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    Re: Silicon Sea 32
    From: Peter Fogg
    Date: 2005 Jun 25, 07:22 +1000

    Mike you wrote on the 25 July 2004 (I hope your query wasn't urgent?):
    
    "Hi folks I need help! Yes it is the " pain" Mike Burkes here with his
    perpetual SS blights! Reference SS 32  no 6. Need to see solution for noon
    LAT! Thanks much!
    Mike Burkes"
    
    Since its been a while since the Silicon Sea voyage was mentioned, I'll
    point out that it consists of a series of navigational exercises that
    appeared here from time to time in the form of a virtual sail around the
    world over an extended period. They are archived at this List's home page:
    http://www.offsoundings.info/navl.htm
    at 'ssea.zip' under ' NAV-L FILES'.
    
    "SS 32  no 6" refers to Leg 32, in the Indian Ocean. Question 6 concerns a
    noon sight.
    To get there, we need to go back to our last established position. Question
    2 asks:
    "2)  What is our Estimated Position(EP) at 0800ZT on 28/06/1996?"
    and the answer supplied is:
    " 2) DR= 11d 27.4'N  62d 20.5'E"
    which seems about right. Q3 is irrelevant to our quest, followed by:
    
    " At Noon We get a break in the clouds and get a LAN sight of the Sun. Use
    the DR for 12:00:00ZT this date.
    4)  What is the Zone Time(ZT) of Meridian Passage of the Sun?"
    
    Running forward the position 4 hours along TC123.2d at 16.1 knots gives me a
    DR position of N10d 52' E063d 15', other methods may give a slightly
    different result. My method, using both meridional parts and meridional
    distances, should be the most accurate (ie; takes most accurately into
    account the somewhat non-spherical shape of the earth).
    
    In the Indian Ocean we are to the east of Greenwich. As the earth makes a
    complete rotation every 24 hours, 360d/24 indicates an hour of time equates
    to 15d of longitude. Since (60/15=4) we are in the time zone centred around
    60d east, 4 hours ahead of GMT.
    
    Sun dials use the apparent movement of the sun across the sky to indicate
    the time. They can be quite accurate (although not particularly precise).
    What they indicate is Local Apparent Time. As does the sun; when at its
    highest point of meridian passage it indicates 1200 LAT. To convert this to
    Zone Time 2 adjustments are necessary. Firstly our DR indicates we are 3 and
    1/4 degrees of longitude further to the east, a time factor of 13 minutes
    earlier. Then we need to account for the earth's eccentric orbit around the
    sun to give us the 'smoothed out' time known as Mean Time. One way to do
    this is with an analemma, and more than you ever wanted to know about this
    can be found at
    http://www.analemma.com
    Suffice it to say for our purposes we need to add 3 minutes of time. Putting
    our 2 adjustments together:
    
      1200 LAT
    - 0013
    + 0003
    = 1150 LMT
    Local Mean Time, or Zone Time.
    
    This is at odds with Q4's answer:
    "4) LAT= 12:30:32ZT"
    which doesn't appear to answer its own question - LAT? - and if it does,
    indicates by itself a position about seven and a half degrees of longitude
    away from 60d, apart from the small correction for the equation of time.
    
    Q5 asks:
    "5)  What is the Zone Description?"
    
    and confirms that it is:
    "5) ZD= -4"
    That's fine.
    
    And so we come to Q6:
    "|6)  What is the Noon LAT if the Sun's Hs was 74d 08.3'?"
    In this case "LAT" apparently means latitude, we are being asked to
    calculate the latitude indicated by the sun's meridian passage, and in the
    absence of contrary instructions shall assume the sun's lower limb was
    observed:
    
    Sextant altitude  74d 08
    Dip (8 foot)      - 03
    Index corr                - 02
    App altitude    74d 03
    Alt corr                  + 16
    Observ alt              74d 19
    Zenith dist             15d 41
    Declination         - 23d 16
    Latitude                07d 35
    
    Which is somewhat at odds with the answer for 6:
    "6) LAT= 10d 49.1'N"
    
    Does this agree with your calculations, Mike?
    
    Nothing and nobody is perfect, including both myself and the Silicon Sea
    series - I made a few blunders while working this out, and there may well be
    a few remaining. The occasional mistakes encountered in the SSs simply serve
    to keep us alert, and not alarmed. It remains a great resource, as I hope
    Mike will confirm.
    
    
    

       
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