A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Ed Popko
Date: 2019 Feb 10, 05:09 -0800
What a wonderful recalculation. I don't deserve it. I consider Venus sight #10 a total fluke. I could never recreate it. Andres' graphing of that day's sight series was very interesting as it showed this back and forth convergence of the series to this last one. This series of lunars are now the most unusual in my sight log.
Venus was not so sharp that I tell you the exact phase. Perhaps I was measuring her center and not even aware of it. I need to wear glasses to correct a very slight astigmatism but I don't wear them when using the sextant, I try to use the scope focus to cancell it though it doesn't completely. The consequence of not wearing glasses and the average quality of optics in my scope (not astronomy pin point quality) is there is a very slight bloom to stars. Stars are not absolutely pin-point light sources. For planets, there is a noticable body but still a slight bloom.
I have been taking a lot of lunars this past year, several hundred by now. The technique I'm using lately is to keep one body dead center in the scope and sweep the other one back and forth across it. The 'trace' of the sweeping body in my mind's eye is what I draw on to bring them in kissing distance. When I feel I have it, I stop sweeping, hold it to check the final position and quickly record the time (pause Frank's GPS Anti Spoofing app). After that, I read the sextant's LD angle.
For myself, I can only do lunars when supported by something - my elbows resting on the hood of my car, shoulders propped against the corner of a building, sitting in a chair etc. If I simply stand and free-hand the shot, the results are always poor. I used my C&P Horizon sextant for a long time but I'm finding the Astra IIIB works just as well at half the weight. With the Astra, however, I have to study the vernier very closely. Unlkie the higher quality C&P vernier which is marked every tenth of a minute, the Astra is only every two tenths. So I visually interpolate the result ot a single tenth of a minute if two vernier marks fall between two frame marks (a Stan Klein trick).
In addition to some luck in the Venus series we are discussing, all the viewing and recording conditions were as good as it gets. All the big sources of error are eliminated: Frank's GPS Anti Spoofing app is a wonderful timer plus giving you a known position and other data. I also use single bright stars to find the sextant IE. You have to know it. If it's small, I don't keep fiddling with the mirrors to zero it out. I'm satisfied as long as it is very small and I know what it is.
Your spreadsheet analysis is quite nice, far more detailed than the algorithms I use for my calculator. Frank's on-line predictor and LD calculator are so easy to use I m hardly using my lunars calculator programs any more.
Thanks for re-running my Venus lunar. Consider adding a "Sight Comments" cell. I would wite 'Fluke' in it.