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    Re: The 57 Navigational Stars (and Zuben'ubi)
    From: Herbert Prinz
    Date: 2005 Apr 21, 21:42 -0400

    Frank Reed wrote:
    > I wonder why they felt it was necessary to fix the list at 57?
    I don't have the faintest idea, but the question reminds me of a debate
    over the reason why Ptolemy chose an 18-year interval for mean motion
    tables in the Almagest (basically, multiplication tables). Toomer put an
    end to it. Ptolemy says that 18 years will "produce symmetry in the
    layout of the tables". Toomer points out that 45 lines was the standard
    height of tables in the Almagest, presumably chosen to conform to a
    standard height of papyrus roll. Loose a couple of lines for headers.
    Ptolemy combines two columns on the same sheet: In the first, a table
    for 12 months and one for 30 days (= 42 entries); in the other column, a
    table for 24 hours and one for n individual years. But 42 - 24 equals
    18. Get it?
    > They added six stars and deleted six others including Polaris, which
    > always seems surprising.
    Indeed, surprising. I never understood the obsession of _modern_
    navigators with the special treatment of Polaris. We don't need to point
    out to this forum the historical significance of Polaris. But nowadays
    it is hardly more than a boy-scouts thing, just like finding north in
    the woods with your watch: same importance and same accuracy.
    It's a myth that one can find latitude from Polaris without knowing
    time: one needs _local sidereal_ time. In modern times, this is
    equivalent to an estimate of longitude and Greenwich time. (When did you
    last use a nocturnal?) If one has such an estimate (and who doesn't?),
    but does not trust it, it is much easier to find latitude with a regular
    two star sight, reduced a la St. Hilaire, and to discard the longitude.
    If the almanac gave GHA and Dec of Polaris, one could use it in the
    regular way, if one really must, and forget the special procedures and
    special tables consuming three pages in the almanac. But why would one
    want to use a star of magnitude 2.1 in the first place?
    It is true, since Polaris is always nearly on the meridian, a shot of
    Polaris can give a very good latitude, if (!) one has local time; but
    this feature is inherent in the geometry of the observation, not in the
    method of reduction. Therefore, there is no need to make head stands for
    a special method of reduction. Besides, since Sumner's great discovery,
    we have come to understand that latitude is just one specially named
    line of position.
    Herbert Prinz

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