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    Re: lat/long from meridian passage
    From: Frank Reed
    Date: 2011 Jan 23, 15:35 -0800

    For any who haven't opened the attachments, here is Patrick Goold's message, 
    slightly edited for formatting, with a table replaced by commas:
    
    BEGIN-----------------------------------------
    I. Determining the time of the meridian passage. Norfolk, Virginia.  22 January 2011.
    
    I was confident it was sometime well after noon local watch time but I found I 
    could not make sense of how to use NA�s equation of time.  NA gives mer. 
    pass. as 12 hrs. 11 m and the equation of time for noon as 11 m 29 sec. (I 
    inferred from Bowditch that I should add these two.  But adding them gives a 
    value outside the range of possible variation.)  The website timeanddate.com 
    put LAN at Norfolk, Va at 12:17.  I used this as my initial estimate.
    
    I recorded five shots, all lower limb sun shots. The first two I made at set 
    times and recorded the angles. The last two I waited for a return of the 
    second and then the first altitudes, recording the times when this occurred.
    
    Sight(WT), Hs, Ha, Ho
    noon, 66� 16�, 33� 08�, 33� 22.8�
    12:15, 66� 36�, 33� 18�, 33� 32.8�
    12:17, 66� 35.6�, 33� 17.8�, 33� 32.6�  !?!
    12:19:23, 66� 36�, 33� 18�, 33� 32.8�
    12:34:20, 66� 16�, 33� 08�, 33� 22.8�
    
    Because I am using an artificial horizon I made no dip or altitude 
    corrections. Ha is just Ha divided by two. Ho is Ha plus 14.8 for lower limb 
    correction.
    
    The noon sight and its mirror altitude sight gave an average of 12:17:10. The 
    12:15 sight and its mirror average to 12:17:12.  The average of those two 
    averages is 12:17:11.  My time zone is GMT plus five. So I made out meridian 
    passage at my site as occurring at 17:17:11 GMT.
    
    
    II. Determining Latitude
    
    Zenith distance = 90� - 33� 32.8� = 56� 27.2� N
    
    Dec (tab) =  S 19� 38.5�
    d = .06   d corr = 0.4�
    Dec = S 19� 38.9�
    
    Latitude = z - Dec = 36� 48.3� N
    
    III. Determining Longitude
    
    GHA (hr) = 72� 06.8�
    GHA m/s = 4� 17.8�
    GHA = 76� 24.6�
    
    Longitude = GHA = 76� 24.6� W
    
    According to my handheld GPS, a Garmin GPSmap 76Cx, my location was actually
    
    36� 50� N
    76� 21.7 W
    
    for a difference of 1.7� of latitude and 2.9� of longitude.
    
    Conditions were somewhat less than ideal and accuracy may have suffered. 
    Obviously there is something wrong with the middle of my five sights. It was 
    a very windy day and the glass covers on the Davis artificial horizon permit 
    some drafts to intrude and disturb the reflective surface. It was a very cold 
    day (-4 C) and this made for some fogging of the AH and perhaps of sighting 
    scope.  Finally, there was a haze becoming clouds and overcast as the 
    sighting progressed.
    
    END-------------------------------------------
    Just to reiterate, everything between the "BEGIN" and "END" above is from Patrick Goold.
    
    -FER
    
    
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