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: 
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*](https://en.wikipedia.org/wiki/Elysium_%28film%29) for a demonstration of the idea.
As others have mentioned, it's a large ring which rotates to generate centrifugal force to simulate gravity on the inner wall. The ring is in orbit around a star like a planet orbits a star (with the star far outside the circumference of the ring itself). In this way, the inhabitants inside the ring can watch the sun rise and set as the ring turns (imagine "midnight" as the time when they are closest to the sun, their feet pointed toward the sun, and it is obscured by the floor. And "noon" as when they are furthest from the sun, their feet pointed away from it, and it can be seen roughly overhead).

Other responses have neglected to mention that the surface of the orbital is also sloped toward the edges so that the inner surface looks like a parenthesis in cross-section. (If you were on the inner surface, with both edges equal distances, you would need to "climb" to escape the ring). This is explained as the principle reason the atmosphere stays inside the ring, as there's no solid ceiling. There is also a layer of artificial dust suspended in a magnetic field to shield the inner wall from solar radiation and create different climates.