Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    Name or NavList Code:
    Email:
       
    Reply
    Exercise #5 Lat/Long near Noon.
    From: Jeremy C
    Date: 2008 May 31, 01:10 -0700

    Latitude and Longitude by Sunline near LAN.
    
        This seems to be one of Frank�s pet sights and is outlined in the
    more recent editions of Bowditch (1995 and newer).  Essentially we are
    taking a range of altitudes over a period of time before, during, and
    after the sun crosses the meridian of the observer (LAN).  The raw
    sextant sights are then plotted on the Y-axis, and time (GMT) is
    plotted on the x-axis.  Once plotted, the curve should be a parabola
    and can be folded so that the two halves mirror each other.  Depending
    on the number of sights and accuracy of those sights, the curve should
    be fairly uniform.  The crease, when crossing the x-axis should show
    you the exact time of LAN.  This time can be then entered into the
    almanac to find the exact GHA of the sun at that moment.  GHA
    converted to Longitude should give the Longitude of the observer at
    the moment of LAN.  This curve will be skewed to a greater or larger
    extent based on the change in declination of the body and the North/
    South component of the ship�s motion.  Because of these factors, if
    you use raw data, a prudent navigator would put only as much faith
    into the position as can be reckoned knowing that errors may exist due
    to these factors.
        To obtain Latitude, the nearest sight to the time of LAN (which
    should be the maximum altitude, or very close) must be converted to Ho
    then the z-distance and declination applied as a straight LAN problem.
        A second method to obtain Latitude and Longitude from these numbers
    is to reduce the data as standard sunlines.  A great number of sun
    lines reduced and plotted should give an accurate position if done
    correctly.  This is best done with a computer reduction and plotter as
    the sheer number of sights to reduce to obtain good accuracy will be a
    burden to attempt using tables or even calculators.
        This method can contain several errors and can cause some
    difficulties.  I think that the best time and place for this sort of
    sight is near the solstice (when the change in declination is quite
    small) and in the tropics (where the change in altitude is fairly
    rapid over time.)  Fast moving ships, especially with Northerly or
    Southerly courses, will also cause a skewing of the curve as the
    altitude will change with the change in Latitude.
    I performed this sight in late May in Latitude 15 deg North, while the
    ship was drifting due to engine problems (crs 270, Spd 0.6 knots).
    This allowed for a change in Altitude of about 35� of arc over a
    period of 24 minutes.  During this time, I shot 31 sunlines and
    graphed them.  One sight proved to be in error as it skewed the curve,
    but not so much as to cause a problem.  The change of declination from
    start to finish was 0.2� so the effects of time were minimal in this
    case.  Near an equinox this error would have been much greater and
    probably would have skewed the curve visibly.
        With the graph and a bit of eyeball interpolation, I was able to
    determine the time of LAN within 2 seconds with a corresponding error
    of Longitude of about 0.7� which is certainly an excellent result.  I
    saw the first change in altitude of the sun 3 seconds after the time
    of LAN computed from the GPS longitude, and used this for my latitude
    line, providing a 0.2� error south is Latitude.  This gave a total
    position error of 0.9 nm which is quite good for Celnav at sea.
        After the graphic reduction, I plugged 25 of the sights (the maximum
    allowed) into the computer and did a standard running fix via
    Celestial LOP�s (lots of sunlines).  I excluded the obviously flawed
    sight and several of the 30-second interval sights near LAN, keeping
    all of the other sights some distance from LAN.  The error in Latitude
    from this method was 0.1� and the error in Longitude was 0.4� This
    gave me an error in position of 0.4 nm at the time of LAN.  This
    method under these circumstances provided a very good position.
        The attached spreadsheet (MS Excel) shows the raw data.  The first
    column shows the number of the sight.  All shots were shot after 12
    PM, and the second column shows the minutes of the noon hour, while
    the third column shows seconds.  The fourth column �time� is decimal
    minutes.  Next is the Hs measure, first in degrees, followed by
    minutes of arc to the nearest tenth.  Next we have Hs in decimal
    minutes.  The last two columns were time and Hs which were used for
    the plot which is below and should be printable as a separate page for
    people to try at a decent scale.  The box on the right is the data in
    a more concise form including errors as computed by sailings.  I also
    included a short form of my Lat by LAN computation.
    
    The conclusion I find with this method is that if it is done at a time
    and place to eliminate most of the variables, it can be utilized at
    sea without too many calculations to determine both latitude and
    longitude.  The real trouble begins if your latitude varies greatly
    from your declinations, you are moving rapidly north to south, or you
    don�t have the hour it takes to shoot and reduce this sight.  I still
    believe that an AM sunline crossed with a LAN is your most convenient
    method to get a noon fix and can be used without thinking of all of
    the variables that will creep in using this method in various
    locations and at times more distant to the Solstice.
    
    I am attaching the MS Excel spreadsheet to my next post that I used
    for the graphic display.  Unfortunately I cannot electronically
    reproduce the sunline method of reduction, but it uses the same data
    as on the spreadsheet.
    
    Jeremy
    --~--~---------~--~----~------------~-------~--~----~
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To , email NavList-@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Get a NavList ID Code

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

    Enter the email address associated with your NavList messages. Your NavList code will be emailed to you immediately.
    Email:

    Email Settings

    NavList ID Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site