NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bowditch: Distance to visible horizon
From: Lu Abel
Date: 2012 Dec 6, 11:30 -0800
From: Lu Abel
Date: 2012 Dec 6, 11:30 -0800
The multiplier for distance to the horizon has seemed to vary over time and texts (even Bowditch, the "gold standard"). Not sure why.
What I find interesting, though, is that we're arguing about the last place in a decimal, so the change from 1.14 to 1.17 is less than 3%. Is that really important in practice? I think that surface effects (haze, etc) have a far greater effect on the visibility of objects close to the theoretical distance-to-the-horizon than slight differences in the last decimal digit.
When I teach coastal piloting, I tell my students to use 8/7 as the multiplier (decimal 1.1429). Close enough to whatever value Bowditch is touting these days and often easier to calculate with than a three-digit decimal multiplier.
What I find interesting, though, is that we're arguing about the last place in a decimal, so the change from 1.14 to 1.17 is less than 3%. Is that really important in practice? I think that surface effects (haze, etc) have a far greater effect on the visibility of objects close to the theoretical distance-to-the-horizon than slight differences in the last decimal digit.
When I teach coastal piloting, I tell my students to use 8/7 as the multiplier (decimal 1.1429). Close enough to whatever value Bowditch is touting these days and often easier to calculate with than a three-digit decimal multiplier.
From: Gary LaPook <garylapook@pacbell.net>
To: NavList@fer3.com
Sent: Thursday, December 6, 2012 1:56 AM
Subject: [NavList] Re: Bowditch: Distance to visible horizon
That's interesting, the 1962 and 1975 editions give it as D=1.144 and the 1938 edition gives it as D=1.15.
gl
--- On Wed, 12/5/12, Robin Stuart <robinstuart@earthlink.net> wrote:
From: Robin Stuart <robinstuart@earthlink.net>
Subject: [NavList] Bowditch: Distance to visible horizon
To: NavList@fer3.com
Date: Wednesday, December 5, 2012, 1:04 PMNot sure whether this has been noted previously but both the 1995( http://fer3.com/arc/imgx/bowditch1995/chapt22.pdf ) and 2002 ( http://fer3.com/arc/imgx/Bowditch-American-Practical-Navigator-2002-(2004).pdf ) editions of Bowditch give the following formulas for the distance to the visible in nautical miles
D = 1.17 sqrt( hf), or
D = 2.07 sqrt(hm)
depending upon whether the height of the eye of the observer is in feet (hf) or meters (hm).Given that hf = 3.281 hm, the second equation is not consistent with the first and should actually be D = 2.12 sqrt(hm). This doesn't seem to be just a simple typo. The formulas above are however what you get using the conversion hf = pi hm----------------------------------------------------------------
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