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    Re: Sudden shift in sextant sights
    From: Frank Reed
    Date: 2012 Mar 28, 17:39 -0700

    Zvi, you wrote:
    "A sudden shift of about 1.7’ . I have not changed anything, and kept moving up the shingle beach with the rising tide to maintain a constant height of eye of two metres. The day was very warm and sunny with clear sky, high pressure and noticeable haze. "

    Tis the season. Was there any breeze? Did you notice a change in the direction of the breeze, on-shore versus off-shore, as the day went on? On Spring days when the water is cold and the air over the land heats up during the day, a breeze will often develop during the afternoon that changes the layering of warm and cool air over the ocean surface within a few miles of the shore. Any unusual layering will change the so-called "terrestrial refraction" and move the horizon up or down. This can easily amount to a couple of minutes of arc and ten minutes of arc is not rare. This doesn't happen on the open ocean except in the vicinity of sharp changes in ocean temperature like the eddies of the Gulf Stream. And both near shore and near these currents, the presence of fog during the day also accompanies conditions that may lead to this anomalous dip during other parts of the day. In other words, if you see fog scudding along in the morning, be suspicious of the dip for sights taken later in the afternoon.

    Alex's suggestion that you should consider blunders first and blunders second is excellent advice. Once you've ruled them out, and assuming the instrument is a good one, the uncertainty of the horizon is next in line.

    -FER

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