Planes are flat, right? And the Material Plane is infinitely large?
I typically describe the sky material plane in a manner similar to that of the Earth: there's a sun in the sky and clouds and things, and at night the sun goes down and there's a moon that has phases and stars. I feel like I have some in-book support for a moon, given werewolves and certain magic items and such, and Pelor is literally the God of the Sun (and there is a Sun domain), but recently I had a player point out that the Sun must smash its way through the whole of the Underdark and there should be a massive tunnel to go exploring in. That idea was cool, so the party is now doing that, but I don't really think that such a catastrophic geography is good for every campaign. Is there any other reasonable explanation as to the location of the sun at night? Is there any explanation given in official material?