Correct above poster, the answer is whatever crazy untested theory they want. Sorry to put words in your guys' mouths, FE'ers, but perhaps it goes something like this:
You see, I observe that the sun always looks like a circle no matter where I am in the world. I build a sphere and a disc, as well as a whole host of other shapes, and I look at them from different angles. It turns out that only the sphere is consistent with what I see when I look at the sun. Does that give weight to the spherical hypothesis of the sun? No, because you see, I want the sun to be a disc, and so instead it must be that it is an optical illusion and my eyes are really deceiving me. There's this thing called refraction in geometric optics that allows parallel rays of light to get redirected at different angles. Change of medium can do this, and so I'm gonna say there's this stuff in the sky that bends light to make a disc look like a sphere everywhere. After all, refraction exists, so I'm going to invoke it whenever and however I want with the convenience that the sky makes discs looks like spheres at long enough distances. How much distance? Who knows, and I've never tested any of this, but it has to be correct. The alternative of the sun simply being a sphere as it looks, after all, is ridiculous, right?