Question: In [Iain M. Banks'](https://en.wikipedia.org/wiki/Iain_Banks), fairly rockin' [Culture novel](https://en.wikipedia.org/wiki/The_Culture), [*Consider Phlebas*](https://en.wikipedia.org/wiki/Consider_Phlebas) a good third of the book takes place on what is called the **"Vavatch Orbital"**. An orbital is apparently a massive artificially created megastructure, similar to the [Death Star](https://en.wikipedia.org/wiki/Death_Star) or the titular [Halo](https://en.wikipedia.org/wiki/Halo_%28megastructure%29).

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 orbitals are rings](https://en.wikipedia.org/wiki/Orbital_%28The_Culture%29), but then why the mention of a sphere, or that Vavatch is not big enough to be a ring?

Answer: 
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](https://en.wikipedia.org/wiki/Ringworld), or a sphere, by which I presume is referring to a [Dyson Sphere](https://en.wikipedia.org/wiki/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.
"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
> 
> 
>