NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
The *&^%$#@ Moon
From: Robert Eno
Date: 2006 Aug 17, 21:26 -0500
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From: Robert Eno
Date: 2006 Aug 17, 21:26 -0500
Gentlemen
I'd like to throw this one out to the
wolves.......er' list members, and see what comes back.
Many navigation texts discuss what was apparently
an "old sailor's tale" about the inaccuracy of moon observations. The story goes
that in the old days, many navigators avoided moon shots because the believed
that moon observations yield inaccurate LOPs; a belief which persists to
this day in some circles. The texts go on to say that moon shots are perfectly
accurate; no less so than any other body that one can observe; that with current
correction tables, moon observations should yield results every bit as accurate
as any other observations.
Notwithstanding the pronouncements of navigators
who have forgotten more than I will ever know, it has been my experience --
after thousands of observations of stars, planets, the sun and the moon, that
moon shots do tend to yield results that are simply not on the money. I have
tried to ignore this and embrace what the textbooks say about how moon shots are
accurate, but the theory just doesn't pan out for me personally. I am one of
those navigators who only uses the moon as a last resort because I fully expect
(and am seldom wrong) that I will be wonky LOPs.
Case in point: the other night, I took the
opportunity to take some star and moon shots. Haven't seen the former since last
April and while the return of the stars also signals the imminent return of old
man winter in my part of the world, I nevertheless welcome the return of my old
friends.
To carry on, I took four observations of the
moon and four of Vega (from a known position) using my Plath Bubble
attachment. As is my practice, when observing from a static platform, I
averaged out the results of the sights, all of which were taken within minutes
of each other.
The average result for the moon shots was 3.9
minutes of arc error, while the results from Vega were only 1.3 minutes of arc
error. To add insult to injury, the Vega shots were extremely difficult to take
because the illumination system in the Plath, even at its dimmest, tends to all
but wash out stars. The moon shot was a breeze but the results were pathetic. I
should add that clouds were moving in at the time. Could clouds have an effect
on refraction?
Ok, Ok, I know that a bubble attachment is simply
not as reliable as a true horizon and that 8 observations do not
constitute a statistically-correct defence to support my assertion
about the inaccuracy of moon shots. I merely presented these data to illustrate
what has been my experience since I first picked up a sextant 23 years ago. It
is more often than not, the same for me: to borrow from the modern pop
culture vernacular: moon shots simply suck. At least for me.
So what gives? Have I been cultivating and
reinforcing an as yet unknown bad habit vis a vis moon observations all of these
years? If so, why would this not be the case for the sun, stars and
planets?
Is there anyone else out there besides me who
simply does not get good results from moon observations?
Is this all in my head??
Robert
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To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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