NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Hewitt Schlereth
Date: 2010 Oct 24, 23:10 -0400
Up till the time I taught a group of three guys over a period of five days ashore and five days at sea this past January, I'd have agreed with you. But after five days practice ashore and five days at sea, using our one and only David Mk 15, they were consistently getting sun LOP intercepts of 1', and often even better. We also got excellent results from naked-eye morning and evening horizon sights - again, intercepts of about 1', and sometimes better, so perhaps conditions were exceptionally benign.
My previous experience with plastic was two years worth of summer-time Atlantic passages with an Ebbco. This was in the '70s, before GPS. My sense then was that the Ebbco was producing sun LOPs of around 2'. But the only check I had was from known positions ashore.
The difference between then and January 2010 was my hand-held Garmin 72. Honestly, I was amazed. The only real difficulty we encountered was with stars. But that was cured by removing the 'scope and just sighting through its mounting rings.
Anyway, now that I've dipped a toe in, I'll be trying more lunars in the future, to test things further.
-Hewitt
Hewitt, you wrote:
"Error in Lunar: 0.9'
Approximate Error in Longitude 28.3'."When I have used a Davis sextant for lunars my results had a range of roughly
+/-2.5'. Sometimes, of course, you can do better just by chance, but in general, inexpensive plastic sextants are not suited to accurate lunars. They're great sextants within that expected range. You can sail oceans with them, no problem, if a couple of minutes of arc is not a serious concern as in traditional small boat line of position navigation.And you wrote (to Douglas):
"This is interesting to me on two counts. First my results are in line with yours, which is heartening. "Seven out of eleven of Douglas's Jupiter lunars were within +/-0.22 minutes of arc --so your results are not actually in line with his. He has concluded that his lunars are much worse than this apparently because there is some kind of bug in his calculator clearing method.
-FER
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