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    Index Error - Comparison to SD per Nautical Almanac
    From: Jim Rives
    Date: 2020 Nov 23, 06:01 -0800

    I have been doing lunars pretty regularly since the beginning of the year.  Normally I take the opportunity to determine my Index Error using the Sun-on-Sun method, as it seems to be more reliable than the split horizon method.  I had expected to be able to confirm my calculation of IE by comparing the resulting calculated Semi-diameter (SD) with the SD published in the Nautical Almanac.  However, I noticed right away that it was off... so initially I thought it was some sort of rounding error.  However, the data for the past six sights shows a calculated SD that is consistently .2 minutes smaller than the published SD.  The IE determined averages +2.6 pretty consistently.   I had thought that I should be able to come to the published SD normally.  Can anyone make any suggestions as to what could be causing this.  Should I be able to agree to the SD per the Nautical Almanac?  

      

       
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