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Hi Jeremy:

> On merchant ships, az calculations are the most common, and really
> the  only, celnav sight still used.

I'm actually surprised to learn that any sort of celnav is still used
on commercial vessels - is it still a requirement that someone on the
deck watch crew know how to do celestial, or is that long gone with the
advent of GPS (etc.) these days?

> You need not point the ship at  the body at all, you just take a
> bearing as you would a terrestrial  sight.  You then compare the
> bearing to the computed Zn and you have your compass error.

Correct (and that really wouldn't be feasible on a ship that size
that's sailing to meet a prescribed schedule), I was thinking more
along the lines of a small sailing yacht (hence the mention of the ASA
certification requirements).

> We use gyro compasses and repeaters so we mark the ships gyro and
> magnetic headings at the same time and then compute declination.

I remember using gyro compasses (and a peloris to take bearings with)
from Navy days (also from the dark ages), but I haven't seen too many
of them on personal cruising yachts.  ;-)

> I will post one of these problems late next week when we head out
> to sea and I shoot a few of them.

Thanks, look forward to that - I was too busy to jump on the earlier
problem, but when I get time I'll go back through the mailing list and
have a go at it.

> Near transit, the body is changing directions very rapidly and
> even a 0.5 degree error can occur quickly.

Agreed, but for yacht navigation (where I doubt seriously if anyone
could read the typical compass to 0.5 degrees), I was wondering if that
would be "good enough".

> Our gyro repeaters have Az circles which use a mirror and a slit for
> sun sights and are accurate to about 1/2 of a degree or so.

No filters for sun sights, or does the slit take care of that for you?

> On a yacht, you can use a hand bearing circle

Not sure what that is, maybe something like the bearing circle on a
plotting sheet? Or maybe something like a hand-bearing compass without
the compass (i.e. that could be aligned with a fore-aft line of the
boat and give a relative bearing to an object?

> or maybe it is easier to just point the bow at the body.

Probably a lot easier to do on a yacht than on a vessel of your size...
 ;-)

--
GregR



--- Anabasis75@aol.com wrote:

> Greg,
>
> On merchant ships, az calculations are the most common, and really
> the  only, celnav sight still used.  You need not point the ship at
> the body at all, you just take a bearing as you would a terrestrial
> sight.  You then compare the bearing to the computed Zn and you have
> your compass error.  We use gyro compasses and repeaters so we mark
> the ships gyro and magnetic headings at the same time and then
> compute declination.  I will post one of these problems
> late next week when we head out to sea and I shoot a few of them.
> We typically shoot Polaris and the sun, although other stars are
> sometimes shot as well.
>
> You can certainly take an amplitude, but this method now takes longer
> then a computer reduced Az.  Bowditch has 2 amplitude tables.  The
> primary calculation is actually easier on a calculator as it requires
> no interpolation.  The other table is used if the amplitude is shot
> with the  center of the sun on the visible horizon rather then the
> celestial horizon (The  sun is on the Celestial horizon when the LL
> is 2/3 of the sun's diameter above  the visible horizon is the rule I

> learned).  If conditions  permit, I will also shoot and post an
> exercise of a sum amplitude shot by both  methods.
>
> As far as timing, you actually want to shoot an Az when the body is
> closer to the horizon when the Zn isn't changing as rapidly in order
> to get a more accurate bearing.  Near transit, the body is changing
> directions very rapidly and even a 0.5 degree error can occur
quickly.

> I typically like to shoot  Az  shots around 8-9 AM or 3-4 PM.
>
> Our gyro repeaters have Az circles which use a mirror and a slit for
> sun  sights and are accurate to about 1/2 of a degree or so.  For
> stars and  amplitudes I use a Alidade (sp?) which is essentially a
> telescope with some  shades that has a sight line and a mirror to
> see the repeater's compass card.  The latter is a bit more accurate
> and I can get a reading accurate  to about a 1/4 of a degree (the
> circles themselves are marked in 1 deg  incriments.
>
> On a yacht, you can use a hand bearing circle or maybe it is easier
> to just point the bow at the body.  I am not really sure on this
> point.
>
> Jeremy
>
>
> In a message dated 5/22/2008 8:41:45 A.M. West Pacific Standard Time,
>
> gregr_ingest@yahoo.com writes:
>
>
> I  was looking at the ASA Celestial Navigation Standard (107) the
> other
> day,  and noticed that one of their requirements is "Calculate the
> true
> bearing  of a low altitude celestial body in order to determine the
> error and  deviation of the compass".
>
> http://www.asa.com/asa_standards/standard_celestial_navigation.html
>
> Back  in the dark ages when I was first learning celnav, the textbook
> method for  doing compass checks at sea was with amplitude tables and
> the sun low on  the horizon (at least if I remember right).
>
> I haven't seen amplitude  tables in years (does Reed's still have
> them?)
> - but it dawned on me that  when you do sight reductions you also
> have a
> true bearing for the celestial  object (Zn), so why not just
> momentarily
> point the bow in that direction  and note what the compass reads?
>
> Even easier, 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.
>
> Can anyone think of a reason why that wouldn't work as  well as using
> amplitude tables, or is my info on that method several years  out of
> date and nobody does it that way any  longer?
>
> --
> GregR
>
>
>
>
>
>
>
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> with
> Tyler Florence" on AOL Food.
>
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> >
>



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