The elegance of conventional Orbitals lay in the fact that their diameter – they were usually about three to four million kilometres across – meant that the speed you had to rotate them at to produce what would feel like standard gravity to your average Culture person – not to mention a generous statistical spread of non-Culture humanoids and other species – also automatically produced a day/night cycle that was right in the middle of what the Culture regarded as the acceptable spectrum of values.

The artificial, inside-out worlds usually orbited their stars on roughly circular paths, generally following orbits set between those of any planets present, though sometimes taking up the same orbit, socketed into safe, non-collisionary lock-step tracks in and around the planet’s Trojan points. Some Orbitals flew more ellipsoidal paths, swinging further out and closer in to their star, to produce seasons, if desired. Spinning them almost but not quite edge-on to their parent sun prevented the far side from eclipsing whatever part lay in sunlight at the time.

The Culture hadn’t invented Orbitals like this, but it had taken them up with an enthusiasm nobody else had ever displayed, the Minds attracted both by the grandiosity of the concept – this was engineering on an epic scale, and even the skinniest full O had the surface area of twenty or thirty standard one-G planets – and by its sheer chronocyclic, material-frugal elegance; compared to planets, Orbitals represented a very matter-cheap way of providing generous amounts of pleasantly rural and even wild-looking living space, plus you could usually build your Orbitals from exactly the asteroidal debris you’d want to get rid of in a solar system anyway, to stop bits of it flying around and hitting your fancy new worlds.

Microrbitals were, as the name implied, much smaller versions of the same basic idea; they could be any diameter you wanted because the spin speed wasn’t rigidly linked to both the force experienced at their surface and their day/night cycle. You spun a microrbital at the appropriate rate to produce whatever apparent gravity you required, set it circumference-on to the sunlight, then put a suite of angled mirrors at its centre to reflect the light, rotating these independently to give you the desired light/dark periodicity.

The microrbital of Bokri was tiny; barely more than a thousand kilometres across. This was so small that the retaining side walls of sheet diamond which kept the atmosphere in would have met in the middle had the world been filled with a significantly denser atmosphere or spun much slower. As it was, the central mirrors were so close to the top of the walls that they rested on struts running along their inside edges so that the whole assemblage looked like the axle and spokes of a giant wheel.