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    Re: Multi-Moon line exercise in 2 parts
    From: Jeremy C
    Date: 2009 Aug 9, 08:16 -0700

    Since at least a few brave souls have faced my challenge, I will give
    not only my calculations, but
    also the GPS fix at 1800L (away from transit) and 1900 (at transit).
    
    My shooting method is rather simple and very similar for both of these
    observations.  As you can
    see, I can shoot one shot every 30 seconds or so of the same body.
    The process is very quick
    when the body is at a known azimuth and altitude.  Between each shot I
    offset my micrometer
    drum about 10' of arc and then, standing at the same place on the
    bridge wing each time, quickly
    aim and rock the sextant to obtain the nadir, then quickly dial in the
    micrometer drum.  As soon
    as I "feel" it is correct, I look down at my watch and subtract 1
    second.  I then walk into the bridge
    where I record the sextant reading and the time, then offset the drum
    and walk back onto the bridge
    wing.  The only added measure I take when shooting in heavier seas is
    wait for the ship to be on an
    even heel before making taking my mark.  I never lean against
    anything, but use my legs as my
    built in gimbal (this is the same process for Lunars as well).
    
    The ship, with the deck of the bridge at 102' at that particular
    draft, is a medium sized container
    ship and height of eye on merchant bridges these days is typically
    about the same for a 200 meter
    long ship.  I am careful to adjust height of eye for both draft (there
    is a chart posted on the bridge) as
    well as my height.  The only un-accounted for variation in this is if
    the ship decides to heave at that
    moment, which will change things a few tenths of a minute.
    
    My results were reduced by my celestial program SkyMate Pro, as
    usual.  I chose 24 sights for my
    near-transit reduction.  My "called" transit was my 18:51:19 sight,
    which was pretty close since it
    gave me an Zn of 180.0.  My determined Latitude was 21 deg 47.9' North
    and Longitude of
    130 deg 04.6' E.  The GPS fix at 1900 (the ECDIS records a position
    every 3 seconds so I could
    back track) was 21 deg 48.7' North and 130 deg 07.5' East.  This was a
    fix error of about 2.8 nm.
    To me this fringes on the unacceptable range.
    
    For my away from Transit I used 25 sights and got a fix of 22 deg
    02.8' North and 130 deg 15.1' East
    The actual position at 1800 via GPS was Latitude 22 deg 01.9' North
    and Longitude 130-10.7' East.
    As you can see, we have a significant error in longitude here and the
    fix error was 4.2 NM.  Peter's
    results are better than mine, so his methodology is superior to mine
    for this type of sight.
    
    The next question is what to make of this data.  Well to me shooting
    the moon as it closes on
    transit isn't an efficient method of determining position.  For the
    amount of work to both shoot
    and reduce this sight, the accuracy just isn't there to make it worth
    while.
    
    For a multi moon sight around transit, we can see that the results are
    less than stellar once
    again, but certainly could be used in a pinch for ocean navigation.
    Still, this sight is highly
    inefficient to gain position.
    
    As a comparison, I shot stars a half an hour later, and obtained a
    1930 fix.  The error was 0.6 nm
    I shot 4 stars in 4 minutes 23 seconds, with perhaps 5 minutes of data
    entry and obtained a far
    more accurate position in far less time.
    
    Jeremy
    
    PS  I would invite Antoine to do a search of the archives for my
    Exercise #17 which is a single-
    body fix using Venus around the time of transit.  It is a mere 17
    sights over 45 minutes, so should
    be easier than this example.
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