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On the subject of quintants and their history, Dan Allen wrote-

>I agree with your thinking George.
>
>I have a qunitant made by Fuji around 1900.  It has no other markings on
>it other than "Fuji #372".  The scale is actually calibrated to 150 degrees,
>slightly past the 144 degrees that qualify it for quintant status.  It is
>very similar to sextants of the period appearance-wise, and if glanced at
>quickly one would never know it is a quintant.
>
>Dan

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At that date, 1900, it seems likely that a quintant would be intended, not
for large-angle lunars, but more for horizontal angles when surveying. It
also gives a navigator a better chance of nulling-out anomalous dip by
measuring two apparent altitudes of a body: the altitude measured up from
the horizon below it, and the altitude measured up from the opposite
horizon. This would become possible for bodies up to 30 deg. altitude.

I wonder if Dan can help, as owner of a quintant. Me, I have never been
able to handle a quintant, or see one at closer quarters than in a museum
case.

As I understand it, there are two important differences between a quintant
and a sextant. The first one is pretty obvious: the arc is wider and has
been cut so as to read up to 144 deg. or more (for Dan's instrument, 150),
as opposed to the sextant which reads up to 120 deg. or perhaps a few
degrees more.

The second difference is a bit more subtle. In a sextant, when it's used
near an angle of 120 deg., the geometry of the mirrors is such that light
is being reflected from the index mirror at (almost) grazing incidence.
Because of this, the field-of view in that mirror is reduced to a very
narrow slot. In a quintant, the relative positions of the two mirrors have
been altered so that the light passing from one mirror to the other is at a
shallower angle to the horizontal, which increases the vertical aperture
angle subtended by the index mirror, when measuring larger angles.

So I ask Dan to check whether, at settings of 144 and even 150 degrees,
there is a useable slot of vision available in the index mirror. And also
whether, when set to these larger angles so as to observe bodies seen
rather behind-and-above his head, his forehead or crown gets in the way of
the light-path at all. The latter might be an important matter when making
back-observations for altitude, but presumably would not be a serious
handicap to meausuring horizontal angles.

George.

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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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