NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Instumental error?
From: Fred Hebard
Date: 2005 Apr 21, 11:27 -0400
From: Fred Hebard
Date: 2005 Apr 21, 11:27 -0400
On Apr 21, 2005, at 9:43 AM, Alexandre Eremenko wrote: > ...verifying ... arc precision > is not a trivial matter. > > If after a year of trials I will say that the arc error is 0.4' > rather than, for example 0.2' as advertized, it can be always > blamed on my poor skill, etc. > The crux of the matter, in my opinion, is to develop your skills before initiating calibration. In my opinion, proficiency has been accomplished when the standard deviation of dry land altitude shots using an artificial horizon is consistently (but not always) under 0.2' of arc, and frequently under 0.1', for three to five observations (increasing the number of observations also can decrease the the standard deviation, up to a point, but numbers of observations cannot substitute for proficiency). Only when proficiency is attained is it appropriate to actually use the observations for calibration, in my opinion. It took me more than a year to become proficient. Now I'm sure there are some out there who can pick up a sextant and start getting good observations in a few weeks, but not me! Embarking on a program of calibration is a good way to develop proficiency. It's just that one has to discard the first few months (or years) of data, prior to the development of proficiency. Here are data for the string of sights when I finally felt I had achieved proficiency on dry land. These are altitude shots using an artificial horizon, showing the mean Hc in decimal degrees and the mean and standard deviation of Ho-Hc in minutes of arc. Date Object mean n Hc(dd) std 09/21/2004 Moon 3 19.070 0.1 0.23 09/22/2004 Sun 3 50.266 -0.1 0.30 09/23/2004 Sun 3 22.184 0.0 0.11 09/23/2004 Moon 3 28.744 -0.5 0.19 09/25/2004 Moon 3 33.045 -0.3 0.07 09/26/2004 Sun 2 30.092 -0.8 0.08 09/26/2004 Sun 5 30.867 -0.4 0.02 09/26/2004 Sun 4 24.766 0.0 0.14 09/29/2004 Sun 3 14.903 -0.5 0.16 09/30/2004 Moon 5 30.222 -0.4 0.15 09/30/2004 Sun 6 21.581 0.0 0.11 10/01/2004 Moon 6 32.747 0.0 0.14 10/01/2004 Sun 6 29.658 0.0 0.10 10/02/2004 Sun 6 29.541 0.0 0.15 10/03/2004 Sun 6 48.553 0.9 0.21 01/26/2004 Sun 3 23.419 -0.9 0.22 As a disclaimer, the only reason to pursue this extreme precision is to calibrate a sextant, and that is useful primarily for lunars, which are as dead as the Dodo, as was said more than 100 years ago. If you can get the standard deviation of Ho-Hc under 0.5' of arc on dry land, that's good enough for use at sea, especially on small vessels.