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Alex wrote

> Observations are takes from three different points near the shore,
> both with upper and lower limb.
> Of course I double chesked and triple checked the index correction
> (using the flat roofs about 0.5 mile away). It is zero!
>
> So where could these -3.5 possibly come from?
> This cannot be any anomalous dip (the whole dip correction
> was between -2.2 (when I sat on a stone just next to the water edge)
> and -3.5). It is hard to believe to such large arc irregularity at
> such moderate angles.

I would look in two areas:

1. Perhaps .5 miles for IC check is OK for a pocket size sextant.  It is not
for a 7/8 or full size sextant.  The key here is the approximate vertical
distance between the centers of the horizon and index mirrors.  That
distance divided by the distance to the roof should give you the tangent of
any parallax error.

2. Wave height can affect dip quite a bit.  Your eye might be 6 ft above the
average water line, but if waves are 6 ft high on the horizon, dip becomes
0.

Bill

   

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