A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Ed Popko
Date: 2017 Nov 30, 15:09 -0800
Frank's excellent Lunars Workshop several years ago sparked my interest and gave me enough to dig deeper. I programmed my HP50g scientific calculator to do the Bowditch-Thompson method taught in class and went on to do additional methods by deBorda, Reed, Letcher, Karl and Young. Programming is my learning style and working old case exampels puts me in a historic context while doing so.
For test cases, about 30, I have mined lunars examples published in 19th century Bowditch Practical Navigator books and other sources. I have had good luck in recreating these examples and it's made me aware of how difficult it was for navigators to take good sights and work them out by hand.
But here is where I am going loony with lunars. It's with comparing the pre-cleared with the cleared and visualizing what is happening with the change in arc length given the change in heights of the two bodies from apparent to observed (with all the corrections for HP, refraction etc.).
I had thought that when the moon's height is greater than the other body's height and subsequent corrections for HP and refraction etc. elevate the moon still higher and corrections lower the other body, I should see a cleared lunar to be a bit larger than its pre-cleared value. But this is generally what what I find. I have many worked many examples where the change was NEGATIVE. That is, the cleared distance is less than the pre-cleared. In fact, the change only goes positive when the moon's height is significanly higher than the other body.
Why is this? It seems counterintuitive.
I can post test cases if they are needed. But perhaps there is a simple explanation for what I'm seeing.