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The following idea may be a bit daft, and if it is I hope Navlist members
will identify the snags and point them out.

What follows is an alternative way to reassemble the basic components of a
sextant in a new geometry, which will give it different characteristics from
the standard instrument. It's difficult to illustrate the notion without
making a perspective drawing (at which I am hopelessly bad), so instead I'll
try to paint a word-picture.

Start by taking a normal sextant and rebuild it. I'm not asking you to
demolish your favourite instrument; just imagine it in your head, which is
all I've done, so far.

The angling of the index mirror has to change. Instead of its plane being
perpendicular to the frame, it's now placed at exactly 45� to the frame, in
such a way that incoming light from the observed body is now reflected to
travel along the pivot line of the arm. The horizon mirror is shifted and
fixed to a bracket that places it so that the pivot line passes through its
centre, with a few centimetres spacing between the two mirrors. The horizon
mirror, too, is angled by exactly 45� to the plane of the frame (about a
vertical axis). In that way light from the observed body, travelling along
the pivot line, is reflected into the telescope, which is aligned to point
to the horizon as normal, and relocated to centralise the horizon mirror in
its view. The horizon mirror is half silvered, as normal, so the observer
sees, in the telescope, a direct view of the horizon and superimposed, a
view of the celestial object, reflected via both mirrors, just as with a
sextant.

The essential feature of the Hadley 2-mirror invention is then retained;
that after two reflections the image of the observed body will shift about
in the view exactly the same as the direct view does, so that motion of the
instrument in a seaway will not cause any relative motion between the two
views.

At the index-arm position when the two mirrors become exactly parallel, it
should be at its zero mark, and the two images of any distant object should
coincide, allowing checking of index zero-error as with the usual sextant.

However, there's another "essential" feature of the normal sextant that will
no longer apply. Normally, as the angle of the index mirror changes, the
deflection of light changes by twice that angle. But that law is only true
when all light paths are kept in the same plane. With this alternative
instrument, the light paths are in very different planes (except at the
zero-check position), such that the deflection of light becomes equal to,
not double, the angling of the mirrors. (If anyone can offer disproof,
please do so).

Horizon shades will be placed in the direct view line through the horizon
glass, as usual. Index shades will go into the space between the two
mirrors.

What would be the consequences of the new geometry? Most obvious is the
doubling of the angular motion of the arm, that's called for to achieve the
same angular range. A standard sextant frame, only 60 � wide, would only be
able to measure up to 60� of arc, not 120� as with the standard sextant, so
this would clearly be insufficient. The arc length would need to be doubled
to subtend 120�, or even (for reasons we will see) 180�. That would lead to
a large, clumsy instrument, except for the following...

Because each measured degree now corresponds to a whole degree subtended on
the engraved arc, and not half a degree as with the normal sextant, the
degree markings, and the teeth of the rack,  now become twice as far apart
as they were. Or, to put it another way, the same precision as before can
now be achieved by halving the radius of the arc. So a standard sextant with
a 60� arc and 18cm. radius could be replaced by one of 180� arc and 9 cm
radius, which would be no less compact.

There would be a few advantages, if only minor ones. Because each mirror
always reflects through 90�, there's no shrinkage in the view to a
letter-box shape, as you get with a sextant at large angles. That implies
you could use the instrument right up to 180�, wall-to-wall between
horizons. to measure dip, as long as one proviso has been met. That is, that
the sideways displacement between the two sight-lines is enough to allow
incoming light from astern to miss the ear of even the most jug-eared
navigator. That's why there has to be a certain minimum spacing between the
two mirrors.

Another minor advantage would be that there's no vertical offset between the
two view-lines, only a horizontal one instead. So instead of calling for a
distant horizon to zero-check on, to avoid parallax errors, that job could
be done with something quite close-up. Also, it means the same instrument
could be used for on-land close-up angular measurements for surveying
purposes, which presently call for a theodolite because of the parallax
error that offset causes..

I haven't yet discovered any major drawbacks to such a redesign, and hope
that Navlist members will point out any there may be. For example, is its
calibration unduly sensitive to the exactness of those 45� twists? Drawbacks
there must
be, I presume, if in all the years of sextant development, nobody has tried,
or even proposed, such an alternative construction. Perhaps someone has,
that I'm unaware of; dicovered the snags, and dropped the notion.

The era of the sextant has been and gone; now is not the moment to be
proposing a redesign, and that's not what I am doing. I am just asking the
question; could sextants have been made in quite a different way, and if
not, why not?

If no serious objections emerge to the principles of what's been suggested,
I can proceed to the practical details of how such a very-different
instrument might have been constructed, which I've been pondering on, a bit.

George.

contact George Huxtable, now at george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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