In fantasy works, it can be interesting to imagine an infinite world. It means you can travel in any direction for as long as you want, always discovering new lands, new cultures, new landscape, etc.
By infinite world, I mean an infinite habitable surface.
However how should such a world be shaped to have a "normal" sun cycle ? That means to have the following features (which the Earth actually have) :
- The sun travels through the sky and when the night comes it disappear behind the horizon.
- There are places on Earth with seasons. For simplicity, assume it only means longer days with the sun higher in the sky during some periods (summer) and shorter days with the sun lower in the sky during others (winter).
- There are places on Earth without seasons.
- The world looks locally flat (there can be geological stuff, like mountains, but no obvious curvature of the world at short scale).
- The fraction of the world which is habitable is infinite.
Note that since it is a fantasy question, the world can
- Have a different sun every day.
- Some part of the world can have behaviour not known on Earth (for examples never ending darkness passed a given point).
- Break any law of physics (for example, the trajectory of the sun can be anything, and the gravity does not need to be explain or even explainable without using magic).
It is clear that given all that, the world can not be flat, because if it was, the sun would never disappear behind the horizon. It is possible to give some kind of curvature to the world to avoid this, but then the question of seasons arise.