from a woodworking newsgroup: Making a backstaff [navigation device]
 

The message below was posted into the
alt.binaries.pictures.woodworking newsgroup. I thought some folks in
this newsgroup might appreciate the information presented by the
author of the content and wish to check out his (I assume) data
provided there. Including the attached image mentioned below (which I
did not retransmit here), the author presents about ten images in
total.


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From: Saville
Subject: Making a backstaff - Intro
Newsgroups: alt.binaries.pictures.woodworking
Date: Sat, 07 Jan 2006 09:43:29 -0500
Message-ID:


I'm in the process of making two backstaffs for a museum in Denmark.

Backstaffs is a device for measuring angles. But the Backstaff
represents a quantum leap in accuracy, user safety, and technology
over the Cross-Staff, or Quadrant. It is not only easier to use; the
Backstaff saved the eyesight of the navigator because he did not have
to look directly at the sun to get a sun sight.

The major technological improvement of the Backstaff was that it
allowed sun sights without the user looking directly at the sun, but,
rather, a shadow cast from the shadow vane blocking the suns rays.
That shadow is lined up with the horizon, so there is no parallax
error. Also, the Backstaff could be used at all latitudes.
Additionally, the backstaff used the diagonal scale for precise angle
measurement.

At noon, the altitude of the sun was taken and this was used to
determine your latitude.

The attached image shows how one used it. Subsequent posts will show
some of the process of building them.

Comments and criticisms appreciated.