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    Re: Swinging the Arc
    From: Joel Jacobs
    Date: 2005 Oct 20, 21:33 +0000
    George,
     
    As much as I would like to accommodate you, I know what I've seen over fifty eight years of blue water experience. Consequently nothing you've said will change my mind because you haven't got all your facts straight.
     
    Most experienced navigators will tell you there is only a single method. The idea that the celestial body is centered at all times requires that the line of sight from the observer to the body be dead on the azimuth, and at that moment, and only that moment, will the sun be stationary when the sextant is rocked.
     
    More often, the body does exactly what the diagrams in Bauer's and Watt's books illustrate. And I'm sure if I had access to my collection of Nav books which are most all in storage, I could come up with other examples illustrating the same thing.
     
    What I do agree with is that in taking high altitude sights that the object tends to jump off the mirrors. This has to do with a rapidly changing azimuth in which rocking the sextant accentuates the change.
     
    Joel Jacobs
     
     
     
     
     
     
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    -------------- Original message from George Huxtable <george@HUXTABLE.U-NET.COM>: --------------


    > Joel Jacobs wrote-
    >
    > > Before we start hunkering down down for Hurricane Wilma, I want to thank
    > > Frank for his scholarly explanation.
    > >
    > > One question remains since I find this statement from the 1918 Bowditch
    > > ambiguous.
    > >
    > > 1918: "the observer should move the instrument to the right and left of
    > > the vertical, swinging it about the line of sight as an axis, taking care
    > > to keep the object in the middle of the field of view. The object will
    > > appear to describe the arc of a circle, and the lowest point of this arc
    > > marks the true vertical."
    > >
    > > Which object is it talking about. If its the celestial body, i! .e., the
    > > sun, then what I visualize would be the same as shown in the diagrams used
    > > to illustrate Method 2.
    >
    > ===================
    >
    > Reply from George.
    >
    > It's simple, really. Yes, the object (the Sun, say) stays in the centre of
    > the field of view, as near as an observer can manage it, as he swings his
    > sextant about an axis which is a line pointing to the Sun. But as the
    > sextant swings, so does that field of view (with the Sun at the middle)
    > appear to swing, superimposed as it is on the view of a part of the horizon
    > in the horizon mirror.
    >
    > So as the sextant swings, it's the horizon view that's racing past the
    > horizon mirror, not the Sun view racing past the index mirror. If you do the
    > job properly, that is. One bit of the horizon looks very much like another,
    > so you don't notice that happening. You would, if there ! was another vessel,
    > on the horizon, to show it up.
    >
    > Say there was a lighthouse, on the horizon exactly below your view line to
    > the Sun. Assume that the Sun is rather high in the sky. If you swing your
    > sextant about the line to the Sun (Frank's method 1), then the Sun would
    > stay still in your view frame, and the lighthouse would shoot from side to
    > side in the horizon mirror as you rocked the sextant to home in. And in the
    > end the measurement would be when the Sun limb grazed the horizon line just
    > at the lighthouse. If you used method 2, you could swing the sextant about
    > the line to the lighthouse, in which case the lighthouse would stay still at
    > the centre of the horizon mirror, and the Sun would shoot about across the
    > field of view, through the index mirror, as you momed in on it. That way
    > would work, but only if you happened to know in advance that the lightho! use
    > was indeed directly below the Sun. In practice, you're never provided with
    > such a convenient marker, and it's impossible to guess which point on the
    > horizon is the one that's directly below a high Sun.
    >
    > That's why method 2 doesn't work, in practice, for a high Sun. You might
    > start off rocking that way, but once the Sun has been captured in the index
    > mirror, then the Sun has to stay in view, and it's the horizon that gets
    > rocked about, using method 1. That's always how the rocking procedure has to
    > end up though perhaps unconsciously.
    >
    > Remember, what started this discussion was Joel's contention, which ran as
    > follows-
    >
    > "The procedure of rotating the sextant is to have the celestial body subtend
    > an arc in which its lowest point just kisses the horizon. At that moment,
    > the sextant is perpendicular, and the reading is taken. To do that > correctly, the object will sweep across the field of view as an arc, and not
    > be centered "at all times" as he suggests."
    >
    > I hope Joel can now accept the combined opinions of Bowditch (1918),
    > Letcher, and Bauer, who all agree with Frank and with me that the object in
    > the sky should remain in the centre of the field of view as the sextant is
    > rocked, and does not "sweep across the field of view as an arc".
    >
    > All this comes about because with a horizon observation, you are trying to
    > find the smallest angle between a point (the observed object) and a line
    > (the horizon). For lunar distances, it's a different matter. Then, you are
    > looking for the angle between two points in the sky. With a lunar, you can
    > capture either object in either view, "horizon" or index, and then swing the
    > sextant about either axis until the other appears in view, and the two brush
    &g! t; together.
    >
    > George.
       
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