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George H wrote:
"1. It wasn't even quite due to get to Local Apparent Noon then, if only 2
seconds short of it.
2. Around noon, the altitude changes so slowly that it's a significant time
before any descent can be noticed, even by the most skilled navigator.
3. At that date in May, the Sun's declination is increasing Northwards by
about half a minute each hour. So at the moment of LAN, the Sun's altitude
will still be increasing at that rate, and it will reach its maximum
somewhat later than LAN, before any fall can even commence."

And Jeremy replied:
"The doubt lies not in the Longitude (dGPS rarely has any error of
significance) but rather in my ability to see the sun ascending or
descending at this time.  What is most probable is that due to the
motion of the ship and my person, I perceived that the sun was
descending and "called noon" incorrectly.  In fact, my biggest issue
with LAN at all is the inability to accurately define it by altitude
alone.  I was simply in error (2 seconds) as to the time of LAN.  This
is the "art" of using the sextant.  Knowing how to set it, when to
call it, and how to align the body and the horizon.  I have seen a
score of navigators shoot the same LAN and it is rare indeed that
anyone calls noon at the same exact moment.  The practical error in
this slight time problem is overshadowed by other potential issues."

Why do you care to "call noon" at all? This is one of those navigational
traditions on commercial vessels (specifically) that people love very much
but its navigational significance is hard to fathom.

George's points above are all right on the money, Jeremy. So let me pose a
problem for YOU that might help you see what George is getting at:
It is around local apparent noon on May 20, 2008. I am in longitude 71d 58.0
West exactly and latitude 40d 00.0 North approximately (off the southern
coast of New England). I am sailing due south at ten knots. What is the
exact GMT when the Sun is on the local meridian (azimuth=180)? What is the
exact GMT when the Sun reaches its maximum altitude? These are not the same.
In addition, how much time elapses after that maximum altitude before the
Sun's altitude changes by one-quarter of a minute of arc? When would you
"call noon"?

And you 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.  This method is in fact outlined in The
American Practical Navigator 1995 edition.  It is horribly impractical
at sea, and still questionable as far as Longitude goes, but it will
certainly give you a slightly better Latitude number."

Why do you say 'horribly impractical'? Do you mean because it would have you
busy for half an hour or more when you could be doing other work (which
would indeed be an issue on a commercial vessel, but then again, what are
you doing playing with a sextant in the first place?? ) or is there
another reason? As for longitude, the biggest problem with using a series of
sights around noon is that they are not really symmetrical unless the
observer is motionless and the date is near one of the solstices. But these
issues can be corrected without a whole lot of trouble and you will then
have a longitude, too. The longitude would not be as accurate as the
latitude but not too bad either. It depends on the details (as Bill noted in
another post).

 -FER
www.HistoricalAtlas.com/lunars



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