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    Re: Sextant accuracy with short distance to horizon
    From: Steven Wepster
    Date: 2001 Jun 25, 8:57 AM

    > Date: Fri, 22 Jun 2001 14:46:56 -0400
    > From- Smith_Peter{at}EMC.COM
    > Subject: Re: Sextant accuracy with short distance to horizon
    > To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    > X-Virus-Scanned: by AMaViS snapshot-20010407
    >
    > Steven Wepster [mailto:wepster{at}MATH.UU.NL] said:
    > >                                   ... I don't know for shure what
    > > Peter and  Russell meant by 'dip short' tables: my 1981 Bowditch
    > > Vol.II has a different table 14, but it has a table 22 'Dip of the
    Sea
    > > Short of the Horizon'. This table gives the dip of objects _in
    front
    > > of_ the horizon, so it should not be used for a normal altitude
    above
    > > the horizon.
    > >...
    >
    > The table for "Dip of the Sea Short of the Horizon" is for just the
    > situation Dan Allen was in: the horizon was blocked by an
    intervening
    > island, so he had to use the point where the island met the water as
    > his horizontal reference instead of the horizon. Normal dip tables,
    as
    > in the Nautical Almanac, give the correction between the horizon and
    > the true horizontal. "Dip of the Sea Short of the Horizon" is for
    the
    > special case, as here, where one is using a point on the sea's
    surface
    > closer than horizon, but at a known distance from the observer.
    > To quote from Bowditch's Explaination of Tables for table 14 (1995
    > edition):
    >
    >       If land, another vessel, or other obstruction is between
    >       the observer and the sea horizon, use the waterline of the
    >       obstruction as the horizontal reference for altitude
    >       measurements, and substitute dip from this table for the
    >       dip of the horizon (height of eye correction) given in the
    >       Nautical Almanac.
    >
    >  -- Peter
    
    I got that. But I missed the point that Dan really _did_ use such a
    short horizon :(.  --Thanks for pointing that out to me.
    _Steven
    

       
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