I render the entire sky including sun and moon using a pp-effect. Eventually the shader works mainly with some simple color-interpolations depending on how close a pixel is to the sun, the position on the hemisphere itself and of course the daytime.
I also use a multi-pass bloom for the whole screen and the well known lightshaft shader depending on a radial-blur (also multi-pass) of the sun-disk (at least I use a bigger and softer disk then the original sun-disk to blur).

I hope this answers your questions