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It's an unanswerable question unless we define the conditions and assumptions we use. Where do we stop?  Is everything a black box because we really don't no the 'ultimate' cause of everything. I say, it's not a black box to me but only because I can derive all the spherical trig equations needed for reduction. After that, I know the basics of a watch/chronometer and certainly all the optical principles.  But that's me.  I would say a sextant would be a black box to me if it were indeed enshrouded in a black box and I had never seen the inside. So, the decision as to whether it's a black box depends entirely on the level of understanding of the user, I MO (space purposely left blank)