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Re: Reducing back sights
From: Bill Noyce
Date: 2004 Aug 11, 10:41 -0400
From: Bill Noyce
Date: 2004 Aug 11, 10:41 -0400
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