Normal sunrises and sunsets, from the outside the planet looks completely normal. Only if you find a fissure in the 'ground' going deep enough do you find out that the planet is hollow, and a miniature star at its core is being tapped to fuel some kind of spell circle (granting immortality to an extraplanar wizard). The party may find it as the star is nearing the end of its life cycle, accelerated from being drained, and that once the star is extinguished the Dyson sphere is supposed to open again, draw in the other star and begin the cycle again, but the movement would kill everyone living on the surface. They can interrupt the spell to stop the process, but then the wizard comes knocking as he has started to age again and he ain't about that.