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I must have missed it, even in my re-reading of Iain M. Banks’ Look to Windward; what is the light source of the Masaq’ orbital?

If I understood the book correctly, the hub Mind sits at the center of the orbital. But what produces the day/night cycle on Masaq’?

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Like most Culture orbitals, Masaq' orbits (not circles) its star, Lacelere. It is slightly tilted, so as it rotates, roughly half of the inside is in starlight and half is in shadow.

Some notes by Banks here:

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.

Of course, the materials used in the construction of something ten million kilometres in circumference spinning once every 24 hours are far beyond anything we can realistically imagine now, and it is quite possible that the physical constraints imposed by the strength of atomic bonds ensure that such structures will prove impossible to construct, but if it is possible to build on a such a scale and subject such structures to forces of these magnitudes, then I'd submit that there is an elegance in using the same rotation to produce both an acceptable day-night cycle and an apparent gravity which makes the idea intrinsically attractive.

...

An idea of how the day-night cycle appears on the surface of an Orbital can be gained by taking an ordinary belt, buckling it so that it forms a circle, and putting your eye to the outside of one of the belt's holes; looking through the hole at a light bulb and slowly rotating the whole belt will give some idea of how a star appears to move across the sky when seen from an Orbital, though it will also leave you looking rather silly.

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This is directly addressed in the novel

“Let me tell you more about the Culture world Masaq’.” The Estodien gathered his robes about him and settled himself further into the cramped curl-pad. “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.

“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.

“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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