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Pat French
Longdon-upon-Tern, Shropshire, UK
The view of the horizon on a disc world would depend in part on the unlikely physics by which you believe it works.
From most viewpoints, the distance would be obscured by the topography, just as it is on an actual planet. Vegetation (if any), hills, mountains and any houses in between would prevent a view of the far distance, including that of any ocean or barrier that might run around the entire edge.
If, somehow, the disc could maintain an atmosphere, then dust and atmospheric movements, for example, might also hinder vision.
However, presuming clear air, as objects receded into the distance, they would appear smaller and smaller until they couldn’t be resolved by eye or other observing mechanisms. Whether they would merge to a line-like horizon would depend upon the nature of the atmosphere, the nature of the illuminating star (sun) and the degree to which the planet contrasted with its sky.
Distance itself wouldn’t be an issue for viewing significant features. Moon craters are visible to the human eye even on an oblique line through our spherical atmosphere.
The opposite shores of seas may be visible, as is Dover from Calais across the English Channel, if the sightings weren’t cluttered by islands or shipping, all of which might merge into a fuzzy line.
Were you to stand on the shore of any surrounding ocean and look outward, your view of the horizon would depend on your geological conjecture. It might be possible to see a retaining feature, such as an ice wall or other barrier.
Otherwise, visibility would depend on the contrast between the ice, or other substance, and the atmosphere that formed the sky in its retaining inverted bowl.
Ron Dippold
San Diego, California, US
Things would disappear into a distant haze if there were any air. Let’s take a disc 13,000 kilometres in diameter – about the same as Earth at the equator – and 1000 km thick. It’s completely flat, with nothing on it. Any known material would slowly curl into a sphere from its own self-gravity, unless you spun the disc, which would then tear itself apart. This is another way we know that Earth isn’t flat, but let’s ignore that.
So, let’s say you are standing on one flat side, in the centre. What do you see? Well, if there’s no air, you can see to the edges of the disc and beyond – there is no geometric horizon. Just a large, flat, boring plain. What if the disc had Earth’s atmosphere? This is also impossible – even in the centre, the gravity is only 10 per cent of Earth’s, since our disc has less mass, and it couldn’t hold on to as much of our thick air. As you go towards the edge, the gravity actually gets stronger – not downwards, but towards the centre of the disc. After all, when you are at the centre, all the sideways gravitational pull from the disc on one side (say, an arbitrary north) is cancelled by all the sideways gravitational pull from the other side (the south). So, all that’s left is the downwards pull. But if you go all the way to the northern edge, you now have the gravity of the entire disc pulling you mostly towards the south. It would feel like you were clinging to a massive cliff face, where the cliff is the entire disc. All the air would get pulled by gravity into a giant dome around the centre of the disc, leaving most of the disc without air. And all those lovely paintings of water cascading off the edge of disc worlds are wrong as well, since all the water would “fall” to the centre. But let’s ignore that again.
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The view of the horizon on a disc world would depend in part on the unlikely physics by which you believe it works
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What would really limit your view is the Rayleigh scattering. Even the cleanest, purest air is made of nitrogen and oxygen molecules (among other things) that scatter more blue light than red. This is why the sky looks blue. Even in the best conditions, you can see only about 300 km through this. Up to 50 km away, everything would look very clear. From about 50 to 150 km, things would start to look bluish and you would lose some details.
From about 150 to 250 km, you would see only silhouettes of tall things like mountains or skyscrapers. At about 300 km, everything would just become a blue-grey haze. This would be your visible horizon – you couldn’t see the edge of the disc unless you were near it. Of course, this is much further than you can normally see on Earth, where the apparent horizon for a 2-metre-tall person is only 5 km away, or 115 km if you are 100 metres up.
If you were about 100 km from the edge, the sideways gravity would mean your view would be like looking up into space from the side of a cliff on Earth. And you would have an apparently bottomless hazy void “below” you.
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