NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2013 Dec 17, 15:22 -0500
Hello George & Peter
I look forward to your results. Next clear night, go out and measure some distances using a pair of the (57) navigational stars.
Once you think you have a good measurement, vary the index arm +/- a few tenths. Can you detect the difference? I couldn't do so reliably and abandoned the effort.
I was attempting to confirm a statement in a geodesy paper published in ~1905. That paper referenced my instrument by serial number and claimed an arc accuracy to 5 arc-seconds. Obviously, that's less than 0.1 arc minutes, about an order of magnitude better than I could resolve. Another instrument in my possession has an inspection certificate that shows ZERO error everywhere on the arc. Its a top end instrument, identical in model to the one Shackleton and Worsely took on that trip to S. Georgia Island. Both Alex and Frank have seen these instruments.
I could just trust the 100 year old assertions. There is little wear on the bearing surfaces, even with heavy use. But I prefer to know!
The star to star distance has tantalized many (including Cmdr Bauer) into thinking the arc could be calibrated this way.
Has anyone ever matched the results published on their certificate from a calibration facility while using star to star distances? Open call: Anyone?
Brad
I agree. When shooting Polaris in the daylight (not with a sextant) it reduces to a distinct tiny white pinpoint, it disappears behind the crosshairs.
-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Peter Monta
Sent: Tuesday, December 17, 2013 2:10 PM
To: george{at}gwbeckerpls.com
Subject: [NavList] Re: Sextant calibration
Hi Brad,
Rather than distinct points, stars are fuzzy blobs.
It might help to illuminate the entire field with some background light---that way the overall contrast of the scene would be reduced, so that the eye would see just the bright cores of the stars against a grey background. Start with no light, acquire the blobs in the field of view, then gradually turn up the light source until the stars are maximally pointlike, but not so much that they're not easily trackable by eye as the sextant is swung and adjusted.
The few times I've looked at Venus during the day with binoculars, I've been struck with how pointlike it seems against the blue sky.
I have some camera images of star-star sights---they don't show the "fuzzy blobs" problem provided the exposure doesn't saturate the stars. At some point I hope to finish the job of estimating my sextant's arc error using these images.
Cheers,
Peter
