A mountain climber on one edge of a deep canyon 800 ft wide sees a large rock fall from the opposite edge at time=0. As he watches the rock plummet downward, his eyes first move slowly, then faster, and than more slowly again. Let a be the angle of depression of his line of sight below the horizontal. At what angle a would seem to be moving the most rapidly. That is, would da/dt be maximal? Remember the rocks acceleration would be -32 ft/sec^2