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    Re: Perpendicularity check
    From: Frank Reed CT
    Date: 2004 Sep 23, 00:01 EDT
    Alex E wrote:
    "I tried star distances (as described in Bruce Bauer for
    testing a used sextant). The distance Arcturus-Dubhe seems to indicate that my sextant error is at most) 0.3'."

    The account of this procedure in Bruce Bauer's book is not so good. First, star-star distances change in an annual cycle due to aberration of star light. Second, some stars have significant proper motions so a table of star-star distances is generally only valid (at the level of accuracy sextants can measure) for a couple of decades even if it includes seasonal tables to account for aberration. Third, it is simply not correct that star-star distances observed when two stars are at the same altitude do not need to be corrected for refraction. The easiest way to see this is to imagine a star which is due east at an altitude of 45 degrees and another star due west at an altitude of 45 degrees. Refraction will push both stars towards the zenith by about 58 arcseconds so the measured distance between them would be close to 89d 58.1'.

    Visit the list archives for April 6th. I posted a couple of messages on star-star sights. The steps for clearing a star-star sight are very similar to lunars so they make a nice introduction to the lunar distance method. More importantly, when they are handled correctly, they are probably the best test available of a sextant's arc error. If you do a star-star sight for every ten degrees of distance and make a table of your cleared results, you will have a valuable tool that you can apply to all other observations you make with your sextant.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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