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From: Jim Thompson (no email)
Date: Thu Feb 03 2005 - 06:14:48 EST
Interesting, Bill. I was just about to do that same exercise today --
comparing definitions in various texts. Obviously there is variation in how
people think about "relative bearing", and there is no clear consensus on
precise definitions. It is important for the user to be clear on his/her
meaning. You are right to suggest that the editors of Bowditch should add
that footnote to these old tables, which have suffered from years of
editing. Clearly the editors have chosen to leave some of the old tables in
the book, in this modern era where navigators can simply punch up those
results on a computer device. But they did so at the expense of shaving off
explanations on how to use those tables.
It is becoming clear to me that some navigation definitions are continually
drifting. Last night at a sailing class I was browsing through a copy of
"The Sailor's Word-Book", an 1867 nautical dictionary by Admiral W.H. Smyth.
He defined "Bearing" in rather vague terms, but cites the points method for
what we would call relative bearings similar to your 2 o'clock analogy,
rather than using degrees. The earlier 1815 Falconer's Marine Dictionary by
Burney, online at www.mysticseaport.org, also gives a similar sense that
relative bearing was merely "ahead, astern, on the beam", rather than 0-360
or 0-180 degrees.
Smyth's 1867 definition of "Horizon" was even more interesting: "The
apparent or visible circle which bounds our vision at sea; it is a line
which is described by the sky and water appearing to meet. This is
designated as the sensible horizon; the rational or true one being a great
circle of the heavens, parallel to the sensible horizon, but passing through
the centre of the earth". Either the author was not reaching for deep
sophistication in his astronomical definitions, or at that time (1860's)
their notions of horizons were somewhat different from the ones described in
Bowditch today. So I checked the 1828 Norie's online at
www.mysticseaport.org: He clearly defined sensible horizon the way Bowditch
2002 does today: a line through the eye of the observer, perpendicular to a
plumb line, and he defined the visible or apparent horizon the way it is
defined in Bowditch 2002. So either Smyth's later definition was incorrect,
or he was following some other semantic convention.
We saw another example of relatively rapid definition drift in the Bowditch
2002 definition of that I quoted in the Table 15 thread. They point out
that, "The geometrical horizon was originally considered identical with the
celestial horizon, but the expression is now more commonly used to refer to
the intersection of the celestial sphere and an infinite number of straight
lines tangent to the earth's surface, and radiating from the eye of the
observer". Today we seem to regard those two horizons as very different
features.
Jim Thompson
www.jimthompson.net
--------------------
Outgoing email scanned by Norton Antivirus
> -----Original Message-----
> From: Navigation Mailing List
> [mailto:]On Behalf Of Bill
> Sent: Wednesday, February 02, 2005 10:40 PM
> To:
> Subject: Re: Bowditch 1995 Table 18
>
>
> >
> > Jim, I don't know where you got this.Relative bearings are
> ALWAYS taken from
> > the head in a counterclockwise(right to left,0 - 360)manner.
> > There is a reason for this.One ship takeing clockwise bearings
> and another
> > takeing c-clockwise bearings is a recipe for disaster.
> > RBs are always taken/given in the above prescribed format.
>
> Interesting.
>
> Jim has quoted Bowditch.
>
> My Annapolis Book of Seamanship uses relative bearing similar to Bowditch,
> angle from the bow (or COG) aft on either side) 0 to 180.
>
> Chapman states, "Relative bearing is the direction of an object from the
> observer measured from the vessel's heading clockwise from 000 to 360.
>
> Dutton states: "Keep in mind that "angle on the bow is not the
> same thing as
> relative bearing. The former is measured from the bow in either direction
> as an angle. whereas relative bearings are measured clockwise from the bow
> through 360."
>
> In no case have I seen the 0-360 use of relative bearing in a
> counterclockwise direction (looking down from aloft). It has always been
> clockwise, same as a compass.
>
> No wonder my shipmates generally give directions to other craft as 1
> o'clock, 9 o'clock etc. Much less confusing, unless of course they have a
> 24 hour watch. <G>
>
> Bill
>
>
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