What’s the surface spectral reflectance of the sun? I know it’s approximately a black body radiator, but clearly it doesn’t follow that it’s pure black. So what is it?
@vruba
I don't think that question has an answer or makes any sense from the physics point of view. you can't ask what percentage of the light emitted by the sun is reflected by it's surface, moreover it doesn't even have a surface.
@rnanclares I’m not asking about light emitted by the sun specifically, and consider that (say) Jupiter doesn’t have a sharp surface either, but does have a reasonably well defined (though variable) reflectance from its outside.
@vruba
Because its atmosphere reflects the sun radiation, not its surface.
You're asking wha's the reflectance of the source of the radiation which IMO doesn't make sense.
@rnanclares Something about the way I have used the word “surface” seems to be causing a miscommunication. Think of it this way: I’m interested in the reflectance of the sun as seen from the outside.
@vruba
Yes, surface reflectance is something different from atmospheric reflectance.
In this case the sun is not reflecting anything because you need to consider a source of electromagnetic radiation which in this case is the sun itself.
What is the emittance of the sun, is IMO the correct question.
