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--- Anabasis75@aol.com wrote:

> Yes, every deck officer is required to be tested on, and pass
> practical assessment in, Celestial Navigation.

Interesting, wonder if the same is required for passenger (i.e. cruise)
ships? I wrote to the Marketing Director of one of the lines and asked
if he could find out what their policy was on that, but never heard
back (he probably didn't understand the question...  ;-)).

> GPS only gives Course Made Good, not heading.

Some of the consumer units have a compass (flux gate?) built in, but
that introduces its own set of variables and unknowns.

> The mirror and slit on the circle actually projects a bar of light
> onto the card. [...] No need for filters or shades when using this.

Makes sense (and the "slit" probably should have been a clue), on the
ones I'm familiar with from Navy days sights were taken by looking
through the peloris itself (though we never did sun sights that way).

> Pointing the bow will give you the declination at that particular
> heading

Perhaps you mean "deviation", or is that called "declination" (i.e. the
error of the compass itself, usually related to the surroundings in
which it's mounted) in other places of the world?

> A series of sights on different bodies as they moved across the
> various points of the compass would eventually get you a decent
> deviation card, but it would take a lot of time and effort.

Actually I would hope that the deviation card would already have been
done, the compass checks only being done to confirm that it's still
showing an accurate magnetic bearing.

> I think I would just swing the boat on a range or try compass
> bearings at a steady heading, and in calm seas if possible.

Agreed.

--
GregR




> In a message dated 5/22/2008 1:41:52 P.M. West Pacific Standard Time,
>
> gregr_ingest@yahoo.com writes:
>
>
> 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?
>
> Yes, every deck officer is required to be tested on, and pass
> practical
> assessment in, Celestial Navigation.  In most parts of the world the
> ship  has GPS
> and Celestial as the back up for positioning at sea outside about 24
> miles
> from land (the outer range of most radar).  Omega, Decca, RDF, and
> most of the
> Loran chains are now gone, so our electronic options in the middle
> of the
> ocean are few.  Additionally, the only way to calculate compass
> error away from
> land is by Celestial observation.  GPS only gives Course  Made Good,
> not
> heading.
> No filters for sun sights, or does the slit take care of that for
> you?
>
> The gyro repeater has gimbals and a bubble level to help maintain the
>  card
> parallel to the sea, it really does matter as when the ship rolls,
> errors  of
> well over 5 degrees can be observed.  The mirror and slit on the
> circle
> actually projects a bar of light onto the card.  The trouble is that
> the  bar is
> about 1/2 deg wide, which is why accuracy is about 1/4 to 1/2 a
> degree by
> eyeballing it.  No need for filters or shades when using this.
>
>
> > 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?
>
>
> A polaris could be used to get relative bearings that would then need
> to be
> converted.  I was thinking more of a hand bearing compass with a
> sight.  I
> have seen them in catalogs.  George points out correctly  that these
> are nearly
> useless with higher altitude bodies however.
>
> > or maybe it is easier to just point the bow at the body.
>
> Pointing the bow will give you the declination at that particular
> heading,
> as George correctly points out.  I admit in my haste of a reply I
> didn't fully
> think that point through.  A series of sights on  different bodies as
> they
> moved across the various points of the compass  would eventually get
> you a
> decent deviation card, but it would take a lot of  time and effort.
> I think I
> would just swing the boat on a range or try  compass bearings at a
> steady heading,
> and in calm seas if possible.
>
> Jeremy
>
> --- 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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