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    My first observations with natural horizon
    From: Alexandre Eremenko
    Date: 2005 Aug 6, 09:14 -0500

    For the first time I had an opportunity to use
    the natural horizon.
    I sailed from Dublin to Kiel, with stops in
    the Scilly Islands, the Isle of White and Helgoland,
    in a 42 feet sailboat, on July 23-August 6.
    Unfortunately, the weather was not good for Cel Nav.
    For several days the Sun was not visible at all,
    on other days I had to use small gaps in the clouds.
    There was no oppornutity for Lunars, and I could make
    only one star observation, in unfavorable conditions.
    The first week was a disappointment (from the Cel
    Nav point of view); we sailed downwind with hudge waves,
    and I could only make observations without a scope.
    The results were very bad, but one has to take into account
    that these were my FIRST observations with natural
    One general remark on observations without a scope.
    I think, one HAS to use the zero-magnification tube.
    Otherwise, it is hard to make sure that your line
    of sight is parallel to the plane of the arc,
    especially when the boat is shaking violently, and
    its course is not stable.
    This introduces a hudge "colimation error"
    which resulted in errors in position line of several minutes,
    once even 2 degrees, though I am not sure what exactly
    caused such blunder.
    In the second week, I achieved a tolerable precision.
    Maybe because of some practice, but more likely,
    because the weather improved.
    Here is the statistics of all my observations in the second
    week (August 1-6).
    I used my SNO-T sextant and Casio watch. Reduction with
    the Almanac and Casio calculator.
    The number in parentheses is the number of observations
    averaged; if there is no number
    in parentheses, this means that only one observation
    was made. The number of minutes
    is the error
    in altitide (using GPS position as AP).
    August 1 Sun, 30 knots wind, waves 2-3 meters,
    course downwind: (5) -0.2'
    August 2 (Sun from a beach, Helgoland)
             (5) +0.5'
             (1)  0.0'
             (2) +0.5'
             (3) +0.5'
    August 4 Sun, good weather (15-17 knots wind, waves less
    than 1 meter): -0.1', -0.1',
    0.1', -0.5',
    0.3', -1.2', -0.8', -0.6', (3) -0.3'.
    The only star observation was on July 25:
    Venus (4), error -1.4' and Arcturus (1) error +3'.
    If anyone is interested, I can post (or send) the full detail
    of each observation.
    I also tested my new Bris sextant which I bought at
    Cassens-Plath factory which I visited early in July.
    They said that the one I bought was the last one,
    its production being discontinued.
    I will write a separate message on the Bris sextant.

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