A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert VanderPol II
Date: 2020 Sep 15, 21:37 -0700
Is there another larger earth involved here? The circumfernece of this earth is 21,639nm so that is the circumerence of the largest circle that can be drawn on it. And 1° azimuth change would involve moving 60.1nm along that circumference. Or am I missing something really big (pun half intended)?.
Letcher's book is my access to HO-208. His instructions indicate that Lat and t should be whole degrees which means that AP is used for the reduction rather than DR/EP. Are there different instructions with the original using DR/EP rather than the reproduction I have?
If you wanted to plot a line of position for the body shot you would need to measure the azimuth (aim the whole boat at the body), apply corrections for variation and deviation.
To plot you would need a very large map to plot on in order for your pencil width to not be 100nm wide.
Mark the GP of the body on the map.
Now comes the tricky party, the line you want to draw is the great circle direction from your location, not the rhumb line. On a mercator projection the rhumbline can be drawn as a straight line, but the great circle LoP is a curve who's shape depends on the latitude of you and the body and the LHA and probably something I'm not thinking of just now. It might be possible to use a gnomic projection, but I haven't played with those much so couldn't say for sure.
I recall reading somewhere that it is posible to plot a very high altitude as an actual circle on a reasonably scale map. The altitude of the body needs to be up 85° or so. If you are able to shoot a body that high accurately, then I can think of several ways to measure azimuth to within a couple degrees. Then you could plot from a single sight, and the azimuth measurement. However shooting that high is extremely difficult.
From: Gary LaPook
Date: 2020 Sep 15, 11:29 -0700