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Henry Halboth wrote-

>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.

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I have a lot of respect for what Henry says, as he has more seagoing
navigational experience under his belt that perhaps the rest of us can
claim when added together!

And he is exactly correct about the accuracy obtainable in timing the Sun
at the moment it passes through (or very close to) the zenith. In that
special circumstance, the altitude increases linearly up to the maximum,
then falls linearly down again, allowing that maximum to be timed
precisely. This is in contrast to the normal situation, in which the Sun
"hangs" for a long time near its maximum altitude, making precise
determination of the moment of that maximum so difficult.

================

>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.

Come on then, Henry! Don't just tease us with that information. Where can
we find that article? Let's see what it has to say.

To argue against myself, I'm aware of one ocean navigator, Eric Forbes, who
sails "Fiona", and contributes articles about his voyages to "Navigator's
Newsletter". And he uses the method of estimating the moment of noon, from
Sun observations near noon, that I have been criticising. Last I heard, he
had completed his third circumnavigation, and was off to the South Atlantic
once again. So he must be doing something right. But then, he carries GPS,
of course.

George.

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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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