Indeed "shadow" is the third of the traditional imagery interpretation criteria ( after "shape"and "size" and before "surface" and "associated features").
Most commercial optical imaging satellites hit at around 10:3O a.m. optimum local sun time for this reason.
Yes , the opposition was also often aware of this way back historically and would hide things away, e.g. back in the hangar., at that time if they knew there was recce about.
