A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Michalis Vaidanis
Date: 2021 Feb 23, 14:06 -0800
After Frank Reed’s and Gary LaPook’s ideas about some modifications of the Rude starfinder (addition of twilight lines, time scales, analemma etc, here: http://fer3.com/arc/m2.aspx/Modifications-2102D-LaPook-apr-2009-g7982 and here: http://fer3.com/arc/m2.aspx/CP300U-USAF-Star-Finder-FrankReed-feb-2021-g49784) and in seeking a way to digitally reproduce the Azimuth/Altitude grids, I stumbled upon the “G.Projector” application from NASA (https://www.giss.nasa.gov/tools/gprojector/). After playing with it a bit, I managed to get grids that fit almost perfectly to the 2102-D blue templates. However, this was not the case with the CP-300/U, the grids of which extend 10 degrees below the horizon.
For comparison, I’m attaching the G.Projector grid, along with its superimposition to the relevant 2102-D and CP-300/U templates. The G.Projector grid was produced with the following parameters: azimuthal equidistant projection, observer at 5 degrees North, azimuth and altitude at 5 degree intervals, lowest altitude at 10 degrees below horizon.
What is not clear to me is why there is that plot difference between 2102-D, CP-300/U and the G.Projector grid, especially to the altitude parallels extending below the horizon. Are the 2102-D and CP-300/U projections the same “plain” azimuthal equidistant or the latter is tweaked somehow?
What am I missing here? Any ideas?