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One circumstance comes immediately to mind which allows the establishment
of position without prior knowledge of either latitude or longitude,
always assuming an accurate chronometer to be at hand - it is, of course,
a special circumstance, not frequently encountered.

When passing under the Sun, i.e., declination very close to observe's
latitude, as evidenced by altitudes approaching 90-degrees, circles of
Zenith Distance established immediately before + at + after transit may
be swept from the Sun's geographical position at time of sight, and their
interception demonstrating the vessel's position with a great degree of
accuracy. Of course, if you should happen to obtain a true altitude of
exactly 90-degrees, the Sun's geographical position at the time of
observation will be the ship's position at that time with no further
work.

It is not particularly easy to observe altitudes of this angular height,
and any effort requires an unobstructed 360-degree view of the horizon,
as azimuth changes extremely rapidly at and about the time of transit. I
have had the opportunity to make this observation on several occasions
off the coasts of Africa and obtained results within the range of
practical accuracy.

On the subject of the determination of longitude by equal altitudes at
and about noon, and again assuming an accurate chronometer, I most
strongly disagree with the comments posted regarding practical accuracy.
I some time ago posted an article, which was absolutely ignored by the
pundits, concerning this method, including a manner of verifying the
accuracy of the circummeridian altitudes as respects equality of observed
time. There is, in my opinion, no more reason to condemn this methodology
than there is to condemn Lunars - again given circumstances and the
relative need to establish a position.

                                Henry

On Sat, 4 Jun 2005 08:41:59 +0100 George Huxtable
 writes:
> Bill Noyce wrote-
>
> >Perhaps the fact that "longitude by noon sun" comes up so often
> >is a good reason that there *should* be a discussion of this
> >method, pointing out why it is a bad idea...
> >
> >Is there any other observation that gives longitude without
> >knowing latitude first?  Maybe this is part of its appeal.
>
> ================
>
> To my mind, finding latitude at noon is a trivial matter that
> presents a
> problem only when the sky is cloudy at noon.
>
> To answer Bill's question, if a time-sight is taken at the moment
> when the
> Sun is due East or due West of the observer, then his latitude isn't
> needed
> at all in calculating local time-by-the-Sun. But that can only
> happen in
> the Summer months.
>
> But why, in this age, does a mariner still ask for separate
> determinations
> of latitude and longitude? It's as if we were still stuck in the
> early
> 1800s, and Sumner and St Hilaire had never invented position lines.
> Why not
> just measure two altitudes of the Sun, at any old times, but times
> which
> are well separated so that the Sun's azimuth has changed
> significantly
> between them. Then draw a couple of position lines from some assumed
> position, allow for vessel's run in the interval, see where they
> cross, and
> that's where you are, in lat and long. Simple as that. Applies to
> any sight
> of any body at any time: a universal way of doing the job. Who needs
> anything different?
>
> George.
>
> ================================================================
> contact George Huxtable by email at george@huxtable.u-net.com, by
> phone at
> 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1
> Sandy
> Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ================================================================
>

   

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