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    Re: Silicon Sea: Leg 82
    From: Eric Haberfellner
    Date: 2001 Dec 02, 8:58 PM

    See Comments below.
    
    >Anybody care to help a beginner?
    
    >I would like to get up to speed from a basis of pilot navigation - which
    >doesn't use identical jargon.
    
    >Unfortunately, that means asking about just about every term used in the
    >problem. If this is too much - tell me - and I'll understand. For example -
    >mag var is the difference beween true north and the direction in which a
    >perfect compass may be expected to point - OK: but deviation: is this the
    >error  in a compass reading due to stray fields remaining after correction,
    >and shown on a compass correction table?
    
    YES.
    
    >Current speed and set: is this a vector like wind speed and direction?
    >Wind direction is the direction from which the wind blows: Set, is
    >this  the direction towards which the current moves? Is it always given as
    >a magnetic direction?
    
    SET is the direction in which the current is moving, not the direction it
    comes from. It can be stated in either True or Magnetic. I am assuming that
    in this problem, the 285d is True.
    
    >Is compass course the compass track between present position and
    destination?
    >  What does LL mean, in the "sun LL" context.
    
    Compass course is the course you see on your compass with no correction for
    deviation or variation. I am assuming that for the purposes of this exercise
    that what is being asked for is the initial compass course for a great
    circle route. This is what the helmsman would need.
    
    LL means Lower Limb. This means that the sextant sight was taken so that the
    bottom of the image of the sun just brushed the horizon. The other
    possibility would be UL (upper limb) which would be the top of the sun.
    
    
    > What does Hs mean?
    
    Height sextant. It is the uncorrected reading of the sextant after taking a
    sight.
    
    >What does Zn mean?
    
    Azimuth. This is the true bearing to the body you are using in the fix.
    
    >What is the RFIX  (referred fix??)
    
    Running fix. It is a technique of advancing Lines of position in time to
    create a fix from two or more lines of position which where calculated at
    different times (perhaps several hours apart).
    
    >HE is head elevation?
    
    Close. Height of eye above the water.
    
    >Index error is a sextant correct? how found??
    
    Almost all sextants will show some index error. It is the angle between the
    index mirror and the horizon glass when the sextant is set to 0d 0.0'. in
    index error of a small number of minutes (less than 5) is not adjusted out,
    but accounted for mathematically when correcting the sight. The easiest (but
    not most accurate) method of deterring index error is to perfectly align the
    horizon so that it shows a straight line in the sextant and then read the
    index error from the arc/drum/vernier of the sextant. A positive index error
    is subtracted when correcting the sight, a negative one is added.
    
    >Sailings? Affect??
    
    Sailings is a general term for calculating course and distance between a
    point of departure and a destination.
    
    >What do I use for charts and plotting - where do I get them?
    
    Since we are in open ocean, you don't need any charts. For plotting, you
    will need plotting sheets. These are essentially charts with nothing on them
    but a compass rose and some lines of latitude and a scale for adding lines
    of longitude. These are available from chandleries, or you can make your
    own. I think that there are some available on the web, but I am not sure
    from where.
    
    >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~``
    
    >Thanks in advance
    
    You are welcome.
    
    >Brian Whatcott
    

       
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