I'd imagine you would see the top of the wall first. Yes, the bottom is closer, but there's significantly less air between you and the top. If you assume a 1000 km height, and use the approximation that above 100 km altitude is essentially vacuum, then if you're looking from 5000 km away then only the first 500 km of your line of sight pass through air at all, and a lot of that will be much thinner (with less haze and clouds) than surface pressure. Even if you assume the 296 km max sea-level visibility applies all the way along the air part of the line of site to the top of the wall, that means that in ideal circumstances you could see the top of the wall from 2960 km away. And allowing for the reduction of extinction with altitude will make this distance even longer.

I imagine the easiest way to see a Ringworld wall from far away would be to look toward the nearer one when you were in local "night" (shaded by a shadow square). The nearest sunlit portions of the wall would probably be quite easy to spot.

On Fri, Jan 24, 2020 at 12:41 PM Bruce Johnson <xxxxxx@pharmacy.arizona.edu> wrote:

This will give the actual mathy goodness needed to calculate this: https://en.wikipedia.org/wiki/Visibility

More generally (from the above linked article)

"At sea level, the Rayleigh atmosphere has an extinction coefficient of approximately 13.2 × 10−6 m−1 at a wavelength of 520 nm. This means that in the cleanest possible atmosphere, visibility is limited to about 296 km.”

So I’m thinking anything more than 600km wide would mean you cannot see them. As you approached the walls they would loom out of the haze, but all the way because there’s no curvature like on a sphere where you would see the peaks first; the walls would reveal themselves form the bottom up, I think since you would be closer to them than the tops. Also, the atmosphere doesn’t reach *nearly* as high as the walls. If you’re doing a rotation that provides 1G the atmosphere won’t reach more than a tenth of that height or so:

https://scied.ucar.edu/shortcontent/earths-atmosphere

"Air becomes so thin at altitudes between 100 and 120 km (62-75 miles) up that for many purposes that range of heights can be considered the boundary between the atmosphere and space. However, there are very thin but measurable traces of atmospheric gases hundreds of kilometers/miles above Earth's surface.”

1000km is MUCH higher than needed to contain atmosphere. you MIGHT actually be able to see the top of the wall with a telescope long before you could see the bottom.

This would be an interesting thing to model. Weather on a ringworld will be vastly different than on a planet, since most of our climate is due to convective equilibration of energy between the hot equator and the cold poles.

ALL of a ringworld is equator.

Maybe that 1000 km of wall contains gigatons of refrigeration equipment to make the rimwall areas ’the poles’ :-)

(it’s been *ages* since I’ve re-read Ringworld, should give it ago again sometime)

> On Jan 23, 2020, at 6:19 PM, Greg Nokes <xxxxxx@nokes.name> wrote:
>
> A question:
>
> Given a ring world, how wide would it have to be so that an observer in the, say, middle 5000 miles of it would not see the shield walls?
>
> IIRC from Niven’s work, the walls are like 1000 miles tall, and the floor is flat, so curvature would not play. I don’t think there is an easy mathy way to solve this...
>
>
>
> Sent from my iPhone
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Bruce Johnson
University of Arizona
College of Pharmacy
Information Technology Group

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