A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Aug 2, 10:16 -0700
Charles McElhill, you wrote (in your message on the "practice horizon bubble" earlier today):
"it's not my location, it's the altitude of the Moon and Jupiter I was referring to when I mentioned uncorrect to get actual results."
That's a lot of work. If you get pleasure from all that monkeying around with calculations, enjoy, of course!
But just a reminder: on my web site there is a web app set up to clear "backyard" lunars that does all of this work for you. You enter your estimated position, and the parameters of your lunar observation including the UT of the sight, and it calculates the altitudes of the bodies (from highly accurate built-in almanac data), the cleared lunar distance, and immediately displays your error. It's extremely fast. You can check a typical lunar observation in thirty seconds. And it's complete -- it includes a couple of parameters that are ignored in many accounts of clearing lunars (including John Karl's version of the standard triangle solution).
I also recommend using my web app or some other well-tested clearing tool to double-check your results even if you prefer doing your own calculations. I wish I had a dollar for every time some self-taught lunars fan (not you, Charles)has emailed me saying that "lunars are no good... I've tried them and I can only get within 1' or 2' in the observed lunar distances, and obviously that's why they weren't more popular historically" and so on and so forth ...only to find that they have hacked together their own tools --some spreadsheet or spaghetti code-- for clearing lunars. The clearing tools are badly flawed. It's one of those cases where it's a good idea to separate the disinct components of the puzzle: instrument, observation, input data, calculation. If you "roll your own" solution from top to bottom, the results can appear to imply observation problems when in reality it's some annoying "sign error" or something similar in the computations. Please note, I'm not saying that you personally have this issue, Charles -- I'm speaking generally about cases I have encountered. :)
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA