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    Re: CHO-T Manual
    From: Bill B
    Date: 2007 Sep 09, 18:05 -0400

    Gary wrote:
    
     > Second, it describes a method of measuring the index error by bringing
    > the top and bottom edges of the sun together twice and then the average
    > of these readings will give you the index error. For example, assume no
    > index error and the semi-diameter is 15'. Bringing the upper limb of the
    > reflected image of the sun into conjunction with the lower limb of the
    > direct image should give you a reading of minus 30'. Then putting the
    > lower limb of the reflected image in conjunction with the upper limb of
    > the direct image should read plus 30'. The average of these reading is
    > zero, no I.E. If the  readings were minus 20' and plus 40' then the I.E.
    > would be plus 10' . This avoids using the horizon.
    
    Keep in mind you need to subtract the off-the-arc reading from 60 to get the
    actual distance from 0.
    
    Also note the sun should be well above the horizon (I like 30 d elevation or
    better) to counter the disc being distorted by refraction.  The textbook
    method would have one hold the sextant horizontally to avoid refraction
    problems.  Thanks to my astigmatism I will get different IE from horizontal
    observations vs upright observations even with the sun high in the sky.
    
    My problem is that I get slightly different IE (approx. 0!3) when using the
    sun method vs a natural horizon or a roofline several miles off.  I
    attribute this to my astigmatism.  Alex also see this difference although he
    claims no astigmatism.
    
    Bill
    
    
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