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In responding to Greg R's postings, I will do my best to maintain civility.

In order to "Calculate the true bearing of a low altitude celestial body in
order to determine the error and deviation of the compass", he suggests that
when you have a true bearing for the celestial object (Zn)  "why not just
momentarily point the bow in that direction and note what the compass
reads?".

Doesn't work. It would give the wrong answer. If there is compass deviation
to be allowed for, then it varies with the vessel's heading. So if the bow
is momentarily pointed  to a different direction, then a different deviation
will be momentarily measured, NOT the deviation that's relevant to the
original course.

Jeremy is right to point out that the correct approach is to sight on the
reference-body directly with the compass, NOT shift the vessel's head.

The point about measuring azimuths of bodies near to the horizon is this: A
compass is only intended to measure such azimuths, not azimuths of bodies up
in the sky. Some compasses have special fittings, such as mirrors, alidades
or shadow-pins, to assist such observations, if they are not too far up.
However, these are critically reliant on the horizontal levelling of the
gimballing, or worse, that of the card; matters that do not significantly
trouble normal compass use at all. Without such fittings, reckoning the
azimuth of bodies far from the horizon calls for estimation and guesswork,
with corresponding loss of accuracy. That's especially true on board a
heeling, rolling, pitching, yawing yacht, the sort of conditions many on
this list will recognise. The higher the body is in the sky, the worse that
problem becomes. Therefore, from that point of view, the proposal to use the
noon azimuth of the Sun is choosing the worst possible moment, in addition
to the fast-changing azimuth pointed out by Jeremy.

"...if you're doing a LAN shot the sun will be either directly north or
directly south of the boat for the time that it "hangs" in the sky, so that
really simplifies the calculations."

Not really true. The Sun "hangs" in the sky for a long time because its
ALTITUDE is unchanging. That makes measurement of altitude easy and
accurate. But its azimuth is changing all the time (and at its fastest).
That doesn't "hang". So the "hanging" altitude hinders, not helps, any
attempt to assess the azimuth near noon from the changing altitude alone. To
assess whether azimuth is truly North or South calls for a knowledge of
Sun's GHA and vessel's longitude, so calculation is called for after all, to
get the moment of Local Apparent Noon..

Sun amplitude (bearing of the Sun when near the horizon) was greatly used by
mariners 200 years ago, both for the reasons explained above, but mainly
because it didn't require any timekeeper on board. Only a rough latitude was
needed, which a mariner would know, and the date. Once the chronometer
became general, then any low body could be used to provide azimuth. It's
common practice, in small-craft celestial navigation, to take a compass
bearing on any low-altitude body being used for a position line, for that
very purpose. Trouble is that, for other good reasons, mariners prefer to
use bodies well up from the horizon for their position lines.

Although the Sun amplitude table lost its main reason for existence in the
19th century, conservative maritime traditions keep it in being, and, no
doubt, retain it as an examination topic.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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