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    Re: Extremely poor conditions??
    From: Marcel Tschudin
    Date: 2012 Mar 20, 11:18 +0200
    Alex,

    Just a short comment. I am still very much engaged in a project.

    At this time of the year the water temperatures are low and the air temperatures can be considerably higher. This leads then to inversions which can, depending on the conditions, be very strong.

    If you know the nearest station (and its code) launching weather balloons you can check the temperature profile for Mar 18, 00Z from e.g. the UWyo Web-page http://weather.uwyo.edu/upperair/sounding.html (Note that they have data from more stations than shown on the map.)

    If you would have taken some photos of the setting sun, you could now recognise the unusual atmospheric condition from the strong deformation of the sun's shape.

    Greg probably can contribute with his observations made some months ago while St. Ana was blowing; this likely corresponded to a similar situation.

    Marcel


    On Tue, Mar 20, 2012 at 2:58 AM, Alexandre Eremenko <eremenko---.edu> wrote:

    Here I post a series of observations that puzzles me very much.
    On March 17, in an unusually warm weather for this season, we spent an
    afternoon with Bill Burchell in St. Joseph harbor on lake Michigan,
    N 42d06'8, W 86d29'5, with his Astra sextant and my TS (pocket sextant).
    We made 7 series of 3-5 observations each of Sun and Venus altitudes over the lake
    from a jetty, approximate height of the eye 12 ft.

    The weather was sunny and warm (80 degrees F by Bill's thermometer).
    The horizon looked very sharp, though there was a lot of glare under the Sun.
    We took a great care in determination of the Index error (from the Sun, from
    the horizon and from a remote roof), in the beginning and in the middle of
    observations. Our determinations varied between 0 and 0'.5, but most were
    less than 0'2.
    Astra had a perceptible side error (about 1/5 of the Sun SD).
    In the following
    I do not correct for the Index or side error.

    My TS has zero Index error to the best of
    my knowledge. I had excellent results with this pocket sextant which I
    posted on the list in the past.
    We observed Sun (lower limb) and Venus altitudes, corrected for
    the height of the eye, semidiameter and refraction, and compared the results with computed altitudes.
    By the "error" I mean the observed and corrected altitude minus the computed
    altitude.

    GMT 21:34, Astra, Sun. Errors: -11'5, -14'2, -9'9, -9'2, -6'3 (Aver.-10.2,SD=2.6)
    GMT 21:46, Asrra, Sun. Errors: -9'9, -10'5, -10'6, -10'5, -10'5 (Bill)
    (Aver.-10.4,SD=0'25)
    GMT 21:54, Astra, Sun. Errors: -11'1, -12'2, -8'7
    (Aver.-10.66,SD=1'5)
    GMT 22:38, Astra, Sun. Errors: -11'4, -8'4, -9'7, -10'5, -9'5, -10'3
    (Aver.-10.0, SD=0'9)
    GMT 23:46, Astra, Venus. Errors: -9'7, -10'0, -9'4, -12'3, -11'4
    (Aver.=-10.6, SD=1.1)
    Sun set in the middle of this observation.
    GMT 23:58, Astra, Venus. Errors: -6'3

    Now pocket sextant observations (w/o telescope)
    GMT 22:14, TS, Sun. Errors: -11.3, -13.8, -10.8, -13.7, -13.4
    (Aver.=-12.6, SD=1'3)
    GMT 00:03 (March 18) TS, Venus. Errors: -11'0, -9'8, -7'8.

    Can anyone propose any plausible explanation of these enormous NEGATIVE
    systematic errors? I never had anything of this magnitude before.
    And as you see, they are pretty stable, about 10-11 minutes.
    I understand there can be a dark strip on the water, mistaken for the
    horizon. But this will lead to POSITIVE errors (overshots). Refraction which
    DECREASES the apparent altitude???
    We are talking of the errors about 1/3 of the diameter of the Sun!
    What could possibly cause this?

    Alex.
    P.S. Bill promises to find the water temperature of water on this day,
    but it is really
    hard to believe in this kind of abnormal refraction.

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