NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Art and Navigation
From: Hewitt Schlereth
Date: 2015 Mar 26, 16:17 -0700
From: Hewitt Schlereth
Date: 2015 Mar 26, 16:17 -0700
Over the years I've had good results with plastic sextants.
The first two years I actually did celestial on offshore passages I used an Ebbco Mk1 which was fine for sun and moon. After that, I got an aluminum sextant with big mirrors and started taking stars routinely.
Greg did an exhaustive set of trials of an Ebbco last summer and his results confirm the utility of plastic. You've got to fuss more, be a hawk about side and index error, but the results are nothing to be ashamed of.
Now that I'm teaching here in San Diego I'm finding the students who have sextants (about half) have Davis 15s or 25s.
I have one of each, plus a Mk3 and am getting results with the Mk3 on a par with the two micrometer drum models. Oddly enough, on the drum models, I get just as good results with the sight tubes as the scopes,.
One of the students in the most recent group preferred to bring the center the of sun to the horizon. That way, there is no semi-diameter correction. (Poof! goes one table from which to pick the wrong number.) I remembered seeing this in an old British text but had never tried it. So I did, and it worked fine.
Hewitt
Tom,
I agree that trying to keep a sextant's index error exactly at zero is futile but this should not be an excuse for never adjusting the horizon mirror. Carly's Mark 3 without a case will be very demanding on index error stability. I will suggest a plastic food container fitted with customized foam as something to try.
Carly,
If you post all your sight reduction data (a picture of working notes should be good enough) then we can check things out.
Greg Rudzinski
From: Tom Sult
Date: 2015 Mar 25, 11:15 -0700CarlyThe goal is not to zero your sextant, but to know the error. I like my sextant to have several min of error ON the arc. Trying to be exactly zero is hard and rarely lasts. Testing error before a round of sights then adding or subtracting it is more practical.
Tom Sult, MDAuthor: JUST BE WELLjustbewell.info
