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    Re: Index Error - Comparison to SD per Nautical Almanac
    From: Jim Rives
    Date: 2020 Nov 24, 12:37 -0800

    Hi Lars, 

    Your reply sent me scurrying to my books to look up sid-error.  No one mentioned this effect.  But it does make sense.  If two circles' centers are offset some distance then as they approach they won't actually touch where they would have, but will continue on a bit until they do further down the arc.  At first this seems like it should affect my determination of IE, but in doing that it is taking the difference between two measurments and, I believe, they cancel out.  I have always heard or read that side-error isn't all that important with respect to accuracy of the sight.  I suspect that that has to do with the circle against a straight line....   The effect of two offset circles meeting is amplified, it seems, by both, and increases rapidly as the offset increases.  

    I don't know whether this .2 minute reconciling item to the NA  SD is considered large or not.  I wonder if it becomes an important factor in doing moon-sun lunars. It appears that my measurement of that LD would be off .2 minutes for the same reason it is using the sun-on-sun method.. two circles of pretty similar size meeting with an offset between centers.  

    Thanks for your answer.   I continue to be amazed at the things I don't know!   

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