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    Re: Star-star distances for arc error
    From: Douglas Denny
    Date: 2009 Jun 20, 04:13 -0700

    Agreed "altitudes" .. not declination for same refraction component. Don't 
    know why I said declinations - brain not in gear whilst typing. I think it is 
    called senility.
    And agreed refraction is not completely negligable for rigorous calculation.
    I also take the point the errors (being in height - altitude) are going to be 
    greatest when the two stars are opposite each other towards 180 degree 
    separation, and less so when the hour angle between then is smaller:  my 
    point is the difference in refractions is going to be much reduced when of 
    comparable altitudes - and I should now say less than 90 degree sepatation in 
    hour angle -  and of a value not entirely way out and comparable to the 
    accuracy of the measurement itself.
    If errors in measuring; and the errors inherent in the sextant itself are 
    comparable with the errors in refraction then it is not worth worrying too 
    much about it, as it is more a theoretical problem.  Interesting to 
    contemplate and correct for, but I would query the practicality anyway for 
    serious measurement - unless lunars are your interest when refraction is 
    incorporated in the procedure anyway.
    Errors in calibration of the scale on a sextant can sometimes exceed a minute 
    of arc for example if poorly divided, but that can only be determined on a 
    proper dividing circle table or optical dividing head.  
    Attempting to use star separations to try to determine scale accuracy for 
    example would not be possible due to the variables in the measurements 
    themselves - including the refraction component even if calculated.
    Checking Polaris altitude (Northern hemisphere) is much simpler for a casual, 
    simple, practical check of the sextant index error if that is all that is 
    Douglas Denny.
    Chcihester. England.
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