Stand in the inhabited belt of Proxima b, and the star rests on the horizon. It does not rise or set. Noon and midnight are not times here; they are places.
Terminus is a series that sits between science fiction and system design: an alien AI asks Earth to design a satellite network that serves a large language model to a young civilization on this planet. Part 2 studies the planet before any satellite goes up, because it breaks most of what a satellite engineer brings from Earth.
What the chapter establishes
A locked planet still rotates. Tidal locking means the rotation period equals the orbital period, 11.2 Earth days here. That matched turn holds the star still in the sky. A planet with no spin at all would see a sunrise every orbit.
Moving air makes the habitable band. A single convection cell rises under the star, crosses to the night side 7 to 10 km up, sinks over the ice, and returns along the ground. The return leg keeps the twilight band temperate and gives it a dayward wind that never reverses. Stall the loop and the night side becomes a cold trap that freezes the atmosphere out of the sky.
Demand is a ribbon. Every user lives within about 20° of the terminator, the line between day and night, and that line never moves across the ground.
Fixed to the ground is not fixed in space. Orbits hold their orientation relative to the distant stars, while the terminator turns 32° per day in that frame. A ring placed over the band today will be gone by tomorrow. Every constellation in the series shows this.
Slow rotation shrinks J2; the lock adds C22. Oblateness goes as the square of the spin rate, so the planet is about 50 times rounder than Earth. The star's permanent tide adds a second degree-2 term, C22: an equator stretched toward the star.
Mercury is a false analog. Its 3:2 spin-orbit resonance gives it a 176-day solar day, and its terminator crawls along the equator at about 1 m/s. A twilight city there would have to walk.
The full chapter derives the planet's J2 from Earth's own measured figure, animates the three spin rules and the wind loop, and shows what a 0.1° launch error costs a polar ring over a decade:
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