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    Re: Star-to-star distance for sextant calibration?
    From: Alexandre Eremenko
    Date: 2019 May 21, 12:41 +0000

    My own experience confirms everything that Brad wrote. I posted many messages on this on
    this list in 2005-2007, but let me re-tell the story shortly.
    
    When I purchased my SNO-T it was "new" from the factory (in fact 12 years old 
    but in factory packing and never used).
    It had all documents including the factory "arc correction table" which made absolutely no sense:
    it showed constant +10" for the whole arc. It is clear that the person who 
    filled this table just did not understand what she was doing:-)
    
    I also have this Russian book which recommends to calibrate the arc with 
    star-to star distances, and gives a short table
    of stars for this. However the book does not even mention that to use this 
    table one has to correct the distances
    for refraction, which leads essentially to the same formulas that are used for 
    Lunar distances. (All this was explained to me on this list).
    I conclude that the author of the book just did not test his own advise:-)
    
    So I programmed these refraction correction formulas on my computer and 
    started to measure star distances, which I did systematically
    during 3 years (many of my results were posted on this list).  My distances were
    generally within 0.2'-0.3' of the true ones, but sometimes I had larger 
    errors. No conclusions about the arc errors could be
    derived from them.
    
    At the same time I had my sextant calibrated by the two places which still did 
    this at that time: Freiberger and Cassens-Plath (I visited their pants).
    Cassens-Plath produced a certificate for my sextant which looks like their 
    standard certificate: "This instrument is free from error for practical use".
    Freiberger produced a certificate filled with some numbers (I was actually 
    present at the process of measuring at Freiberger, while Cassens Plath did 
    not permit me
    to see the process). The table that Freiberger produced looks like this
    0    10     20     30      40      50      60       70     80       90     100     110       120
    0    -4"     -4"   +10"   +6"     -1"     +4"    +5"    -5"      +4"    +4"     +3"       +14"
    I measured enormous number of star (and Lunar) distances, but the results show 
    no correlation with this table.
    Of course, the table does not contradict the Cassens-Plath statement, and on 
    the other hand it has no practical use.
    
    The general conclusion is that modern manufacturing process produces arcs 
    which are "free from error for practical distances",
    and you cannot detect any non-uniformity by observing star-to-star distances. 
    On the other hand, from historical literature I know that
    in 18th century people indeed were able to correct the arcs by observing star 
    distances. My conclusion is that 18th century arcs were
    just inferior, and had larger errors, at least some of them.
    
    
    Alex.
    
    

       
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