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    Re: Altitudes, close to 90
    From: Trevor Kenchington
    Date: 2004 Nov 26, 16:18 -0400

    You wrote:
    > Everyone knows that measuring altitudes close to 90 is
    > hard, however I have not seen the precise explanation of this
    > in the books on navigation that I read.
    > As I think this is of more relevance to
    > practical navigators than my previous message,
    > I include a quantitative explanation.
    > The source of error is that you cannot determine precisely
    > the point on the horizon which is exactly "below" the body.
    I think you have that wrong. The point on the horizon that is exactly
    below the body is the point that the body is brought down to when the
    sextant frame is exactly vertical. The problem is ensuring that verticality.
    With a body of moderate altitude, one can swing the arc and fairly
    easily see when the body reaches the lowest point, which is when the
    sextant is vertical. As the altitude increases towards 90 degrees, the
    apparent radius of the swung arc increases and the point along it
    corresponding to the vertical becomes harder to determine.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus{at}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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