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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Horizon mirrors: was [NAV-L] Question about Davis Mk 25 sextant beam converger
From: George Huxtable
Date: 2006 May 10, 11:20 +0100
From: George Huxtable
Date: 2006 May 10, 11:20 +0100
On May 03, GregR asked- "I bought a Mk 25 sextant a couple of weeks ago, and am wondering about the accuracy of the beam converger (it's a full-horizon model vs. split-mirror) " I think between us Nav-L members have resolved GregG's problems for him, but it may be worth discussing again the values of what Davis call a "beam-converger", and others call a full-horizon or full-view mirror, as opposed to the traditional half-horizon mirror, silvered across only half its width. What's brought it to mind is a thoughtful note, (together with an abstract of the Davis patent about their version) in the Spring issue (No 91) of "The Navigator's Newsletter", by its new Editor, David Burch (of Starpath). That quarterly journal, issued to members of the Foundation for the Promotion of the Art of Navigation, has been through a very lean patch, but has recently been thoroughly transformed, I'm pleased to say. A few Nav-l members pop up in that journal. For example, Bill Murdoch (a name familiar to Nav-l readers) has contributed about that Davis patent. I won't copy details of that note, except David's "nutshell summary", which goes as follows- "Full-view mirrors make the easy sights easier, and the hard sights harder." That view corresponds exactly with the reports of the "traditional" navigators, using lunar distances, in the 2001 re-enactment for the BBC of a leg of Cook's first circumnavigation. They found that in daytime a high, pale, Moon was invisible against a milky sky using a full-view mirror, but showed up clearly with a split mirror. It was a question of contrast. For anyone interested, the foundation is run by Terry Carraway, and the subscription for members in the US is (I think) $30. The website address is- www.navigationfoundation.org the address is PO box 1126, Rockville, Maryland 20850, USA. Phone or fax is 301-622-6448 navigate1@comcast.net for emails. George Huxtable. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.