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    Re: Sextant accuracies
    From: Trevor Kenchington
    Date: 2003 Mar 17, 22:26 -0400

    Doug Royer wrote:
    > In coastal pilotting these are known as Horizontal distance bearings.Pick 2
    > or more terrestial objects.Turn the sextant on it's side and take the angles
    > between the 2 or 3 objects by superimposing one of the objects over one of
    > the remaining objects.Dip is not a consideration in taking Horizontal
    > angles. Write the angles down and go to the chart.With dividers find the
    > distances between each of the objects.Your position will be tangental to the
    > distance between the objects.Calculate the distance off,using dividers from
    > each point draw in the radius and where the lines intersect is your
    > position.
    Not quite.
    It is possible to find distance off by observing the vertical angle, if
    the elevation of some charted object is known, but the method Doug is
    trying to describe is more commonly known as "horizontal sextant
    angles". It does not involve estimating distance, nor bearings.
    Pick three charted objects. For convenience here, I'll name them a, b
    and c, from left to right as you view them. Using your sextant on its
    side, measure the angle between your lines of sight to a and to b, then
    likewise the angle between b and c.
    On the chart, draw a construction line from a to b. Subtract your a-to-b
    observation from 90 degrees and draw another line through a that makes
    an angle (90 minus a-to-b) with the line from a to b. Do the same thing
    with a second line passing through b, thus forming an isosceles
    triangle. Where the two lines cut at the apex of the triangle is the
    centre of a circle that passes through a, b and your position. Draw that
    circle on your chart.
    Repeat that plotting for a second circle passing through b and c, using
    your observation of the b-to-c angle.
    Since your position is on both circles, it must be where they cross.
    There are some extra wrinkles, dealing with cases where a, b,c and you
    are all on (or close to) the same circle or where your observation is
    greater than 90 degrees but the above is the essentials of the method.
    As has been noted, it is much the most accurate of the non-electronic
    position-fixing methods available to a navigator and was much used by
    hydrographic surveyors for that reason. Indeed, I rather suspect that
    the more upmarket sextant manufacturers are kept in the business by
    surveyors' needs for precise instruments rather than by the few seagoing
    officers who are willing to pay for something more accurate than an Astra.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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