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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: Sextant precision
From: George Huxtable
Date: 2004 Sep 30, 20:12 +0100

```Alex Eremenko points out that you could use an artificial horizon on a ship
beset in ice. That's true; I hadn't thought of it. Important for polar
explorers, and for equipment on Russian (and perhaps Canadian) vessels.
Nansen could have used that technique on "Fram" in his "Farthest North"
voyage, until his mercury froze. He doesn't say much (not anything, really,
that I can find) about how he determined the position of Fram, though at
one point he mentions carrying theodolites.

=============

Robert Gainer writes-

>One of my sextants was made by Brandis & Sons and has an arc with a radius
>of 5-1/2 inches to the graduations on the limb. The limb has a silver insert
>graduated in 20-minute increments from ?5 degrees to 185 degrees. The
>vernier is graduated in 30 second divisions from ?30 seconds to 20 minuets
>30 seconds. The largest reading you can make is 162 degrees before you hit
>the index arm stop. I can read it to a precision of 15 seconds of arc by
>estimating the vernier. I have not used this instrument in 25 years and did
>not know how to measure the accuracy at the time. I pulled it off the self
>yesterday and if time permits I will test it this Sunday.

===============

That sounds like an interesting and unusual instrument.

I don't think we need to take too seriously the graduation up to 185
degrees, at which point the observer would be trying to look back through
his own forehead. Especially as the index arm hits the stop at 162 degrees.

But I wonder if this instrument qualifies to be a "quintant", rather than a
sextant. On a quintant, the working scale subtends a fifth of a circle, or
72 degrees, being graduated from 0 to at least 144 degrees.

Mostly, quintants were made for marine surveying, for measuring horizontal
sextant angles. Not much advantage over a sextant for lunar distances,
which as far as I know were only ever tabulated out to 120 degrees.

Lecky, in his "Wrinkles" was a strong advocate of the use of the quintant
by mariners; I think because, being designed for surveyor's use, they were
"the best" in all respects.

But I wonder whether Robert's "Brandis" was actually useable over its full
range to 162 degrees. Two things happened at large angles:

1. The available slot in the vertical field-of-view would shrink until at
some angle, at which the light was tangential to the index mirror, the
letter-box shape of the field of view would shrink to zero vertical
aperture.

2. The top of the observer's head or forehead would get in the way and
block the field of view.

So when Robert tests out this unusual instrument, it would be interesting
to know how far it's practically useable, in terms of its maximum angle.

George.

================================================================
contact George Huxtable by email at george---.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

```
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