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    Re: Sun sights in foggy weather
    From: Frank Reed
    Date: 2019 Dec 12, 09:50 -0800

    Igor S, you wrote:
    "Last Sunday I was driving in a foggy weather but nevertheless I've clearly seen the sun disc"

    These are interesting opportunities for sextant sights. Of course if the fog is on the surface, the horizon may be obscured, but often the fog is high enough (a thin low cloud layer) that you can see the Sun's disk clearly and the horizon, too. Then you can shoot the Sun with no shades and bring its sharp limb right down to the horizon.

    There are other times when there's more scattering, and the Sun becomes a "blob" without a clearly defined limb. It's still possible to shoot the Sun in those cases with 5-10' accuracy by bringing the center of the "blob" down to the horizon. In that case there's no correction for semi-diameter (or if you have "Sun correction" tables for UL and LL sights, you average them).

    By the way (a question for anyone), what's the difference in water droplet and/or ice crystal sizes for these two cases? When is the Sun's limb sharp when seen through fog or clouds? When does the limb disappear leaving a diffuse image or "fuzzy blob" for the image of the Sun?

    And you asked:
    "would I have an appreciable error in measured altitude (due to atmospheric refraction)?"

    I didn't quite catch what you were asking when you first posted this ten days ago. I think I do now. The refraction under such conditions is almost exactly identical to the refraction in clear skies. There is a very small change in refraction dependent on the water vapor content in the atmosphere, but it's never large enough to affect manual celestial navigation sights. Certainly the clouds themselves (as distinct from the water vapor content, which is not visible) contribute nothing to refraction.

    Frank Reed

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