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    Re: Assumed altitude method
    From: Dave Walden
    Date: 2009 Jul 10, 14:50 -0700

    >Dave,
    >Can you describe it briefly? Thanks.
    >
    >-FER
    
    Well, perhaps not briefly. :)
    
    I have read "The Method of Assumed Altitudes: A New Approach to an Old Art" by 
    T.D. Davies (from the ION disk).  I don't yet have any of Davies' Tables 
    themselves.  Based on the article, I have worked up the attached spread sheet 
    which reproduces his Figure 3, a sample page from the Tables.  (I did use 
    current RA's and Dec's vice his 1980, but more about this later.  And thanks 
    to Andres Ruiz for his program which produces nicely formated lists of names, 
    RA's, and Dec's. )  This leads me to have some confidence that I am 
    understanding his method so far.
    
    The method requires this special table.  The procedure is to observe a bright 
    star.  (Here comes the perhaps too bold assumption that it is one of the 42 
    stars included in the Table.)  The Table is then entered with assumed 
    latitude, the nearest whole degree to the DR latitude, and the observed 
    altitude, to the nearest whole degree.  On the latitude page, one scans down 
    the altitude column for the LHA of Aries value closest to the current value.  
    (If the assumption above is true, a close value will be found.)  One takes 
    out from the Table, the LHA of Aries and the true azimuth.  One now 
    calculates an assumed Longitude based on the known GHA of Aries to yield this 
    LHA of Aries.  From this assumed position, plot the azimuth given in the 
    Table and the intercept based on the difference between the obseved altitude 
    and the nearest whole degree altitude (the column heading).
    
    An example is given:
    
    At sea (well, on station), at 13h 00m 00s, 7 July 2009, observed altitude of a 
    bright star bearing NNE is corrected to be 21 degrees 43.6 minutes.  (The 
    USNO Ice program is used to simulate the sextant observation. Scan attached.) 
     Approximate position, 39 degrees 00 minutes North, 77 degrees 00 minutes 
    West.
    
    Required: LOP by Method of Assumed Altitudes, optionally, the name of the star.
    
    
    
    Nautical Almanac (well, Ephemerides nautiques, scan attached) gives GHA Aries 
    123 degrees, 35.2 minutes. Minus assumed Longitude West, gives LHA Aries 46 
    degrees 35.2 minutes.  Now open Table to Latitude 39 degrees North, and turn 
    to column for 22 degrees altitude. (Scan attached.)  Moving down, one comes 
    to the (cryptic?) entry, "46-55B 27".  This is taken to mean that if the 
    Latitude were 39 degrees North, and if the altitude of star "B" were exactly 
    22 degrees, the LHA of Aries would, of necessity, be 46 degrees, 55 minutes.  
    One then subtracts this LHA of Aries from the Almanc vaule of GHA Aries to 
    get the assumed Longitude of 76 degrees 40.2 minutes West.  From this AP, one 
    lays down the azimuth in the tablulated 27 degree direction.  The intercept 
    is the difference between the observed and the 22 degree nearest value or 
    16.4 nm.  (Scan of plot attached.)  In the explanatory material with the 
    Table (or spread sheet), one learns that "B" is Dubhe.
    
    
    A second example:
    
    Immediately (!) following, at sea (well, on station), at 13h 00m 00s, 7 July 
    2009, observed altitude of a bright star bearing SE is corrected to be 34 
    degrees 8.5 minutes.  (The USNO Ice program is used to simulate sextant 
    observation. Scan attached.)  Approximate position, 39 degrees 00 minutes 
    North, 77 degrees 00 minutes West.
    
    Required: LOP by Method of Assumed Altitudes, optionally, the name of the star.
    
    LEFT TO THE READER.
    
    Corrections/comments welcome.
    
    
    
    
    
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    File: 109070.t10julydw2.xls
       
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