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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.