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    Re: sextant index error measurement
    From: Frank Reed CT
    Date: 2006 Nov 5, 18:52 EST
    George H wrote:
    "When you check index error with a horizon, it's true that the
    direction of the horizon is displaced, due to refraction, but that
    displacement is almost exactly the same for the direct view through
    the horizon mirror, as it is for the reflected view through the index
    mirror, from a point only a couple of inches higher up. So that any
    refraction has a quite negligible effect on such error checking. For
    that purpose dip tables are irrelevant."
    Here's another fun thing to consider on this topic...
    The index mirror has a higher "height of eye" than the horizon glass. So that means that dip is a tiny bit larger for the reflected view of the horizon than the direct view. Supposing the index mirror is three inches above the direct sight line, how close to the surface would you have to be (how low a height of eye) so that the difference in dip would be larger than 0.1 minutes of arc? Looking at it another way, how close to the water would I have to be before the visible horizon itself is too close for a proper index correction?
    42.0N 87.7W, or 41.4N 72.1W.

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