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    Re: Fw: Chichester's Calculations
    From: Gary LaPook
    Date: 2008 Dec 08, 00:52 -0800

    I am attaching a marked a up version of page 235, page 29 from Volume 3 
    of H.O. 249 and the correction table from that volume. To make it easier 
    to follow this explanation I have marked up page 235 with red boxes 
    labeled A through G.
    "A" show his computation of GMT or Zulu time for entering the Almanac. 
    12:11:22 is his watch time and the watch is obviously set to GMT. 02:30 
    + 1/2 is the correction for his watch error which is running that many 
    minutes fast on GMT, Adding these two numbers produces the GMT of the 
    observation of 12:13:52 (he dropped the extra half second.)
    "B" shows the computation of LHA or entry into H.O. 249. Taking the 
    entry of the sun's GHA (Greenwich Hour Angle) for 12:00:00 GMT he takes 
    out the GHA of 000º 53.1'. since the sight was taken 13 minutes and 52 
    seconds after 12:00 o'clock you look in the increments table in the 
    Nautical Almanac or in the Air Almanac and take out 3º 28' additional 
    for that extra time which you add to the GHA at 12:00: to find the GHA 
    of the Sun at the time of the observation to be 004º 21'. Chichester 
    automatically added 360º to this and wrote down 364º21' to make it 
    easier for the next step of subtraction his assumed longitude. Since the 
    DR position is 43N -  25W ("C") he choses an assumed longitude of 25º 
    21' so when he subtracts this from the GHA he ends up with a whole 
    number of degrees of LHA which is 339º.
    "C" shows the DR position.
    "F" shows the declination of the sun taken out of the Almanac at the 
    same time that the value of GHA was obtained.
    "D" show the computation of Hc using H.O. 249. Looking on page 29 of 
    volume 3 for latitude 43 and declinations 15-29 same name, we go down 
    the column for 20º declination and come across from 339º LHA and we take 
    out the tabulated Hc of 61º 02', the "d" correction value of +50 and the 
    azimuth of 136. The tabulated Hc is for 20º declination exactly. Since 
    the declination of the sun was actually 6 minutes more at the time of 
    observation we must correct this tabulated Hc for this difference. We go 
    to the correction table and under the 50 column (the "d" value) we read 
    down to the 6 minutes of extra declination line and extract 5' which we 
    add to the tabulated Hc of 61º 02' to determine the actual Hc of 61º 07'.
    "E" shows the computation of Ho. Starting with the HS (sextant altitude) 
    of 60º 50' we add the correction  for semi diameter (this is obviously a 
    lower limb shot), subtract refraction and subtract dip which Chichester 
    has combined into one correction of + 13'. (The SD alone is + 16 and 
    refraction for this Hs is - 1' making a +15'. Since Chichester uses +13' 
    he is including a dip correction of -2'.) On the next line he applied 
    the Index correction (IC) of + 1 to arrive at the Ho of 61º 04'.
    "G" shows him subtracting the Ho from Hc to arrive at the intercept of 3 
    away since Ho was less than Hc.
    The other examples he gives are done the same way although it is 
    interesting that in three of the examples he also combines the IC with 
    the other corrections to Hs writing down +14 total correction.
    I will get to page 234 tomorrow.
    Beverley Maxwell wrote:
    > Gary,
    > Thank you.  I am sending page 235, which Chichester refers to as the 
    > short method, and page 234 he calls the long old method.
    > Frank M.
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