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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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