From: roy@mchip00.med.nyu.edu (Roy Smith)
Newsgroups: rec.boats
Subject: Re: Celestial vs. GPS (was Re: Bermuda Trip - 7/'93)
Date: 16 Dec 1992 20:37:56 GMT
Organization: New York University, School of Medicine
NNTP-Posting-Host: mchip00.med.nyu.edu

briang@atlastele.com (Brian Godfrey) writes:
>   So what if they drop and break their sextant, or drop it in the drink?
>   What's the diff, really?  They're still relying on a device.  The
>   important key to survival is not this or that technology you decide to
>   use, it's knowledge, understanding, and resourcefulness.  

	This is absolutely true.  However, let's assume you understand
celestial navigation and are handy enough to improvise tools with whatever
materials you have.  If you then lost your sextant, you'd still be able to
build some sort of altitude measuring device, perhaps using the graduations
on a compass as a template for the elevation scale.  If you're lucky enough
to have a good set of drawing tools (say, the nice Plath compass and divider
set and straightedge that live in your nav table drawer) and a steady hand
and lots of patience, you should be able to construct a 60-degree angle and
repeatedly bisect it using geometric constructions (30, 15, 7.5, etc; 8 such
bisections get you to 60/64ths of a degree).  Even if you could only get 1
degree accuracy, that's enough to have *some* idea of where you are.  Could
you find, for example, Hawaii, or some part of it, only knowing your
position to +/- 1 degree?  Probably.

	What if you lost your copy of HO-whatever-it-is?  Well, if you
really understood the math involved, you should be able, perhaps after some
trial and error, to recreate the spherical trig formulas, and given enough
time and patience (both of which should be in good supply in the middle of
the ocean) work out sine and cosine tables (you do remember the formulas,
don't you?  I think it's sin(x) = x + x^3/3! + x^5/5! + ...; if it's not
that, it's something like that, and given enough time I'm sure I could work
it out by trial and error).  Obviously, you don't need to work anything out
to the 0.1 minute accuracy HO-249 uses.  If you could get 10 minute accuracy
that should be more than good enough for emergency navigation.  Even 1
degree accuracy would be useful.

	What if you lost your Nautical Almanac?  Well, if you just had an
analema (maybe have one tatoo'ed onto your thigh before you leave on an
ocean trip? :-)) you would have a reasonable table of sun's declination vs.
time of year.  Probably more than accurate enough to let you follow a
lattitude line (parallel sailing, just like Columbus did) until you hit
land.

	I guarantee, however, that no matter how clever you are, or how well
you understand the inner workings of a Loran or GPS receiver, you won't be
able to build one, even a rough one that's awkward to use and yields
terrible accuracy, with the materials you have on hand.

	I wouldn't be surprised, however, if you could hand-craft some sort
of directional antenna and turn your SSB or Ham radio into an RDF.
-- 
Roy Smith <roy@nyu.edu>
Hippocrates Project, Department of Microbiology, Coles 202
NYU School of Medicine, 550 First Avenue, New York, NY 10016
"This never happened to Bart Simpson."
Newsgroups: rec.boats