I am still thinking about world building in my game and I was thinking about Oerth, the Greyhawk world. In Greyhawk the sun goes around the world, as it did in some dark ages beliefs.
Now here is the thing. Or two rather. First, in today's age we tend to think of ourselves as very progressive and smart and forget that a lot of the things we know now, people also knew then. Sure there are some "fantasy" style beliefs. After all the Greeks knew a lot about Astronomy and even knew the size of the Earth (and the fact that it was round) as far back as the 4th century BC. Sure today some people still think the world is flat or created by magic, but for the most part people are and were smart.
Secondly I like my universe to make some sort of sense. Afterall I still would like to play a Greyhawk 3000 game someday and that might be harder if my sun is really just a burning mountain going around a flat earth.
So yeah, I like magic, vampires and ghosts in my games, but draw the line at Earth-centered cosmologies.
Maybe I can have parallel cosmologies. If one goes to the sun via magic, they find the realm of Pelor there, but if you launch a probe, it is just a ball of nuclear fusion.
What do you all do in your FRPGs? Do you even bother? What is your cosmos like?