NavList:

In truth, whenever we take a sight we are determining ourselves to be somewhere on a circle D = 60 * (90-Hc) nautical miles away from the GP of the body.

In the case of most "normal" sights, D is a large number.  If we were to try to actually measure D on a chart we'd have to use such a small-scale chart that we'd not have a very precise location (not to mention the fact that scale (and hence distance) is not constant on the most common type of chart projection, Mercator).    Fortunately we have those neat spherical trig formulas that we either find worked out in HO229 or 249 or these days we plug into our  pocket calculators.

But when Hc is near 90 degrees (say, above 87 or 88), it's entirely feasible to actually plot a circle of position.   Plot GP of the body on the chart, open dividers (or, better, a draftsman's compass) to D, and draw at least the part of the circle of position near one's DR.

Lu Abel

Greg R. wrote:

--- frankreed@HistoricalAtlas.net wrote:

  

That's a good point about sights very close to the zenith. I had
mentioned previously on the list that there may be a special case
when the Sun is close to the zenith. I still haven't thought through
whether it really screws up the graphical technique or merely
requires more stringent rules for its application.
    

Something sticks in my mind from way back when I was first learning
celnav that for objects near the observer's zenith (i.e. Hc ~89 degrees
or greater) the resulting LOP should be plotted as a circle of position
(do we call that a COP?) around the object's GP vs. a straight line as
we normally do. But I don't have any personal experience doing that, so
I can't vouch for the technique or its accuracy.

--
GregR




  

I made one last try at convincing George Kaplan (a name some of you
know) to
join us in Mystic this week, but alas, it's not in the cards.

While I was at it, I chatted in my email a bit about longitude around
noon
and asked him this:
 "Which leads to a question: is there an established name in the
literature,
or even in your own jargon, for a fix resulting from a series of ten
or
twelve sights taken over a relatively short period of time? I've been

calling it a "rapid-fire fix". Do you know another name?"

His reply:
 "I don't know of a special name.  You're correct, of course, if you
can get
a bunch of sights on either side of noon, you can get good enough
geometry
to get a 2-D position.  It works with the LOPs, too, in that they
provide a
good spread of azimuth around then.  There is a slight catch,
however, and
that is, the higher the Sun is in the sky (and therefore the more
rapid the
altitude and azimuth change near noon) the more you have to worry
about the
curvature of the LOPs.  In some near-degenerate cases (sun within
several
degrees of the zenith), the usual straight-line plotting -- or math
that
assumes straight-line LOPs -- may not provide the right fix."

That's a good point about sights very close to the zenith. I had
mentioned
previously on the list that there may be a special case when the Sun
is
close to the zenith. I still haven't thought through whether it
really
screws up the graphical technique or merely requires more stringent
rules
for its application.

 -FER



    

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