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Sorry I missed out on the resulting discussion real-time.  How do I get out
of digest mode on this Listserver?

For me, the curved earth/parallel rays is as essential to understanding
celestial navigation (everything fits.. . measured Ho versus theoretical Hc,
reason for varying refraction, time/arc conversion, height of eye corr., no
need for height of eye corr. with artificial horizon,...) as the
heliocentric model of the solar system is to understanding the motion of the
sun, planets, and moon relative to the stars. No more planets doing bizarre
loopty-loops in the sky... the complexity disolves... it's a simple,
beautiful phenomonon.
Rather than the flagpole diagram as an introduction to new CN students
(which I believe is one I saw in my first CN book way back when, that
initially prevented me from comprehending how CN worked... thus my being
easily upset over seeing it again in "A star to steer her by") I find a
diagram showing a sunray directly overhead of a circular earth with an Ho =
90 and a parallel sunray tangential to the same circular earth (as in
sunrise/sunset) with an Ho = 0 is a better first introduction to new CN
students. Everybody can relate to the Sun at high noon and the Sun at
sunset.  You can then magnify the earth's surface showing a 1 degree
latitude segment separation, equating the 1 degree difference in angle with
the horizon (90-89) to 60 nautical miles in distance. Then repeated with a
magnification to show a 1 minute segment separation = 1 nm.  You thought you
were "here" [Hc], but the sextent tells you "you are there"[Ho]... you get
the idea.  Then using the same diagram (moving the North pole to the center
of the circle), you can put the earth in rotation and using 360 degrees = 24
hours (again something everyone can relate to), develop the role of time in
finding longitude.
Everything one needs to understand celestial navigation was learned in grade
school.  (i.e. Declination = revolution of an earth tilted on it's axis).
Better to start with that then to start with an erroneous analogy.  Start
with a flagpole if you must... just don't put a star on top of it.  In the
student's mind it may be stuck there for too long a time and that (IMHO) is
not a star to steer her by.

-- Joe Shields - South Coast 23 #56 - (40d34m N, 80d04m W)

   

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