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    Re: Compass Checks at Sea
    From: Jeremy C
    Date: 2008 May 22, 05:54 EDT
     
     
    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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    >
    > >
    >


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