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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 B
Date: 2005 Feb 2, 16:41 -0500

> 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.

Interesting, and to a degree true as one should be working from COG as the
bow may not be pointed in exactly the same direction as course made good.

My point is that "angle on bow" and "relative bearing" have two different
meanings, especially when course plus bearing are equal to or greater than
180.

It should be specific.  Something like, "angle on bow corrected by
difference between course steered and course over ground."

I also find the phrase, "To determine the distance of an object as a vessel
on a steady course passes it,..."  misleading.  You do not have to "pass" it
(have it abeam) at some time for the tables to work.

Whether the above constitutes "error" is a matter of perspective and
semantics.  I feel when it can be made clear with the addition of a
recognized term, it is an error to omit it--leave ambiguity to politicians,
not pilots.  I don't feel there is any wiggle room in using the term "error"
for the table 15 formula presented in the 1995 edition.

Bill

>> 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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