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Bruce,

Thanks for your comments on this procedure; they are specific enough
with respect to the sequence of tasks to allow us to try it the next
time the opportunity presents itself. Your tables take care of the lunar
and the almanac converts arc to time, the next challenge for me is
developing the formulas and a form with which to derive LAN from the
timed sun sight.

I look forward to reading your method for calculating the altitudes
without exact time.

Regards,
Arthur

-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of Bruce Stark
Sent: Saturday, October 12, 2002 1:35 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude

Good posting from Arthur and George. Anyone interested in the navigation
of
that era should go over that posting several times to absorb not only
what
they've said, but the implications.

The procedure Arthur suggested will work today for someone who's lost
Greenwich time and is uncertain of the dead reckoning. I'll lay that out
in
more detail:

Take and work a time sight in the morning. You won't get accurate time
from
it, not unless the sun was due east, or the DR latitude correct, but
you'll
get it close enough to know, within a few minutes, when to start
monitoring
the sun's altitude for the noon latitude. Anyone who's tried to get noon
latitude with only vague idea of when the sun will "dip" will appreciate
the
value of this first, approximate, working of the time sight.

Once you've got the correct latitude, work the time sight again to find
exactly how fast or slow the watch is on local apparent time. Be sure to
write that down because, until you get the next time sight, it will be
the
basis of your calculations.

That's all the regular "sight reduction" you'll have to do, and the only
thing you took from the Almanac was the sun's declination. At the very
most,
that never changes more than 1' per hour. Your estimated GMT would have
to be
a long way from the truth to get you in trouble.

Now let's say you get a set of distances of the sun from the moon. If
you are
able to take the altitudes before and after the set of distances you can
go
ahead and clear it, find, from the Almanac, the GMT that fits, and apply
the
equation of time to convert GMT to GAT.

To the average watch time of the lunar observation apply the correction
you
found with the time sight: so many hours minutes and seconds to be added
or
subtracted to convert watch time to local apparent time. The difference
between the LAT and GAT of the lunar is the longitude of the place where
you
took the time sight.

Keep that in mind. The time you're using is specific to the meridian
where
you took the time sight, so the longitude you find is specific to that
meridian also.

You've found latitude and longitude, and the lack of accurate GMT was no
hindrance whatever in working the observations. Besides the noon
latitude and
lunar, which took no more work than if you'd had accurate GMT, you've
worked
a time sight twice, using different latitudes. That's exactly what was
required in order to plot one Sumner line, using an accurate
chronometer.

Modern navigators find this hard to swallow. In the system they've been
taught, everything is founded on, and must begin with, accurate GMT.
They've
come to accept, as a bedrock truth, that to work observations
successfully
you have to have accurate GMT. If you don't have it the only hope, in
their
view, is to flounder toward it by iteration.

So much for now.

In case a list member is wondering what to do when he can't measure
altitudes
for the lunar, I recently stumbled on a way of calculating them that
fits
present procedures, but is no more dependent on accurate GMT than the
method
posted under "It Works." Given time, I'll explain it soon.

Bruce

   

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