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    Re: Was Bowditch Table 15, now confused
    From: Jim Thompson
    Date: 2005 Jan 28, 20:29 -0400

    How about this.
    Stand and look out to sea, but look above the sea/sky interface, on a line
    perpendicular to the line that your body makes from feet to head: you are
    looking out along the imaginary sensible horizon.  Then "dip" your eyes down
    to the sea/sky interface, which is the visible horizon.  The angle traversed
    by your eyes as they dip is the "Dip".
    The "horizon" that we are intuitively used to calling a horizon from
    childhood (the sea/sky interface) is not part of the Horizon(tal) Coordinate
    System, which is made up of imaginary lines.  The sensible horizon is,
    however, part of the Horizonal Coordinate System.  The Horizontal Coordinate
    System's horizons are perpendicular to a line from the center of the earth
    through the point of the surface of the earth on which you are standing, to
    a point on the celestial sphere above your head.  That frame of reference
    moves as you move.
    Since you can see only the visibile horizon, then you have to apply the Dip
    and Refraction corrections to convert your sextant measurement to the
    Horizontal Coordinate System's frame of reference, in order to reduce a
    The Horizontal Coordinate System connected to the Terrestrial and Celestial
    Coordinate Systems, and hence to the data in the Nautical Alamanc.
    Jim Thompson
    Outgoing email scanned by Norton Antivirus
    > -----Original Message-----
    > From: Navigation Mailing List
    > Sent: Friday, January 28, 2005 6:34 PM
    > Subject: Re: Was Bowditch Table 15, now confused
    > > Perhaps a clearer way to put it is that the Sensible Horizon
    > > is perpendicular to the vertical.  As it happens, measurements
    > > using a pool of oil or mercury are relative to the Sensible
    > > Horizon.
    > Not clear how to interpret that.
    > Is the sensible horizon is a plane perpendicular to the vertical (line
    > through center of the Earth and viewer), AT THE VIEWER'S HEIGHT
    > OF EYE, as I
    > understand Jim's drawing?
    > General instructions for a pan of oil etc. instruct the user to
    > place it on
    > the ground.  No dip correction.  If it relates to the sensible horizon and
    > my understanding of the definition is true, then dip correction would be
    > required if the pan were placed on a stool?  That doesn't seem
    > right to me.
    > I do clearly understand I am deeply confused.
    > Bill

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