A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Ed Popko
Date: 2018 Feb 7, 14:46 -0800
Thanks for the review of my sights and suggesting metrics for evaluating a series.
On lunars, It was hard not to do them, day or night - clear sky during the day and really dark at night, mild-temps, no wind, leisure setup with car hood as desk and GPS Anti Spoof Pro on a propped up tablet (illuminating my notes as well). What's not to like. Pre-calculating or estimating the LD is a huge head start, especially for crescent moon-sun shots where the moon is not all that bright.
I have a slight astigmatism, corrected with glasses, which makes stars bloom a small amount when I'm not wearing them. But I don't like wearing glasses when using my sextant. So I experimented with focusing the 7x35 differently and swinging the star or sun back and forth across the moon's limb till I was satisfied they were just kissing. And with GPS Anti Spoofing running, it was a simple matter of touching the screen to lock in the time. My Astra IIIB is light and easier to use for lunars than my C&P Horizon, and that helpes too.
BTW, this swinging technique also worked well in taking superimposed sun shots with an artificial horizon. I shot a lot of sun lines over my vacation and for some, I wanted to try bubble sextant refraction tables which required center-sun elevation.
It's very hard to simply superimpose the images of the sun in an artificial horizon. You are never sure at what point they precisely overlap. However, if you keep swinging one image past the other while concentrating exclusively on the alighment of their ULs or LLs, you can easily see the precise moment when they are perfectly tangent and thus both superimposed.
I took your lunars workshop several years back. I left with enough how-to skill to start taking lunars and clearing them with Bowditch-Thompson method. I was able to program some of the workshop algorithms into my calculator as well. Since then, I have added several more clearing methods. Your web tools for clearing and predicting are very helpful too.
I find lunars historically and technially fascinating. Unlike other forms of celnav where GMT starts the process, lunars ends with GMT. And because you record the times of your sights, you know how you are doing.