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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Swinging the Arc
From: Frank Reed CT
Date: 2004 Dec 5, 18:23 EST
From: Frank Reed CT
Date: 2004 Dec 5, 18:23 EST
Alex wrote:
"How can you possibly do this in practice??
Except by invereting your sextant and looking at the Sun through
the horizon mirror?"
Except by invereting your sextant and looking at the Sun through
the horizon mirror?"
Try it. Use a light on your ceiling and a window sill as surrogates for Sun
and horizon. Suppose the angle is 45 degrees. Can you see that you can rotate
the sextant all the way around a complete circle starting at the horizon (window
sill), rolling left to horizontal, right through straight overhead, back through
horizontal on the right, and finally back to the horizon from the other side? In
order to do this, the sextant has to be rotated about an axis that points to the
Sun (celing light). In the real case, the axis of rotation can be well outside
the sextant, but in this model case, the axis will have to pass nearly through
the index mirror because the object is so close (parallax from shifting a few
inches).
And:
"Maskelyne also asks the same question and concludes that one
has to rotate the sextant about BOTH horizontal and vertical axes."
"Maskelyne also asks the same question and concludes that one
has to rotate the sextant about BOTH horizontal and vertical axes."
I don't think that makes any sense. The rotation is about a single axis.
I'm pretty sure that the mathematical and physical analysis of solid-body
rotation was worked out after Maskelyne's time. The wording he used may have
reflected looser terminology from that earlier era.
And:
"When the altitude is small it is enough to rotate about horizontal axis, when it is large, the main part of the motion is about the vertical axis."
"When the altitude is small it is enough to rotate about horizontal axis, when it is large, the main part of the motion is about the vertical axis."
And try this with the real Sun: Place the reflected image of the Sun on
some horizontal line (assuming you don't have a sea horizon handy). Rotate the
sextant so that the Sun's image stays centered in the field of view. How is the
sextant turning? I think you'll find that it's rotating about the axis that
points to the Sun. Wouldn't you agree that this is the rotation required to find
the vertical angle?
You have Bruce Bauer's book, right? Read his section on this topic. He's
clearly describing something very different. And that description is similar to
what you'll find in some other navigation textbooks. It must work to some extent
under some conditions. That's why I was proposing that three method comparison
earlier.
By the way, the other method works perfectly if the sextant is used
upside-down. Were some of the early sextant designs meant to be used with the
object in the fixed mirror (our "horizon" glass) and the horizon
brought to it via rotation of the index mirror? If so, that might explain some
of the confusion.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
[ ] Mystic, Connecticut
[X] Chicago, Illinois