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    Re: Jupiter's moons
    From: George Huxtable
    Date: 2002 Mar 21, 00:51 +0000

    >Herbert wrote magnification do/did
    >these sighting pieces have?>
    >I am afraid I have not, in UK decent old sextants have been priced out of
    >navigators' markets by being bought up by chi-chi antique dealers to sell
    >to people to hang on a wall.  My own 'good' sextant is a modern Zeiss but
    >with only one eyepiece.
    >I looked in my copies of Nories Epitome (one 1805 and the other 1896) which
    >give good, long and lucid descriptions of sextants. Both descriptions say
    >that a sextant is normally equipped with two magnifying eyepieces of
    >different strengths but neither sadly give any indication of what the
    >stengths are.
    George Huxtable comments-
    Herbert Prinz' question was more relevant than Nigel Gardner's reply. I am
    not aware of any observer who has claimed to see a moon of Jupiter from on
    board a vessel at sea, using any sort of telescope, sextant-mounted or not.
    The magnification required is incompatible with the motion of the vessel.
    The notion was (reluctantly) exploded by Maskelyne in voyages to St Helena
    in 1761 and to Barbados in 1764. I don't think anyone has been more
    successful since.
    Satellites of Jupiter can not be seen in a telescope unless it is firmly
    rooted ashore. When this could be done, timing of occultations was a valid,
    accurate, and widely-used method of obtaining longitudes both inland and at
    landfalls. When the boundaries of France were remapped by this technique,
    the King complained that he had lost more of his kingdom to his astronomers
    than he had ever lost to his enemies.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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