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    Determining latitude by length of day
    From: Mal Misuraca
    Date: 1997 Nov 13, 11:02 PM

      Except at or near the equinox, when the length of day is essentially equal
    at all latitudes, the length of day is a reasonably accurate means to
    determine latitude.  And, depending on the accuracy of the vessel's dead
    reckoning between sunrise and sunset, from which the length of day is
    calculated, a reasonable longitude can be calculated by the same equal
    altitudes method we have been discussing in these exchanges.  In other words,
    one half the time between sunrise and sunset is by definition local apparent
    noon, for which the GHA (and longitude of the boat) can be determined from the
    almanac.
    
      If the vessel is moving during the day, corrections for that movement must
    be made, but they are not so complicated as they sound, and there is a decent
    formula for them.
    
      Then, there is a very good means to navigate to virtually any spot by
    pointing directly at a star that passes overhead that spot at precisely the
    instant when the star is directly above your destination.  So, for example, if
    you are en route to Kaneohe Bay on the east side of Oahu, and you are sailing
    in the summer months, the star Kornephoros (also known as Rutilicus or Beta
    Hercules) will pass directly over a spot due east of the entrance to the
    channel into the bay once each night.  You need only predetermine what time
    each night, then point the boat at Kornephoros at the required time, and
    repeat each night of the passage, and you will be on a celestial track on a
    great circle course to Kaneohe.
    
      This method was incorporated into an emergency navigation kit for the
    sailors in the Pacific Cup race two years ago, the race from San Francisco to
    Kaneohe that is held each two years, and the kit will be updated in 1998.
    
      How to pick out Kornephoros (or any other star).  If it's a major star in a
    constellation, this should not be a problem.  Get a star chart (sometimes the
    chart in the almanac is good enough), and assess where the star is in relation
    to others in the constellations around it.  Learn to pick it out easily.  Some
    stars are easier than others.  Saiph, in Orion, is very easy to spot, and it
    passes about one nautical mile due north of one of the Marquesas chain---and
    would make a good guide star for sailing to that island.
    
      The Polynesians used a comparable method, but had no means to know when
    their guide stars passed over their destination.  They knew only that when the
    star rose at night, its general direction was an indication of what course
    they should sail either to one side or the other of its rising azimuth.
    Knowing GMT and the time the star passes over where you are going is much more
    exciting, because at just the right instant each night, that star becomes your
    own personal lighthouse or Star of Bethlehem, because it sits at that instant
    precisely overhead of your destination.  Point at the star, and you point at
    your destination.  And the higher the star rises in the sky, the closer you
    are.  When the star is at your zenith (90 degrees from the horizon, directly
    overhead), keep a close lookout, because you are within a short distance of
    your landfall.
    
       Mal Misuraca
       Celestial in a Day
       Aliscafi---.com
    
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