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Hi Paul

To determine the index error, you should use either the natural sea
horizon or the sun itself.

Since your inland sea is frozen over (Lake Michigan), you must use the
sun.  With the proper shading in place, observe the sun.  There are
two methods

Method 1: superimpose image A directly upon image B of the sun.  Move
you index arm away and back until you are sure of the overlap.  Read
the drum.  If its on the arc, in the future you will deduct this
corr'n.  If its off the arc, you will add this corr'n.

Method 2: measure the sun's limbs from either side of the sun.  In one
case you will be on the arc, in the other you will be off the arc.
Which ever is greater will determine the on the arc off the arc
corr'n.  The delta between them (abs value) will be the numerical
value.

Best Regards
Brad

On Thu, Feb 28, 2013 at 4:32 PM, Paul Fithian  wrote:
> ________________________________
>
> While playing around with the sextant, I've noticed a big difference in
> index error when viewing things far away vs. near.
>
> For example, if I adjust the horizon mirror so that a near (5' away)
> horizontal line matches up through the glass and the mirror images, these
> images don't line up when I view a horizontal line that is several hundred
> feet away.
>
> Which one do I choose to apply the right index correction for the artificial
> horizon readings? I know I have to take the Hs, apply index correction, then
> take 1/2 of this reading to get Ho.
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