In Iain M. Banks, fairly rockin' culture novel, Consider Phlebas a good third of the book takes place on what is called the "Vavatch Orbital". An orbital is apparently any massive artificially created megastructure, similar to the Death Star or the titular Halo.

At one point in the book the Vavatch Orbital is described as a minor orbital, not as big as a ring or a sphere. Then later it is almost directly described as having a ring shape. Still earlier in the story it is described as a bucket being swung in circles, though this description maybe used only to describe how the orbital uses centrifugal force instead of actual gravity.

Whatever the size, the orbital has to have enough sea space for a Megaship (a city sized ocean ship) to travel for more than a century before reaching land again.

Wikipedia tends to suggest that all orbital's are rings, but then why the mention of a sphere, or that Vavatch is not big enough to be a ring?

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    The Vavatch was actually bigger than most orbitals. – Solemnity Feb 24 '13 at 23:08
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    "An orbital is apparently any massive artificially created megastructure, similar to the Death Star or the titular Halo" No. An orbital means a ring. Can't recall right off which book that is in, but an orbital is a ring. – dmckee --- ex-moderator kitten Feb 25 '13 at 6:50
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    Also, the Bungie team when originally releasing Halo commented that they'd based the Halo ring design on Iain M. Bank's orbitals. Worth reading: vavatch.co.uk/books/banks/cultnote.htm – Alex Aug 7 '14 at 3:08

There's an illustration (by Mark Salwowski) of Vavatch on the cover of the 1988 Orbit (UK) paperback edition of the novel:

The first description of the Orbital in the book also gives its dimensions:

Vavatch lay in space like a god’s bracelet. The fourteen-million kilometre hoop glittered and sparkled, blue and gold against the jet-black gulf of space beyond. As the Clear Air Turbulence warped in towards the Orbital, most of the Company watched their goal approach on the main screen in the mess. The aquamarine sea, which covered most of the surface of the artefact’s ultradense base material, was spattered with white puffs of cloud, collected in huge storm systems or vast banks, some of which seemed to stretch right across the full thirty-five-thousand-kilometre breadth of the slowly turning Orbital.

So it's 35,000 km wide and 14,000,000 km in circumference (4,500,000 km in diameter), giving it a surface area of 4.9 × 1011 km2, about 960 times the surface area of Earth. A ship taking an (Earth) century to circumnavigate Vavatch would need to travel at an average speed of about 16 km/hour (8.6 kt).

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  • so about 3/4 the diameter of Neptune – endolith Dec 5 '15 at 2:52
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    Wait, I read that wrong. It's way huger than Neptune. The breadth of the ring is similar to the diameter of Neptune, and the diameter is 4.5 million km?? It's 3.2 times the diameter of the Sun! – endolith Dec 5 '15 at 5:17
  • A very impressive structure indeed! – Cupit Feb 1 '19 at 15:12

Here's how Banks describes Orbitals himself, in his essay A Few Notes On The Culture :

Perhaps the easiest way to envisage an Orbital is to compare it to the idea that inspired it (this sounds better than saying; Here's where I stole it from). If you know what a Ringworld is - invented by Larry Niven; a segment of a Dyson Sphere - then just discard the shadow-squares, shrink the whole thing till it's about three million kilometres across, and place in orbit around a suitable star, tilted just off the ecliptic; spin it to produce one gravity and that gives you an automatic 24-hour day-night cycle (roughly; the Culture's day is actually a bit longer). An elliptical orbit provides seasons.

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  • This is the most accurate representation, explaining every aspect of the device including its weather, gravity and seasons. My only worry is the size and energy output of the star. I am left to assume there is a means to protect the populace from the high levels of radiation from a star the size of our sun. – Thaddeus Howze Nov 11 '14 at 21:34
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    @Thaddeus The ring is in orbit around a star, it doesn't have a star in the center of the ring. It's in an orbit similar to that of a planet. "One thing almost every Orbital ... does have, is a Hub. ... the Hub sits in the centre of the Orbital, equidistant from all parts of the main circumferential structure (but not physically joined to it, normally). The Hub is where the Orbital's controlling AI (often a Mind) usually exists" – endolith Dec 5 '15 at 5:18
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    Don't forget that The Culture have crazy forcefield tech. To the that point that GSVs don't even have physical hulls anymore, it's all forecfields. Solar radiation would be trivial to them. – Cupit Feb 1 '19 at 15:23

I think it is clear from the Culture novels that there are various mega-structures that are found in the culture universe, including spheres, rings, orbitals, shellworlds and mega-ships like GSVs that can accommodate billions of people themselves.

It seems clear from their regular appearance in the novels, that orbitals are relatively common. @DanielRoseman's excellent answer quoting Banks describes that pretty well - but from this description and the novels we find that Orbitals are ring shaped structures, in orbit around a star (not surrounding a star). They are sometimes complete and continuous (like Vavatch) and sometimes not complete or finished, and comprise 'plates' that people live on - such as the Chiark Orbital from Player Of Games. Orbitals tend to have a 'hub' comprising a mind located at the point that it revolves around.

I think the description in Consider Phlebas is a little confusing, but it is meant to distinguish orbitals (these relatively small structures in orbit around a star) from a 'true' ring in the sense of a Niven Ringworld, or a sphere, by which I presume is referring to a Dyson Sphere. A Niven Ringworld is a vastly bigger structure than an orbital - a ring at Earth orbital distance around a star would be over 900 million kilometers in circumference - much larger than the 3 million kilometers that Banks describes an orbital. Rings if they are rotated for gravity, unfortunately take the laws of physics to breaking point - Niven had to invent magical new materials for it to be made from. In the Culture however, perhaps (equally magical) force fields can be used in place of unrealistically strong materials.

I think we are meant to presume that rings and spheres are known in the Culture Universe, but perhaps much, much less common than orbitals.

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    Here's an illustration of ring vs orbital: orionsarm.com/eg-article/5151b9b79834e orionsarm.com/eg-article/4845ef5c4ca7c – endolith Dec 5 '15 at 3:15
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    Capital-R Rings seem like an artifact of the early series before he had fully worked out the size and scope of the civilization - given what we learn later, they're far too large for the Culture to bother building for itself (it might be able to, but wouldn't justify ruining a solar system to achieve it). The Culture's total population is later revealed to be similar to that of the Ring from Ringworld (which was post-apocalyptic and considered effectively deserted). – user36551 Feb 28 '17 at 16:22

I pictured it as a HUGE ring, but ring in the sense of the thing on your finger, it orbits itself, the slow rotation causes things to fall, but doesn't generate gravity due to having limited mass (compared to a GSV or planet), the "surface" is either the inside "up" or the outside "down", I'm guessing the outside would be unlivable.

The "inside", if you will, is a huge expanse slowly curving upwards, like our planets horizon curves downwards, but as it is so vast the curvature happens outside humans fields of vision, appearing to be a flat ocean etc. Imagine a perfect skateboard ramp circle, with open ends, and possibly the width of a planet, but with nothing Inside but space & air.. (maybe not even air in the middle)..

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  • GSVs would not have significant gravity either. – iandotkelly Feb 25 '13 at 1:33
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    They use fields on GSV's for artificial gravity, which is awesome, cause then they can suspend it at will. – Grizly Feb 25 '13 at 2:47

There is definitely no air in the middle, since the rotation pushes the air towards the inner surface.

However, you do not need a ceiling; only walls to keep the air in.

See Elysium for a demonstration of the idea.

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"Consider Phlebas" calls Vavatch a hoop on page 255:

Vavatch, that fourteen-million-kilometre hoop, was starting to uncoil. A chain, it had been cut

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