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### The view on the inside of a Ringworld

Posted: Thu Feb 18, 2016 2:11 pm UTC
I'm considering a story where the protagonists live on a Ringworld. As in, native inhabitants.

Specifically, it's a <1AU diameter Ring made of unobtanium and rotates at a rate around the Sun to produce 1G gravity on the inner surface by centripetal force. Day/night cycles are generated by gigantic solar shades in a much lower orbit (yes, this means that night time looks like an eclipse), so the Sun never rises and sets. It just disappears occasionally.
-> These shades also collect the power required for stationkeeping
-> To a large extent, the original starlifting apparatus used to get the material needed to build the Ring is still present on the shades; they can operate like photon drives in order to keep the sun in the middle of the Ring; or in times of greater necessity, restart starlifting to get reaction mass for greater thrust
-> This is of course related to why the star is quite a bit smaller and cooler than our Sun and why the Ring isn't 1AU but is still in the water zone

There are few questions that might need answering, any help?
1. How high should the side walls be?
- Presumably you can't have a flat band of unobtanium since the atmosphere would escape around the edges. What's a decent minimum height (I would like the walls to be as unobtrusive as possible)
- The side walls might be close enough to actually see them from somewhere in the middle? (see qn 2)
- Alternately, the Ring could just be non-flat, like a slice out of a Sphere, at a width at which the atmosphere stays inside. That would provide a gentle slope up towards the edge, a bit like a mountain going to space.

2. What's a reasonable width of the Ring? Presumably, a Ring width = Earth diameter is going to be fine, but could the Ring be wider without affecting the conditions at the edges too much?
- Once the Ring starts to get a few degrees of arc away from the orbital plane, the sun isn't going to be directly overhead anymore.

3. What would the Ring look like on the 'ground'? Our character stands on the surface at sea level and looks up.
- Presumably they aren't going to see a moon.
- Will stars be visible or will they be hidden by the penumbra of the sun even at night?
- What about the other sections of Ring? Since not all of the Ring is 'night', would the day portions of the Ring be bright enough to be visible in the night portion's sky? Will this affect the stars?
- The Ring is probably too big for the curvature to be seen. We're talking millions of kilometres in circumference after all. (by a poor approximation, a 1AU ring would curve upwards by 1 meter every 546 km, so any visible curve should be quite obscured by visibility limits)
- A smaller sun would have a different emission spectrum, would it just be a bit more red?

4. Weather on the Ring
- Again, there probably won't be seasons, unless artificially controlled by the sunshades
- On the other hand, the atmosphere is also a couple of million times larger than Earth's, could that lead to some ridiculously huge weather effects? Hurricanes?

### Re: The view on the inside of a Ringworld

Posted: Thu Feb 18, 2016 2:19 pm UTC
i feel all of these questions were answered by larry niven already.

### Re: The view on the inside of a Ringworld

Posted: Thu Feb 18, 2016 8:12 pm UTC
Yeah, Niven basically answered all of these. But short-form responses:

jseah wrote:1. How high should the side walls be?

Depends on atmo pressure and makeup. Assuming Earth-like, Ringworld had them 1000 miles tall.

- The side walls might be close enough to actually see them from somewhere in the middle? (see qn 2)

Depends on the width. 1000 miles is visible a good bit out, but on the Ringworld they were invisible across most of the area.

Alternately, the Ring could just be non-flat, like a slice out of a Sphere, at a width at which the atmosphere stays inside. That would provide a gentle slope up towards the edge, a bit like a mountain going to space.

That doesn't remove the need for walls. At best, the slope is just acting the same as the wall, and the atmosphere rarifies as you get further out until it joins with space.

2. What's a reasonable width of the Ring? Presumably, a Ring width = Earth diameter is going to be fine, but could the Ring be wider without affecting the conditions at the edges too much?

Ringworld is just short of a million miles wide, far bigger than Earth's diameter.

- Once the Ring starts to get a few degrees of arc away from the orbital plane, the sun isn't going to be directly overhead anymore.

This affect obviously depends on your radius. For Ringworld at 1AU, its million-mile-wide ribbon was about .6 degrees of arc, if I'm doing my math right. If you're too wide relative to radius you'll start to experience a noticeable sideways drift at the edges, anyway, which isn't good.

3. What would the Ring look like on the 'ground'? Our character stands on the surface at sea level and looks up.

They appear to be on flat land stretching in all directions, and see an arch stretching across the sky. On Ringworld, the far end is about 14px wide (using CSS units, if I got my math right), so it's still visible as a line about as tall as one line of this text. (1 million miles at a distance of 190million miles is about .3 degrees, and a px is about .0213 degrees, so approximately 14px.)

- Presumably they aren't going to see a moon.

Not unless there's a moon in an orbit that makes it visible, no.

- Will stars be visible or will they be hidden by the penumbra of the sun even at night?

