What if the world was a glorious donut?

October 26, 2009 8:09 AM

If I'm sitting on the terraformed surface of a Culture orbital, what does the horizon look like?

For those unfamiliar with Iain Banks' Culture novels, imagine an artificial ring spinning on its axis and revolving around a star. People live on the inside (concave) surface of the ring.

Ostensibly, the curvature of the ring is gradual enough that the ground beneath them appears flat, as it does on the convex surface of the Earth. But what do they see when they look to the horizon? Can they see the far edge of the ring? Every time I try and picture this, my brain stops working.

Thanks for indulging a nerd.

posted by reverend cuttle to Science & Nature (12 answers total)

In the Halo series, when you stand on the surface of one of the Installations, you see the very distant dges of the ring from a tiny acute angle to the surface you stand on through the point of perpendicularity to that surface, whereupon they disappear beyond your field of view. Generally, glare from a sun or other object in space precludes your view of the ring directly above you, though on the Ark structure I think you can see some things joining directly overhead.
posted by Inspector.Gadget at 8:14 AM on October 26, 2009

Larry Niven addressed this question in "Ringworld"

"The Ringworld had no horizon. There was no line where the land curved away from the sky. Rather, earth and sky seemed to merge in a region where details the size of continents would have been mere points, where vanishing point held his eyes fixed."

posted by zarq at 8:29 AM on October 26, 2009

On a culture orbital, you probably wouldn't notice any deviation from flatness. With a circumference of 10 million kilometers, you'd need to see thousands of kilometers for any noticeable curvature, which is unlikely in an earth-like atmosphere.
posted by themel at 8:38 AM on October 26, 2009

This site has some interesting renders of Niven's Ringworld, and there's a good image at the bottom of this page that I think is a fairly accurate depiction of what you'd see standing on the surface.

In the Ringworld books, the natives called what they could see of the Ringworld "The Arch". The atmospheric haze obscures the ground in the distance so you can't see it curving upwards, but you can see most of the rest of it as an arch over the top of the sky. I imagine the sunlit areas would look as bright as the Moon does to us, so you could still see them during the day.
posted by lucidium at 8:42 AM on October 26, 2009

But ringworlds are much much much larger than orbitals.

Ringworlds go all the way around a star. Orbitals are more like small gyroscopes that revolve around a star in the same way planets do. It's like an annular slice of a planet, spinning like a top while orbiting a star.

That said... I imagine the 'cross-ring' (short way) horizon would be pretty obvious, since the width is less than an earth width (I think?), and the horizon would curve up to the sides (since you're inside the ring) the same way our horizon curves down at the edges (since we're outside the sphere).

If you mean the long way, looking toward the inner edge of the ring itself... well that would be a line curving straight up and tapering to a fading point, or if you could see far enough, curving all the way up, around, and back again behind you.

How much of it you could see would depends what the sunlight is hitting, I suppose. Could be some nifty nighttime views, too.
posted by rokusan at 8:42 AM on October 26, 2009

Ah yeah, the Ringworld is way larger than Culture orbitals. I just remembered seeing those Ringworld renders before and I couldn't seem to find an accurate render of an orbital.

Going from that wikipedia article, the circumference of an orbital is around three orders of magnitude larger than the Earth's, and the width is on the scale of thousands of kms. So I think the long horizon, and possibly even the short horizon, would still be obscured by the sheer thickness of atmosphere.
posted by lucidium at 9:25 AM on October 26, 2009

You can see a visualization of this (poorly reproduced here) in a book called Space Colonies edited by Stewart Brand. It deals mainly with the 1970s space colony ideas advanced by Gerard O'Neill. He abandoned the donut shape for a cylinder shape that gives an even stranger horizon, imo.
posted by mattbucher at 9:27 AM on October 26, 2009

Best answer: From ground level or nearby, it doesn't look appreciably different from a planet, with a few exceptions.

Orbitals are big things. Your typical orbital is a band several thousand kilometers wide. That's easily far enough for atmospheric haze to be the Limiting Factor, even without all the pollution you primitives pump into the atmosphere.

During the day, the view on an Orbital looks pretty much like the view from a planet: terrain moving away from you, fading and blueing as the atmosphere obscures more and more details, and then you can't see any further. Your sight horizon would probably be farther than on a planet, though. And, if you look in the day sky very carefully, you might be able to make out the tiny, thin dark band of the other side of the Orbital, but at that distance -- in the neighborhood of three million kilometers -- there won't be any detail at all. You wouldn't normally see an "arch" or the terrain rising up; that would be too far away to be visible and at the distance required the Orbital would already be just a string.

At night, you can't see the terrain much anyway, but you'd get a pretty normal night sky with a shining, bright string through the zenith. Again, without much in the way of details. The other side is about as wide as your moon, or maybe twice as wide, but it's twelve times farther away than your moon is.

Other than being able to make out the other side of the Orbital, about the only other changes would involve things like ships traveling. On a planet, a sailing vessel might be visible popping over the curved horizon masttops-first. On an Orbital, the same ship would be visible farther away and would just slowly resolve itself out of the gloom, all at once.
posted by ROU_Xenophobe at 9:29 AM on October 26, 2009 [5 favorites]

Ah, I just came back to correct myself, but ROU_Xenophobe has already explained it.
If I've worked it out right, the other side of a ten million km diameter orbital with a width of a thousand km would have an angular diameter of around 1 arcminute. By comparison the Moon's angular diameter is around 30 arcminutes.
posted by lucidium at 9:53 AM on October 26, 2009

All depends on scale but BERG made a great visualization of Manhattan without a horizon, which I think would come pretty close to that of standing on an Orbital.

So enjoy!
posted by Static Vagabond at 10:51 AM on October 26, 2009

Best answer: Here are some visualizations of what an orbital ringworld would look like from the surface:

CGI
Illustration
Screenshot from Halo
Another screenshot

(The Halo rings are only ~10,000km in diameter, btw).
posted by Rhaomi at 12:45 PM on October 26, 2009

Other than being able to make out the other side of the Orbital, about the only other changes would involve things like ships traveling.

Well, that and walking off the edge of the planet if you travel laterally, rather than around the ring, sure.

The horizon when looking across the short width of the ring is very different than the infinite forward/around, since there's an actual edge there.
posted by rokusan at 9:15 AM on October 27, 2009