Each spherical shell is too thin to be seen face on. It is only when looking at the edges that our line of sight passes through enough of the shell to make it visible. This is known as the Limb Effect and is why the Sun looks darker at the edges.I'm still having trouble with only being able to see the portion of the shell that is "edge on." Why can't we see the rest of the sphere. Wouldn't it just look like a giant ball?