NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
[Discussion] Hypothetical Exercise - Abduced By Aliens
From: Greg R_
Date: 2008 Jun 15, 23:01 -0700
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From: Greg R_
Date: 2008 Jun 15, 23:01 -0700
Someone once mentioned on the list that if you
only knew what ocean you were in, the rest of the "Unknown DR" problem could be
solved fairly easily in just a few iterations - and I was surprised at how fast
it actually solves on a computer. Usually just 2 iterations yielded a very
accurate position from any AP with all 3 stars in view
at the time the sights were taken (and the "Object Not In View" warning was a
big help in tossing out wildly inaccurate guesses at a possible DR
position).
But we don't have the luxury of electronic
computation for this one, so we're left to solve it manually with whatever we
have available.
Narrowing down the location to a rough AP seems
fairly straightforward. Based on the GHA and Declination values at the time of
the sights, if I did the GHA math right we would appear to be somewhere in the
middle latitudes and western longitudes (or more specifically, somewhere in the
Pacific Ocean).
And if we knew our
approximate latitude (which we don't in this case), we could also do a
time-to-arc conversion (based on our sunset or twilight time) to get a rough
longitude.
Another limiting factor that helps to narrow down
our location somewhat is that celestial objects are only above the horizon for
half of the world (i.e. 180° of GHA/LHA).
I suppose the way I would solve this would be to
pick some random location that would have all 3 stars in view at the time of the
sights (probably on some even 10-degree boundary of latitude and longitude
for ease of plotting later), and draw what would be the very long intercepts on
a small-scale chart (or maybe even a world map) with a compass - yielding
circles of position, since we're dealing with a very small scale for the first
iteration.
Then using the intersection of the 3 circles as
the new AP, reduce the sights again (maybe several times, if necessary) until a
good fix is obtained.
Just for reference, here's how Navigator solved
this one:
Using 40°00.0'N / 160°00.0'W as a "random" AP to
start the process, the first iteration produces a fairly large cocked-hat (as
expected) with 22°43.8'N / 156°27.2'W
as the resulting "fix".
Then using first iteration as AP for the second
iteration yields 23°18.7'N / 157°17.6'W, which is almost spot-on to the real position of 23° 18.4 N /
157° 17.7 W.
--
GregR
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