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Greg R wrote-

| I appreciate your attempt at civility, I will do my best to
| reciprocate.

Good. Mutual respect appears to be restored. I hope it will stay that way,
and we can put past events behind us. Peace breaks out...

| >> "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.
|
| You have a point, but I'm assuming that the vessel also has a compass
| correction card - so wouldn't lining the bow up with a known azimuth
| serve as a good check on whatever compass bearing the sight happened to
| coincide with (of course, also doing the math required to change from
| true to magnetic bearing)?

"Having a compass correction card" presumes that you already know what the
magnetic deviation is, at all headings. But remember, the object of the
exercise was to "Calculate the true bearing of a low altitude celestial body
in order to determine the error and deviation of the compass". Which
presumes that the deviation, at the heading being followed at that time, was
unknown and to be determined. By making a complete turn, through all
headings, you could create a complete correction card. But otherwise, you
can't determine deviation at one heading from the deviation at another
heading; which is what was suggested.

| > 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.
|
| Agreed on that point as well, but whenever I do sextant sights I drop
| an imaginary line from the celestial object to the horizon so I'll know
| where to find it in the sextant - the same method should also be at
| least semi-accurate for verifying a compass bearing.

One problem with that is that one bit of the horizon looks very like another
bit, in the absence of conveniently placed markers.

| Not pinpoint accuracy, but probably within a few degrees (which is
| about the best we could hope for with the average yacht compass).

The higher the body, the less accurate that procedure is, in finding its
compass bearing. That's why noon is the worst moment for measuring a Sun
azimuth, as I pointed out.

| > 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).
|
| I know that the sun's azimuth changes rapidly both before and after
| LAN, but is that also true of the time of LAN itself?

Yes. At the moment of Local Apparent Noon the Sun's azimuth is changing at
its fastest, though not a lot faster than its rate shortly-before or
shortly-after noon. The motion in azimuth does not pause, or "hang".
Altitude does, because before noon, it was rising, and after it, falling; so
there has to be a turn-around when the altitude stops changing.

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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