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George Huxtable asks:

> A further question now arises: what does Jim Thompson
> wish to use an octant backsight FOR?

I had a different reading of the question.  I assumed Jim was
using a sextant to measure the altitude of a body high in the
sky, but without a suitable horizon under it.  He's measuring
the altitude from the opposite horizon instead.  In that case
he has the procedure exactly correct.

David Weilacher raises a concern about differing refraction
between UL and LL corrections.  I think Jim's procedure works
correctly, as long as he remembers that what looks like a LL
measurement in the backsight is really measuring the (supplement
of the) altitude of the body's LL, and vice versa.  The key is
to perform all the corrections for the body itself *after*
subtracting the reading from 180.  Jim is correct that index
error and dip corrections should be applied *before* subtracting
from 180.  After correcting for dip and index error, and
subtracting the result from 180, the result is just what you
would have gotten from a normal forward sight after the same
corrections.

        -- Bill

   

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