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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
    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.
    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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