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    Re: My first observations with natural horizon
    From: Joel Jacobs
    Date: 2005 Aug 9, 18:12 +0000
    Alex,
     
    Since this was your first attempt to shoot from a small boat in rough conditions, it would be intersting to know what procedures you used to get your self postioned so that you were not concerned about falling or going overboard, and what else you did to get the best sight possible.
     
    I guess your skipper was more concerned in getting to his destination. Frequently, a change in course or heaving to, provides a much better platform.
     
    Joel Jacobs
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    -------------- Original message from Alexandre E Eremenko <eremenko{at}MATH.PURDUE.EDU>: --------------


    > 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 ver! y bad, but one has to take into account
    > that these were my FIRST observations with natural
    > horizon.
    >
    > 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 C! asio 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 a! lso 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.
    >
    > Alex.
       
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