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    Re: Pointers
    From: Mike Burkes
    Date: 2004 Oct 9, 09:03 -0700
    Hi Alex,
    The " Pointers" refer to the 2 stars, Dubhe and Merak, of the  Big Dipper that " point" to Polaris the North Star.
    Mike Burkes
    ----- Original Message -----
    Sent: Saturday, October 09, 2004 6:33 AM
    Subject: Pointers

    Can anyone explain me that the "Pointers" are?
    I mean this is something in the sky,
    probably a group of fixed stars.

    I just read a funny paper:
    Joel Brenner, Determination of latitude in an emergency,
    American Math. Monthly, 51, N 6 (1944) 343-244.

    The author recommends a method of determination of latitude
    without almanach, without any tables, using only a sextant
    and Polaris (and the "Pointers").

    In general to find lalitude from Polaris altitude with accuracy
    of few minutes, one only needs its LHA, (or LHA Aries, or LHA of
    any fixed star).
    (See the table in the top on p. 274-276 of the 2004 almanac).
    One can neglest the a_1 and a_2 corrections in the
    middle and bottom of this table, which are very small.
    And knowing LHA Aries with only 10 degree accuracy may be OK
    "in emergency". This is because Polaris is so close to the
    North Pole that its altitude changes very slowly with LHA.
    The author
    proposes to use the position of
    the "Pointers" in the sky to find LHA Aries and thus LHA
    Polaris. Instead of using the almanac table, he proposes
    (in emergency) to find the correction graphically,
    just by drawing a picture as explained below.

    Then, of course, refraction remains. On this the author remarks:
    "A good navigator knows the refraction corrections.
    For observed altitudes 45d, 30d, 20d, 15d, the corrections are
    -1', -2', -3', -4', respectively.

    Here is an Example from this paper:

    "As sea, charts lost, clock stopped, tables illegible,
    sextant working, navigator takes altitude of Polaris, 44d30'20",
    IC -0'20", no dip. The refraction can be neglected.
    After consultation
    with the crew, it is decided that the figure shows the approximate
    direction of the pointers [the author includes the figure].
    A circle is sketched and drawn free hand (or by the use of
    two holes on a
    card, or by the use of a coin);
    the hour angle of Polaris next estimated [according to the author,
    Polaris is 135 degrees anticlockwise from the Pointers]
    (an angle of 135d=90d+45d is easy to draw) and last the value of cos
    of the hour angle of Polaris can be read to one place
    of the decimals from
    the figure.
    The latitude equals 43d42' plus or minus 4'

    [End of citation].


    So what are the Pointers?
    The article does not explain this, apparently assuming that this was
    common knowledge in 1944:-)

    Alex.
       
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