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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Old Sextant on German money
From: George Huxtable
Date: 2007 Mar 10, 17:32 -0000

```Frank's brilliant bit of sleuthing to find those pictures of the Gauss
instrument, and Wolfgang's description of it, are enabling us to put
two and two together. It seems that Alex's description of what he
could make out from the banknote were correct, and my own comments on
the digital image were mostly wrong.

So how did the thing work, then? I will offer a guess, for what it's
worth.

Wolfgang wrote-

"As far as I understand the description the Vize-Heliotrop is a
sextant
equipped with an additional mirror which reflects the image of the sun
to a
distant point. It is not used for measuring then but for marking the
position of the observer for another observer at the distant point."

It seems to me, then, that it's intended to solve the same problem
that faces distressed mariners, who are advised to use a mirror to
flash reflected sunlight at a possible rescuer, in an attempt to
attract attention. But how does he decide to angle the mirror, to
ensure that the light reaches its target? Gauss's instrument solves
that same problem, scientifically.

To avoid confusion, let's call the man holding the instrument the
observer, and the man at the distant point (another observer) the
target, at whom the reflected sunlight is going to be aimed.

To do its job, the normal to the mirror (that is, the line drawn
perpendicular to its plane) must be in the plane containing the
observer, the Sun, and the target, and it must bisect the angle
between Sun and target, as seen from the observer. For that purpose,
the mirror is added to a sextant, as a piggy-back on the index mirror,
fixed to it, but twisted a bit in angle, as Frank's photographs
clearly show. As I see it, it has to be carefully mounted to be
perpendicular to the frame of the sextant, just like the index mirror
is. But that added mirror plays no part in the the observer's
proceedings; he simply aligns the sextant to look at the target point
via the horizon mirror, and the Sun via the (shaded) index mirror, as
though he was measuring the angle between them. There's no call to
record that angle, however.

Now the (filtered) light from the Sun passes, via the normal index
mirror and the horizon mirror, into the telescope. Similarly, unshaded
sunlight strikes the added mirror, with full brilliance, and is
reflected along a different path; still in the plane of the sextant
frame, but angled differently, depending on the offset angle between
the index mirror and the added mirror. What value should be chosen for
that offset angle ? As I see it, if it's made exactly equal to the
(fixed) tilt angle of the normal to the horizon mirror, with respect
to the telescope axis, then the light will be reflected from the added
mirror, not toward the horizon mirror, but directly toward the target.
In that way, it will fulfil the requirement of sending a flash of
sunlight to the second observer at the distant target, and show him
the position of the first observer, with his Gauss instrument.

It may be worthwhile making the added mirror less than perfectly flat;
perhaps very slightly curved or rippled, so as to diffuse the
reflection a bit and relax the need for precise alignment.

Does all that make any sense? Have I got it right? It does nothing to
explain the curious mechanism that's shown by the horizon mirror,
however.

George.

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.

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