I have been away from blender for a few years, but I just picked up Avastar so I"m giving myself a crash course refresher on it, and this thread got me back up to speed fast. Nowhere else has all the things I missed laid out so nicely.
This is not the right place for this, but I cannot find a better thread. There is a group of national security people who engage in crowd sourcing photos to determine the geo-location where a picture was taken. Once they figure out where a picture was taken and the orientation of the camera, the next step is to figure out the time of day that the picture was taken.
One of the tools being explored is to use the sun position and the fspy add-ons in blender to approximate the position of the sun based on the shadows in a photograph. Once they get the position of the sun, they can work backwards to get the time of day.