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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
sight.

The Horizontal Coordinate System connected to the Terrestrial and Celestial
Coordinate Systems, and hence to the data in the Nautical Alamanc.

http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
--------------------
Outgoing email scanned by Norton Antivirus

> -----Original Message-----
> From: Navigation Mailing List
> [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Bill
> Sent: Friday, January 28, 2005 6:34 PM
> To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> 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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