NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
FW: Re: Chronometer Suggestions
From: Irv Haworth
Date: 2009 Jan 9, 14:35 -0800
From: Irv Haworth [mailto:irvhaworth@shaw.ca]
Sent: January 8, 2009 3:11 PM
To: . Subject: RE: [NavList 6952] Re: Chronometer Suggestions
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From: Irv Haworth
Date: 2009 Jan 9, 14:35 -0800
From: Irv Haworth [mailto:irvhaworth@shaw.ca]
Sent: January 8, 2009 3:11 PM
To: . Subject: RE: [NavList 6952] Re: Chronometer Suggestions
I don't think this
forum was intended to be used as a teaching platform , but in
deference to a novice I'll take a chance.
Re G.L . comments...my humble reply is really directed to
Bill Sellar as a novice celestial navigator who simply wanted to know the
effects of time error on distance (fix). U.K. George is quite right in that we
have probably confused him with somewhat irrelevant information-errors. Let me
explain ,in simple terms, what I think is the
information you need to get on with it..
A pound (mass) is not a pound the world around but a 1' of
arc is (currently) 6076.11549...... feet
on the surface of the earth regardless of location. (consider a
Great Circle).
Simply stated
one either from spherical trig formulae or sight reduction
tables one obtains Hc (height computed) . From this he applies his
measured etc Ho (sextant height observed and corrected) . .If Ho is equal
to Hc (unlikely unless his DR is bang on( consider use of HO211 ),then his ultimate 3
body fix is his DR. Using the modern short methods tables (HO214 /HO229 etc) he
would then use Ho-Hc to obtain an altitude intercept = a. (If Ho is greater than
Hc then he is closer to the body (T) if les than he is further away from the
body (A). Since both Ho and Hc are angular measurements the difference
will be in minutes ( ' ) of
arc. and thus in nautical mile(s).
Turning to GL 's statement , there is no question that the
length of 1 ' of arc varies as the cos. of the Latitude and if he used a
spheroid (oblate spheroid for earth) he could plot his fix ( position ) directly on a globe . (Our simple
statement would now become Ho-Hc= a x cos L). Since this technique , having been
tried -and dismissed- in
WW11 and much earlier ,
is not feasible the we must fall back on Marc Saint Hilaire's technique
(altitude difference or altitude intercept) and take it that Bowditch is right
in asserting "for practical navigation purposes 1 NM is considered the
length of 1' of latitude on any great circle on earth regardless of
location.
Given the G.L.'s statement is true how do we
manage to obtain a fix using Marc St. Hilaire's technique...Gerhard Mercator
solved this for us by developing the Mercator chart. Thus after obtaining the
results from a 3 body fix we construct ( or
purchase) a Plotting sheet and use the mid (horizontal)
line as our mid latitude and plot the LOP's . This fix is then transferred
(plotted) on to our Mercator chart.
I will now sit back
and wait for the flack -hopefully - flavoured with some construction
criticism as well as any correction(s)
.
Irvin F.
Haworth
From:
NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of
Gary LaPook
Sent: January 6, 2009 3:25 PM
To: NavList@fer3.com
Subject: [NavList 6952] Re: Chronometer Suggestions
Sent: January 6, 2009 3:25 PM
To: NavList@fer3.com
Subject: [NavList 6952] Re: Chronometer Suggestions
Hi Hi de KA9UHH K --- On Tue, 1/6/09, Irv Haworth <irvhaworth@shaw.ca> wrote: From: Irv Haworth <irvhaworth@shaw.ca> |
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