervisibility points on earth?

Factual Questions
1

That is, the two points where two observers can see each other, over the curvature of the earth and the relief of the surface in between.

2

Naked eye, or can they use optics?

3

Are you more interested in places that are theoretically in direct line of sight, or are you looking for places where one could practically expect to reliably transmit signals back and forth? E.g. could a place that is generally shrouded in cloud cover/fog count?

Does the bottom of the ocean count? e.g. is the bottom of the Marianas trench considered to be in line of sight with things above the ocean, even though practically the water blocks most light?

4

<tongue in cheek>Does fibre optics count?

5

I’m pretty sure we’ve covered this on the Dope before…

My guess is that Mt. Denali is going to be one of the points. If not there, then it’s going to be a pair of mountains in the Andes. Maybe the Himalayas, but probably not: Everything’s high up there, including the land in between.

6

Or, can we put a mirror on the moon?

7

There’s a ton of information on this site, which suggest that Denali to Mt. Sandford at 227 miles is right up there.

There’s an interesting thread here discussing this, if you’re interested.

8

Here’s a thread from 2012 discussing the subject. According to the pages quoted in that thread, the longest line of sight possible in principle is almost 500 km across the Colombian plains; however, atmospheric distortions are likely to make it impossible to ever actually see this with one’s own eyes.

9

Atmospheric distortions work both ways. Firstly,most of the lines of sight on that website assume a normal amount of atmospheric distortion, so that things that would be over the horizon on a planet with no atmosphere will become visible as the light rays bend.

But secondly the atmosphere itself is opaque at such large distances. 500km of atmosphere at sea level would be completely opaque to image-forming light, even if some scattered light gets through.

10

Would the fact the Earth bulges in the middle play a role? If so, then how much of an advantage would you get by being on the summit of say Mt Kilimanjaro or Mt Kenya or Mt Aconcagua looking north/south versus doing the same at Denali or Everest?

11

Here is what purports to be a photo of a mountain about 195 miles (315 km) away:

While atmosphere distortion/occlusion would often occur, it is not necessarily a given.