Good question. The Spiral serves as a universe, while worlds are like galaxies in my opinion. These galaxies have smaller areas inside of them which also float, like planets. So it would go: Spiral>World>Island is equal to Universe>Galaxy>Planet. Seeing that IRL there are still many galaxies left undiscovered, there are mysteries to these galaxies. I'm not even going to go into the whole "life on other planets" thing, but galaxies (if there was life there) far away from ours would need a sun to sustain life. Such as worlds would need their own sun, as there is no visible "one sun" seen in any of the Spiral maps. So to sum it all up, my opinion is that each world would have their own sun.
Wow this was longer than expected.

Ruthless Rebecca Radcliffe