This is related to my previous Phys.SE question on the derivation of the phase function - upon sifting and scanning through 600 pages of John Strutt's collected work, there is absolutely no mention of Rayleigh's phase function (related to Rayleigh scattering) quoted by a million other sources with no derivation or justification.
I know it is some sort of godforsaken approximation for which nobody on the Web has a justification or derivation for and it's really bothering me. I have no problem with empirical data reconstruction, but I want to see this "empirical data" which everyone just throws around.
Why do people say something is "defined". Nothing in physics is defined, it's not a recipe for an exotic top-of-the-head meal, throw three cosines, five g and you're there. It has to be constrained by nature in some way.