A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Ritchie
Date: 2020 Feb 15, 15:23 +1300
Sorry I can’t help you with John Karl’s Bx method. However, you did ask if there is another/easier method. With Astron, I use the following steps which may be of interest to you.
1. If you want an advanced position line, advance the assumed position accordingly, (Advanced AP).
2. Using Advanced AP and great circle formula, apply the Capitaine Marq de Saint Hilaire Azimuth (Az) and Intercept to calculate a datum position (Datum Pos) on the desired LOP.
3. Calculate the Observed Distance (Obs Dist) of the body in nm. Obs Dist = ((90 - Ho) * 60). Ho is after all sextant altitude corrections.
4. Using Datum Pos and great circle formula, apply the reciprocal of Az and Obs Dist to calculate the body’s assumed Geographical Position, (Ass GP). This is, of course, a hypothetical position assuming perfect Azimuth and Intercept.
4. Using Ass GP and great circle formula, apply a bearing of (say) 000° and distance Obs Dist to calculate a spot position on the desired LOP.
5. Keep repeating 4/. with bearing increments of (say) ¼ degree. Each time join this spot position to previous one with a straight line.
Astron always calculates the entire 360° LOP so that it continues to work with zenith sights and high zoom-outs. You will probably choose a lesser arc for step 5. The bearing increments will also vary depending on desired zoom, etc.
You can see it working on the Sight Log/Plotter page of Astron, both on my chart and Google Maps. https://vigilanceofbrixham.co.uk/Astron/Astron.html
If you are interested in this method, contact me on my gmail address, 1billritchie and I can send you the individual build files as the combined Astron.js is huge.
Normally 50N 003W, presently 38S 176E.
Dear Alex, thank you very much for bringing my attention to those formulas! Looks like they are the key to understand the way how to build the Weem's Star Altitude Curves (my recent topic of interest).
Now, having looked at the problem, I think the difficulties you at times hit are because for some certain Bx values the curve has two points with the same LHA but different Lat. I do not know how to deal with this (yet).