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> It will add a certain extra spice that they are coming from a professional
> in the big-ship world. I hope that some of them can be based on real
> experience, though presumably others will be invented scenarios.

You can rest assured that all of the exercises that I will post will
be directly from my navigation notebook and will not be fabricated.  I
may invent a bit of a side story to liven up the problem, but all
numbers will be from my sextant and watch.  When I get home I may post
a few of the US Coast Guard problems that I found interesting, but I
will certainly put a disclaimer in for those.

>
> His first problem raises, from me, one question, and in the traditions of
> this list, a minor pedantic quibble.
>
> Here's my question. I see that the navigator is working on zone time. Is
> that standard practice, for navigators in the merchant marine? In my
> experience, small-craft navigators, particularly if they are using astronav,
> usually keep a timekeeper for the purpose which is always set to GMT, winter
> or summer, and always work on that. Of course, you need zone time as well,
> from another clock, to tell you when the pubs will be open and to work the
> tide tables.

This ship�s bridge keeps clocks in three time zones, UTC, local (as
determined solely by the Master), and US East Coast (where the owners
are located.)  In reality these days in the US Merchant Service UTC is
kept only for radio traffic.  All other shifts and times are kept in
local.  Celnav, when done at all, is marked in local time, and then a
quick check on the UTC clock will confirm the actual UTC hour.  In
fact, when I mark my sights on my notebook, I merely note the minutes
and seconds.  LAN is the only time where it is important to know your
zone description because if we are keeping a different ZT then is
dictated by the Longitude, LAN can be around 11 AM or 1 PM.  On this
ship, it occurs quite a bit as we delay changing clocks so that the
captain does not have to stay up as late to send his radio traffic.

>
> And here's my quibble. Jeremy has calculated the time of Local Apparent Noon
> to be at 12h 13m 42s. And yet, he says-
> "I shot the sun at 12hr 13m 40s as I observed start to descend from its
> highest altitude."
> But it's unlikely he could have observed it starting to descend, at that
> moment, for three reasons-
> 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.
>
> So, if the Sun really was observed to have started descending at the time
> Jeremy says, it throws some doubt on his quoted longitude.

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.
    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.
    We must also remember that in mid-ocean sailing, errors of latitude
of 1 to 2 nautical miles is acceptable as you can fairly easily make
landfall with such an error.  For me, an error of 1.3 nm is what I
rate a �fair� sight.  For the record, the source of this error is most
likely due to the slightly hazy horizon on that date and my inability
to put the limb exactly on the correct plane.  It is just what you
have to deal with out here.  I am quite sure that the Captain would
not complain about such an error at sea.

Jeremy

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