And thinking further. f95toli, you say:
But that is literally from an optical point of view. It's what we see using a lens and a retina. If we had a very large light sensitive surface, quite flat, arranged at the point of the earth in space and perpendicular to the plane of the earth/sun, would not all points on that surface over a disc the same size as the sun receive the same amount of light ?
In fact, I suspect that the surface would receive the same amount for the same sized disc, and then less according to some ratio as we travel further beyond that point because beyond that point the surface is effectively tilted to the rays that strike it.
Which brings us back to my very first point that the effect of the diminution in power over the curvature of the theoretical sphere's surface is not strictly an effect of the relationship between a disc and a sphere.