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    Re: Pointers
    From: George Huxtable
    Date: 2004 Oct 9, 20:53 +0100

    Alex Eremenko asked
    >Can anyone explain me that the "Pointers" are?
    >I mean this is something in the sky,
    >probably a group of fixed stars.
    Yes, these are the two stars at the working-end of the "Plough" (Ursa
    Major), that point roughly toward the North Pole. They are alpha (or Dubhe)
    and beta of Ursa Major. This indeed is common knowledge; every school-kid
    ought to know it, but in Russia, no doubt, the name was different.
    Beware, though, that in the 60 years that have elapsed since the paper Alex
    refers to, precession has brought Polaris some 10 arc-minutes nearer to the
    pole, so an allowance would have to be made to keep the result within the
    claimed 4-minute error. The angle subtended at the pole, between the
    pointers and Polaris, will also have changed somewhat from 135 degrees.
    Things were more difficult in the great days of the Portuguese navigators
    around 1500, when Polaris went round in a circle that was more than 3
    degrees from the pole, so the correction was much more important to get
    right (though I doubt that Columbus ever mastered it!).
    In those days, the orientation of a line joining two stars known as "The
    Guards" was used, both for telling the time at night and for correcting the
    altitude of Polaris. The guards were beta (or "Kochab") and gamma of Ursa
    Minor. If the line joining the guards was vertical, then the altitude of
    Polaris was taken to be 90 degrees (which no longer applies). At other
    times of the night it was corrected according to the slope of that line
    between the Guards.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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