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    Re: Sextant Terms
    From: George Huxtable
    Date: 2007 Oct 21, 19:57 +0100

    Dan Allen asked about a matter which seems to puzzle many, justifiably. It's
    a result of inconsistent naming.
    | A sextant has an arc of 60 degrees; most (all?) sextants use the
    | double reflecting principle to measure up to an angle of 120 degrees.
    | A quintant has an arc of 72 degrees; these instruments can measure up
    | to an angle of 144 degrees.
    | It is my understanding that earlier instruments, which did not have
    | the double reflecting principle, could only measure their arc's
    | worth, so to speak.
    | So a quadrant is 90 degrees; the instrument of the same name could
    | only measure 90 degrees.
    | An octant is only 45 degrees; these could only measure 45 degrees,
    | but did not some octants use the double reflecting principle to
    | measure 90 degrees?
    Dan is absolutely right, that until double reflection came in, instruments
    could only measure a range of angles which corresponded to the geometrical
    length of their arc. So the astronomer's quadrant could measure only to 90
    degrees. similarly with the mariner's quadrant, when this was no more than a
    90-degree arc with a sight-tube attached and a plumb-bob to show the
    vertical. Other quadrants, such as the Davis, appeared, and these all had an
    arc, or two combined arcs, 90 degrees in length, to measure altitudes up to
    the vertical. No doubling.
    Then Hadley's and Godfrey's inventions appeared in 1730 (though these had
    been preceded by a double-reflecting instrument, using the same principle,
    by Newton, which had been used at sea by Halley before 1700). In all these
    double-refecting instruments the measurable angle was double the actual
    scale length. Lunar distances had not yet become a practical possibility, so
    there was no call to measure angles greater than 90 degrees, and for these
    instruments a scale-length of 45 degrees would suffice. Nevertheless, the
    Hadley instrument , measuring up to 90, was still for that reason called a
    Hadley quadrant, even though its arc was only a half-quadrant, and from
    that, much misunderstanding followed.
    When lunar distances became practicable, it became useful to have an
    instrument which could read angles up to 120, so the arc of the "Hadley" was
    expanded to subtend 60 degrees, about 1760, and it became known for that
    reason as the sextant, a sixth of a circle, though that was logically
    inconsistent with the previous naming of the "Hadley quadrant". The earlier
    instruments continued to be made and used, for the next 100 years and more,
    but the illogicality of their naming was recognised, and they became known
    not as quadrants but as octants, because the arc subtended one-eighth of a
    So to summarise; reflecting quadrants, Hadley quadrants, and "Hadleys" were
    exactly the same thing as octants, under a different name. All are
    double-reflecting. They all had an arc of 45 degrees, and all could measure
    angles up to 90 degrees.
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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