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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Single-body fix method
From: Christian Scheele
Date: 2009 Aug 6, 22:03 +0200
From: Christian Scheele
Date: 2009 Aug 6, 22:03 +0200
" Alas, I am not in the air or near the poles"
That makes two of us ;-) Thanks for the exercise, I'll leap at the
opportunity.
Christian Scheele
----- Original Message -----From: Anabasis75@aol.comTo: NavList@fer3.comSent: Thursday, August 06, 2009 3:47 PMSubject: [NavList 9387] Re: Single-body fix methodIf I may humbly direct you to my "multi-moon" exercise posted only a few hours ago. This is a post showing two real world examples of using a single body to determine position, both around and away from transit. Alas, I am not in the air or near the poles; but it is a decent, if time consuming, method of determining position if the navigator knows and accept's the method's limitations and has the time to both shoot and then "reduce" the numbers to derive the position.Jeremy.In a message dated 8/6/2009 1:10:11 A.M. Central Asia Standard Time, scheele@telkomsa.net writes:
Can anybody point me to an open-access source dealing with Byrd's and Weems'
attempts to make use of the "single-body fix method"? I believe Weems
undertook these tests to aid Byrd in his air navigation in polar regions.
The "single body fix method" has been the subject of one, perhaps several
threads on this site, which I have only been paying attention to recently.
James N. Wilson has also written an article about it which is on the ION CD,
but I am not familiar with the work as I do not have a professional interest
in celestial navigation or any of the natural sciences. I understand that
the method's weakness are the requirement of knowledge of the body's azimuth
and the limitations it places on the fast-moving observer. I saw a
digestible -speaking in very personal terms - description of it complete
with a derivation of the required formulae in the online ION Newsletter in
an article about half way down the page by Joe Portney entitled "Portney's
Corner: The Lost Sub Quick Fix". Here is the adddress:
http://www.ion.org/newsletter/v11n1.html
A follow-up reader's letter under the heading "Pondering Portney's
Ponderables" in the subsequent online issue claimed/reported on errors that
appeared in an embedded diagram and in the derivation of the formulae in the
earlier article. The web address at which this second article can be found
is:
http://www.ion.org/newsletter/v11n2.html
The first article merely alludes to the Byrd and Weems trials involving this
procedure, involving sights of the sun taken through "the open hatch of a
seaplane."
I am still pondering the method. To this end I am reading a chapter in
Charles Cottter's "The Elements of Navigation" on the subject of "rates of
change" (of celestial bodies' altitudes), although this particular
description is limited to meridian observations and is only indirectly
related to the subject of the "single-body fix method", Cotter's objective
here being the determination of the sun's maximum altitude. At present I am
unsure as to why Cotter puts observed changes in altitude of a celestial
body, a function of the combination of movement of the celestial body and
the observer's own movement, into a formula that is a "partial integral" (my
own term), covering the rate of change in one minute of time in one linear
equation, rather than a "true" integral function, but maybe that's because
we are talking about a slow-moving observer (on a ship) and it is therefore
good enough to use an approximation. In any event, I imagine that practical
application of the method (if, indeed, it is practical at all), where the
period of observation of a body's rate of change may be a minute or a half a
minute (for as much as I know), must by necessity involve the "partial",
rather than the "real" derivative, but I think this is a different
consideration. To those readers who are familiar with this cited chapter, is
the method which Cotter explains an approximation as I suspect, or is it
complete and to be used without reservation as suggested?
Christian Scheele
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