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Anabasis wrote:
> Peter is correct in saying that you can shoot a number of sights over
> say 15-20 minutes and smooth a curve to determine not only latitude,
> but also longitude by LAN.

Peter responded (if I understand correctly):
> Just to be clear about this, my understanding is that observations over some
> hours, not minutes, both before and after LAN, are required to establish the
> parabolic curve that Jeremy describes.  What I was proposing was sights over
> a few minutes, when for practical purposes this curve ressembles a
> horizontal line.

Bill adds:

I ran a large number of scenarios on the curve near LAN many years ago (I
cannot locate the data--probably in storage) and it can be very situational.
For mid latitude sailors during the warmer months, 15 to 30 minutes either
side of AP LAN  produces a curve that is usually more than sufficient.  The
duration of the flat line varies depending on the relationship between
latitude and declination.

Imagine if you will (or better yet use your favorite online resource or
electronic gizmo) to track the rate of change in elevation near LAN of the
Sun at or close to the NP in late June, or on the equator near an equinox.
Plot both cases. Two different animals (two entirely different results).

My interest in the rate of change around LAN was sparked when I finally got
around to attempting to learn cel nav (a long time goal). Along with my
first (non-cardboard) sextant, an Astro IIIB, I also ordered the practice
bubble scope.  I wanted to calibrate bubble correction and it seemed the
easiest way to do this was to observe the Sun during LAN and make as many
observations as possible while the elevation hung within the my self-imposed
man/machine limitations of 0d 0!3 under ideal conditions.  That prompted me
to determine how long the optimal viewing period lasted at my latitude--with
declination as the variable.

It is not my intention to argue Peter's straight line, but rather to issue a
caution regarding how long the slope is flat for practical usage. If you
intend to assume a portion of the plot line is horizontal--either in
back-yard sights (when possible given the double angle) with an artificial
horizon or on the water--first check on the rate of change around LAN for
your location and date.  Then decide on how much change is tolerable for
your level of precision.

Bill



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