A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Francis Upchurch
Date: 2016 May 24, 19:00 +0100
Just invented same(thought I was clever but as usual, I’m generally second!) to compare my lovely Francis Barker little Box sextant with my trusty Astra B which for many years has been perfect.
Method works. (Box sextant pretty good considering size etc. Well +/- about 1-2’ arc max, usually better compared to Astra which is spot on). Hope to use the Box sextant to re-enact some of the Chichester stuff later, if my poor ship, recently crushed by a 500 ton cargo vessel ias repaired on time.(Cornish have a word “directly” meaning any time soon or maybe in the next hundred years. Bless them. It is one reason I chose to live down here after escaping from the madness of London.
Greg Rudzinski, you wrote:
"It is a great deal simpler to compare sextant to sextant directly. No timing, sight reduction, or trips to the beach. The Bris can now be calibrated in the back yard. "
Yes! Very nice. I've written on something quite similar before... If you take any two sextants and set them to the same angle, and then set them on a table index mirror to index mirror (some fiddling around required but it should be obvious), then the test for calibration reduces to a plain index error test. The idea here is that the well-calibrated instrument "feeds" light which is already deflected by the angle showing on the arc to the second instrument. The second instrument takes out that deflection if it is set to the same angle. Then you line up some very distant object in both reflected and direct view (just like an index error test). Any difference on the displayed angle on the second instrument is the calibration error of that instrument. Naturally this won't work unless the calibration of the first is reliable. It's a method for transferring calibration from one sextant to another. It will work with a Bris as easily as a Davis ...or a Tamaya ...or an Astra.
Conanicut Island USA