Thanks for posting the interesting screenshot Lois99.
Now I can picture it in my mind that it's like a big 3D object made of blackness which we can only see one 2D slice of. The part where the ground (or anything else) intersects the blackness. Unfortunately it seems that in influencing the shape of it we must rely on trial-and-error which is like "feeling in the dark" as Wes said.

In my mind I picture all the area between the two meshes filled with black, and that shape being extended along the sun's path to the ground.

I really like the experiment. Stuff like that is important as a reference, we should try to get some of these copied onto the wiki, and maybe we can get it sticky link to that, so we can refer the newcomers to the FAQ, because this will come up again and again, I can guarantee it.