I understood turbidity is about the scattering due to dust in the atmosphere?
Overcast would be about a global illumination rather than a slightly scattered point source.
Perhaps you should use an HDR instead.
Edit:
Actually after reading up some about the Moons apparent size near the horizon it is an illusion.
Of course the size does vary a little with the distance of the planets from each other but the angular measure for both the sun and moon is pretty constant across the sky at about 0.53 degrees
Still I wouldn't mind a slider for the Sun/Moon size , dunno, maybe 0.9x to 1.7x the default dia ?? would 'look better' for some renders
A discussion of the illusion here
http://retina.anatomy.upenn.edu/~bart/scriptie.html....
There is a little graph in there showing a linear like change in the perception of the dia with elevation which would make it easy to implement in code for animations. I anticipate an 'enable apparent size' button
I note that a full Moon is about 1/500,000 the brightness of the Sun...

Anyone up for rendering a solar eclipse
