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From: George Huxtable (no email)
Date: Wed May 10 2006 - 06:20:51 EDT
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
for emails.
George Huxtable.
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
5HX, UK.
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