NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What is the best accuracy you can expect from a'dip-short' table
From: Lu Abel
Date: 2014 Oct 23, 16:48 -0700
From: Lu Abel
Date: 2014 Oct 23, 16:48 -0700
Stan beat me to it ! All this esoteric talk about how one might
use dip short offshore avoided talking about one of the most common
uses -- teaching nascent celestial navigators.
Not all of us can afford to ship aboard a vessel and go out of sight of land. Sometimes we're lucky if even part of our land-based view of the ocean has a natural horizon.
Here in San Francisco Bay my favorite spot to take students for sights has about 100 degrees of natural horizon from South to WNW. But by doing dip-short, I can add add 30 or 40 degrees to that. Anything east of dead south is dip-short, so if I want to cross the moon with the sun for a fix it's most easily done with a dip-short shot to the southeast. Or how about the easiest fix: two sun shots at least 3 hours apart? (technically an advanced LOP, but we're actually standing still on the shore). If I don't take one of those shots before noon (and therefore to the SE), I risk having SF's afternoon fog or haze obscure my horizon.
Having said all that, I wonder if it's worth worrying if dip short tables are accurate to a tenth of a minute or not. It's my experience that there are many factors that contribute to sights being off by more than that, so plus or minus a tenth is not worth worrying about.
Not all of us can afford to ship aboard a vessel and go out of sight of land. Sometimes we're lucky if even part of our land-based view of the ocean has a natural horizon.
Here in San Francisco Bay my favorite spot to take students for sights has about 100 degrees of natural horizon from South to WNW. But by doing dip-short, I can add add 30 or 40 degrees to that. Anything east of dead south is dip-short, so if I want to cross the moon with the sun for a fix it's most easily done with a dip-short shot to the southeast. Or how about the easiest fix: two sun shots at least 3 hours apart? (technically an advanced LOP, but we're actually standing still on the shore). If I don't take one of those shots before noon (and therefore to the SE), I risk having SF's afternoon fog or haze obscure my horizon.
Having said all that, I wonder if it's worth worrying if dip short tables are accurate to a tenth of a minute or not. It's my experience that there are many factors that contribute to sights being off by more than that, so plus or minus a tenth is not worth worrying about.
On 10/22/2014 7:01 PM, Stan K wrote:
How about for someone learning to use a marine sextant for ocean navigation? Better than an artificial horizon, no?
Stan
-----Original Message-----
From: Gary LaPook <NoReply_LaPook@fer3.com>
To: slk1000 <slk1000---.com>
Sent: Tue, Oct 21, 2014 1:12 pm
Subject: [NavList] Re: What is the best accuracy you can expect from a 'dip-short' table
As long we are talking about "dip-short," what was the purpose for these tables? If you are shooting a star while standing on the shore or anchored then you already know where you are, you need to know this to know the distance to the waterline you are using for reference line, The only reason I can come up with is for use in convoys where another ship is blocking the horizon below the celestial object being observed. Any other ideas?
gl
From: Greg Licfi <NoReply_GregLicfi@fer3.com>
To: garylapook---.net
Sent: Tuesday, October 21, 2014 4:31 PM
Subject: [NavList] What is the best accuracy you can expect from a 'dip-short' table
Hi all, Does anyone know what is the best accuracy you can expect from a 'dip-short' table is? There are many sources of tables (kinda like opinions) is any one better than another? Should you use an equation, and is there one particular equation that is more accurate than another? Thanks, ~Greg
