I've always liked the idea that Equestria's cosmology was more like the ancient concept of celestial spheres ([link]) than real-life solar systems. Of course, that wouldn't explain why the moon is sitting in a crater in a desert on the edge of the world, but it's still somewhat relevant, lol.
I read the related chapter... -_- (let's just say that the other chapters were WAY better... but, it's not the place to discuss that)
Anyway... Since the planet is round (and Equestria is a country/Nation), it would be very weird if the Moon and Sun were just casually sitting on the planet itself, and 1/1000000th of their size(or so). And, there's the fact that, in the series, they often mentioned the Moon/Sun *in the sky*, ad not ''waiting on the ground''.