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: 
[Look to Windward](https://en.wikipedia.org/wiki/Look_to_Windward) contains a description of an Orbital in chapter 12

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> 'It is what they call an Orbital; a band of matter in the shape of a very thin bracelet, orbiting around a sun - in this case the star Lacelere - in the same zone one would expect to find an habitable planet.
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> 'Orbitals are on a different scale from our own space habitats; Masaq', like most Culture Orbitals, has a diameter of approximately three million kilometers and therefore a circumference of nearly ten million kilometers. Its width at the foot of its containing walls is about six thousand kilometers. Those walls are about a thousand kilometers high, and open at the top; the atmosphere is held in by the apparent gravity created by the world's spin.
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> 'The size of the structure is not arbitrary; Culture Orbitals are built so that the same speed of revolution which produces one standard gravity also creates a day-night cycle of one of their standard days. Local night is produced when any given part of the Orbital's interior is facing directly away from the sun. They are made from exotic materials and held together principally by force fields.
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> [...]
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> 'Each Orbital is different and each Hub has its own personality. Some Orbitals have only a few components of land; these are usually square parcels of ground and sea called Plates. On an Orbital as broad as Masaq' these are normally synonymous with continents. Before an Orbital is finished, in the sense of forming a closed loop like Masaq', they can be as small as two Plates, still three million kilometers apart but joined only by force fields. Such an Orbital might have a total population of just ten million humans. Masaq' is toward the other end of the scale, with over fifty billion people.
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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)..