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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Was Bowditch Table 15, now confused
From: Jim Thompson
Date: 2005 Jan 26, 07:55 -0400

```> -----Original Message-----
> From: Bill
> your site brings an interesting term,
> until now sight unseen for me, into my vocabulary.  That being, "Sensible
> Horizon."  Which I take to mean visible horizon. I can appreciate but not
> calculate how that differs from the ? horizon (refraction(s)
> etc.)  Can you

Bill,

I am far from done with Table 15.  I have printed out everyone's posts and I
am still working on the problems of understanding (a) what the Table's
result means, and (b) how that formula was derived (likely beyond my
abilities, but I am getting some insights).  Hopefully my muddle at
http://jimthompson.net/boating/CelestialNav/CelestNotes/Bowditch15.htm
eventually will become true and meaningful.

Regarding the Horizon Coordinate System, see
http://jimthompson.net/boating/CelestialNav/CelestNotes/Coordinates.htm#Hori
zonCoordinates
but I have an image just below
http://jimthompson.net/boating/CelestialNav/CelestNotes/SightReduction.htm#C
orrections
which shows the relationship between the Sensible, Visible and Geometric
Horizons, and one just above that link which shows Dip.

Those images are better than plain text, but basically:

1. The Visible Horizon is the one you see with your naked eye when you look
at the apparent bounday between the sky and sea (or earth).  A line from
your eye to that horizon is called the Visible Horizon.
2. The Geometric Horizon is the real line between your eye and the actual
boundary between sky and sea/earth.  Since light bends between the horizon
and your eye, then the Visibile Horizon is rarely coincident with the
Geometric Horizon.  The difference is accounted for mainly by terrestrial
refraction.
3. The Sensible Horizon is part of the Horizon Coordinate System.  The
Sensible Horizon is a line from your eye that runs out into space parallel
to the Celestial Horizon.  The angle between the Sensible Horizon and the
Visible Horizon is called the Dip.  The Sensible Horizon is a very
non-intuitive concept for learners.

special Dip section to my CN Study Notes pages.  I am working on that too.

This stuff is extremely humbling.

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

```
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