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Re: Summary of Bowditch Table 15
From: Jim Thompson
Date: 2005 Jan 27, 16:24 -0400

```> -----Original Message-----
> From: George Huxtable
>
Jim Thompson wrote, about Bowditch Table (9 or 15)
> >D increases, if D is the distance from the observer's horizon to the
> >object.
>
George answered: No. From table 9, as the height-difference H-h increases as
the observer
> descends his mast, then the increasing vertical angle corresponds to the
> same distance d as before, because that hasn't changed. The distance d is
> NOT the distance from the observer's horizon to the object. That was
> someone's misunderstanding; Trevor's, perhaps, that has since been
> recanted. d is the distance between the observer and the object.

Is that:
(1) The distance over the curve of the sea, between the observer's ship's
hull and the waterline at the base of the lighthouse?
(2) The length of the line from the oberver's eye to the top of the
lighthouse?

Jim wrote,
> >Doh!  So what is "D" then, and how does one use the table?
> >D is, I thought, the distance from the sea horizon to the top of
> the object.
> >I presumed that I should add my distance to my horizon (6.4 NM) and the
> >distance from the horizon to the top of the object (3.8 NM) to
> get a total
> >distance of 10.2 NM, with a 0slight vertical dogleg two thirds of the way
> >along.  Is that incorrect?
>
George clarified, hopefully,
> All quite incorrect.
> I hope it's clearer now.

As clear as my night-shift-soggy brain can make it.  I'll get there, though,
like our little terrier trying to get at the marrow of her discarded bone.

In our other email exchange about Bowditch 2002's article 2202 and Table
16's title (your 1977 Bowditch's Table 41), I finally realized how to read
the title of that table, thanks to your clear explanation.

Let me take a stab now at the title of Table 15 (9), which reads, "Distance
by Vertical Angle Measured Between Sea Horizon and Top of Object Beyond Sea
Horizon".  In plain English with some useful commas placed as a result of
your mental hand-holding down this path of lateral thinking, I now believe
that the title translates to, "Distance between the observer and an object,
determined by measuring the vertical angle between the observer's sea
horizon and the top of the object, when the object lies beyond the sea
horizon".  Did the penny drop, finally?

Jim

```
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