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Thank you George for your contribution.

May I just add some further comments

> Marcel's question puzzled me considerably, until it emerged that was
> referring to bubble-sextant observations, with respect to the true
> horizontal, not altitudes measured up from the observed horizon with an
> ordinary sextant.

This comment reminds me of something I forgot to think of. Yes, the observed
(apparent) horizon is not the physical (true) horizon. Due to the
terrestrial refraction the apparent horizon is already something below the
physical one.

>>
>>R(-2?)  =  R(0?)  +  (  R(0?) - R(+2?)  )
>>

The idea behind this approximation is that in the case of negative altitudes
the light has to pass the lower air masses twice, once incoming until e.g.
the tangent point at the earth surface and a second time outgoing from the
tangent point to the observer at the elevated position.

Marcel

P.S. Greetings to Abingdon, where I lived something over twenty years ago...

   

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