NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Suitable Sextants - Mirrors
From: Joel Jacobs
Date: 2005 Oct 14, 21:05 +0000
From: Joel Jacobs
Date: 2005 Oct 14, 21:05 +0000
Herb,
Thank you for your long and detailed explanation which I am way too old to have any interest in picking apart because you're not even on course. No bubbles have been burst here other than George's and likely your own. However, I would like you to point me to some books on navigation that substantiate your group's points of view since many of us learned our navigation, not as academicians, but as seamen.
We do not swing a sextant around the horzion as you say. We know the azimuth before we ever go on deck. Our sextant is frequently preset to altitude. We line up on the azimuth, and before taking the sight rock the sextant to determine perpendicularity which give the correct reading. It is second nature. Unless you go out and do it, you will never undertand what I am talking about.
Since you're also down on Bauer (whose book has been quoted on this list favorably in the past), here is another reference that I selected for George H's benefit since the author, now deceased, was English and well respected in his profession as a mariner. He had the knack of saying a lot in few words. See Fig 12 page 26 of "The Sextant Simplified" by Oswald M. Watts which shows a diagram the essence of what's in Bauer's book and which I explained to your group's dismay.
Watts, a distinguished professional mariner, navigator and author says this in the last paragraph on page 23: ...."so the sextant must be rotated slightly to the right and left (pivoted on the handle by the wrist...pendulum fashion), so that the reflected sun is made to lift off the horizon each side, describing the lower part of a circle exactly as in figure 12. His illustration is so good, that I photographed it and added it to Bauer's so everyone one can see.
Thanks Herb for another chuckle.
If I'm wrong, I consider myself in good company,
Joel Jacobs
-------------- Original message from Herbert Prinz <ml@HPRINZ.US>: --------------
> Yourname Here wrote:
>
> > here is a link to photographs of page 108, Figure 6-4 of Bauer's " The
> > Sextant Handbook". [...] For those interested, Bauer goes into great
> > detail in the methodology of swining an arc on pages 105-108 which
> > are the same as my own.
> >
> > http://www.villagephotos.com/pubbrowse.asp?folder_id=1485991
> >
>
> Joel,
>
> I hate to burst your bubble, but Bruce Bauer has it all wrong, and so do
> you. His figure 6-4 does not show clearly how he is swinging. But he
> tells us on p.106, which you chose not to reproduce in your post:
>
> "Swinging the arc is also called rocking the sextant and simply means
> rotating th! e instrument from side to side around the line of sight to
> the horizon."
>
> I think that there has long been a consensus on this list (at least
> since Frank Reed brought the attention to the subject a while ago) that
> this is the wrong way of doing it. Actually, it's an impossible way of
> doing it (see below).
>
> Instead of trusting a particular book, let's use common sense. The goal
> of swinging the arc is to find the exact point where the vertical line
> through the star intersects the horizon. If you pick a point on the
> horizon arbitrarily and then start swinging the sextant around the line
> of sight to this point, how do you ever find the correct point??
>
> That this wrong way does not work is further evidenced by Bauer's own
> claim that he cannot make it work for any other than medium altitudes.
> See p. 108, "One more wrinkle on swinging arcs [..! .]", full quote on the
> page you posted. Bauer says "At high altitudes the arc becomes hard to
> manage because of its sharp curve." Not at all. The very problem with a
> star at high altitude is that it is difficult to guess the azimuth
> because the curve is so _flat_! Of course, if you decide on the wrong
> azimuth prematurely, then face that direction and start swinging the
> arc on a horizontal axis, the star shoots out quickly on eiter side of
> the mirror. Neither will its arc ever touch the horizon - it may
> intersect. Even a Venetian wall mirror on your sextant will not help you
> solve this problem.
>
> Why Bauer says that swinging does not work for low altitudes, because of
> the arc flattening out, is beyond me. It makes me wonder whether he ever
> swung a sextant. For sufficiently low altitudes one can actually swing
> the object full circle (!) without loosing it! s view.
>
> Herbert Prinz
