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George:

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

>> "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)?

> 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.

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

> 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?

> 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.

Which brings us back full-circle (and via a great circle route?...
;-)) to my original question - does anyone know what's acceptable to
ASA as a compass check if amplitude tables aren't being used these
days?

--
GregR



--- George Huxtable  wrote:

>
> 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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