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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: Bowditch 1995 Table 18
From: Bill Noyce
Date: 2005 Feb 2, 09:53 -0500

```> Course 2d true, object and shoreline to port
> First bearing 295d true, difference 293 d

There was nothing in the instructions you quoted, at least,
to indicate which bearing to subtract from the other.
I would have computed this "the short way around" as
(2+360)-295 = 67 degrees

> Second bearing, 245d true, difference 243d

Ditto here, (2+360)-245 = 117 degrees.

Of course these are the same as your "bearing on the bow",
but Bowditch's phrasing avoids assuming the bow points in
the direction you're traveling.

I'm sure 67 and 117 degrees appear in the table.

To see how the table is constructed, you can draw the two
right triangles whose sides are:
- the perpendicular from object to course line, which hits
it at the point of closest approach
(this side is common to both triangles)
- segment of the course from there to observation point
- sight line of the observation.
The angles needed are the one between these last two sides, for
each triangle.  From symmetry it should be clear it doesn't
matter whether it's measured clockwise or counterclockwise.

I have a hard time calling this an error.  Perhaps the instructions
could be clarified.  In any case, it's not a bias toward the
starboard side.  If you were travling northwest (300 degrees)
and observed an object off the starboard beam (30 degrees) would
you report the difference as 270 degrees, 90 degrees, or something
negative?
-- Bill

```
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