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: Alexandre Eremenko
Date: 2012 Dec 6, 15:23 -0500
From: Alexandre Eremenko
Date: 2012 Dec 6, 15:23 -0500
On Thu, 6 Dec 2012, Lu Abel wrote: > The multiplier for distance to the horizon has seemed > to vary over time and texts (even Bowditch, the "gold standard"). Not > sure why. I guess, this is because the formula incorporates some refraction correction. So it is not just the "geometric" distance. Refraction correction actually varies (we discussed this a lot on the list in the past), and which model of refraction to incorporate in the table is probably the author's choice. That's why some authorities recommended to use a dip-meter instead of the dip tables. Same applies to the distance to the horizon, though no dip-meter helps here. Alex. > 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>> 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 wrote: >> >> >>> From: Robin Stuart >>> Subject: [NavList] Bowditch: Distance to visible horizon >>> To: NavList---org >>> Date: Wednesday, December 5, 2012, 1:04 PM >>> >>> >>> Not 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 >>> ---------------------------------------------------------------- >>> NavList message boards and member settings: www.fer3.com/NavList >>> Members may optionally receive posts by email. >>> To cancel email delivery, send a message to NoMail[at]fer3.com >>> ---------------------------------------------------------------- >> >> > > > > : http://fer3.com/arc/m2.aspx?i=121313 > > >