If it's dark enough to be called "night", stars will be visible. Exact amounts depend on amount of light blocked, but you can see some stars at dusk or in bright cities.

- What about the other sections of Ring? Since not all of the Ring is 'night', would the day portions of the Ring be bright enough to be visible in the night portion's sky? Will this affect the stars?

Yes, the arch will glow. I don't know the math offhand to figure out how much reflection we're talking about and what effect it would have.

- The Ring is probably too big for the curvature to be seen. We're talking millions of kilometres in circumference after all. (by a poor approximation, a 1AU ring would curve upwards by 1 meter every 546 km, so any visible curve should be quite obscured by visibility limits)

The Earth is flat enough to fool dumb people even today, and since flatness is proportional to radius, any ringworld around a star is guaranteed to be flatter than Earth. On a clear day you can only see a few hundred miles before the atmosphere blocks your vision.

- A smaller sun would have a different emission spectrum, would it just be a bit more red?

Depends on the star. ^_^ A white dwarf emits more in the blue, a red emits more in the red, etc. It depends on the temperature, and there are guides to what colors different temperatures are.

- Again, there probably won't be seasons, unless artificially controlled by the sunshades

Correct. Seasons are caused by the amount of solar radiation changing due to the earth's axis moving relative to the sun. This won't happen on a ringworld unless it's done purposely.

- On the other hand, the atmosphere is also a couple of million times larger than Earth's, could that lead to some ridiculously huge weather effects? Hurricanes?

Yes. Weather is *influenced* by seasons, but they're not the sole cause. All temperature gradients and heat flow contribute - day vs night, water vs ground, wind and water currents (which are also *caused by* weather), etc. Most of those still occur on a ringworld, so you'll have plenty of weather. On Ringworld, for example, there was a perpetual world-spanning hurricane caused by the asteroid-punch mountain fucking up wind patterns.

### Re: The view on the inside of a Ringworld

Posted: Fri Feb 19, 2016 12:04 pm UTC
Xanthir wrote:
- Once the Ring starts to get a few degrees of arc away from the orbital plane, the sun isn't going to be directly overhead anymore.

This affect obviously depends on your radius. For Ringworld at 1AU, its million-mile-wide ribbon was about .6 degrees of arc, if I'm doing my math right. If you're too wide relative to radius you'll start to experience a noticeable sideways drift at the edges, anyway, which isn't good.

Why? Assuming a cylinder shape ringworld (a flat ribbon with walls) the only forces acting on a creature standing on the sun side would be
1. "downwards" due to centrifugal force. Directly away from the axis of rotation.
2. "upwards and slightly to the center of the ring" due to the gravity from the sun (but that is unlikely to be much)
3. "towards the center of the ring" due to the own gravity of the ring (but for a sufficiently thin ring this is unlikely to be much)
(and of course the reactionary forces, mostly normal force)

Why would there be a noticeable sideways force?

(A sphere section ringworld would have a noticeable amount of "sideways force" because the floor is not going to be aligned with the axis of rotation. But why would one ever built that?)

### Re: The view on the inside of a Ringworld

Posted: Fri Feb 19, 2016 3:57 pm UTC
There's not, I'm crazy, never mind. ^_^

### Re: The view on the inside of a Ringworld

Posted: Fri Feb 19, 2016 3:59 pm UTC
Welcome to the club. I went crazy a long time ago. It is fun on this side of the line. We have unicorns and tasty food that doesn't make you fat!

### Re: The view on the inside of a Ringworld

Posted: Sat Jun 04, 2016 2:59 am UTC
Minor ressurection...
Neil_Boekend wrote:(A sphere section ringworld would have a noticeable amount of "sideways force" because the floor is not going to be aligned with the axis of rotation. But why would one ever built that?)

Structurally, either a sphere-section or perhaps a sphere-section-like shape (something akin to a catenary, swept in rotation around the ring) might be preferable.

Two disc-rim walls, their barycentres marginally offset either way from that of the host star, kept separated by a cylinder-wall 'floor', may be subject to forces in variousplaces that need over-engineering, especially the wall/floor right-angled join, but elsewhere too, especially considering construction needs.

Two 'rim-ropes' kept circular by tensional forces due to spin, could be kept forced apart by an inverted 'ground arch', looking like a continuous slung hamock but actually acting in compression (at least where it matters), when taking into account the complexity of the various forces and loadings. At the very least, no sharp corners to concentrate forces or require extra (and/or rounded) 'elbow' bracing.

You might trust to your unobtanium's strength, but no point in taking the mickey. And in the close-to-catenary form it would help contain the 'pool' of the atmosphere in the depths of the dip, the splayed walls outward and upward as 'mountain ridges into space' as effectively as you like.

(Less volume than a square-sectioned dip, and less 'near sea-level' ground area, give or take how you are prepared to first load the structural under-oval rim with filler bedrock to suit your simulated surface 'geology'.)

But this is just fom a mental model of the potentially proposed system, so I might be missing something.