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    Re: Introduction
    From: George Huxtable
    Date: 2008 May 2, 09:29 +0100

    First, I would like to add my own welcome to Bruce Hamilton, who clearly
    knows what he's talking about, and asks several intelligent questions.
    
    He has already had some useful responses from other list members, and I have
    a few bits to add.
    
    He asked-
    | 2) It is supposed to be possible to get exact GMT by observing the moons
    | of Jupiter, but I have yet to find any tables that have more than a
    | minute accuracy.  Has anyone else tried and had better luck?
    
    Herbert Prinz has pointed to the sources of precise timings of Jupiter
    satellite events, from Paris, where they have long specialised in such
    matters, from the Cassini days in the late 17th century. In those days, the
    French were the experts in remapping longitudes, on land, using Jupiter
    satellite events, and the French king complained that he had lost far more
    of his kingdom to his geographers than he had ever lost to his enemies.
    
    The best events to use for timing are when the inner satellites disappear
    into Jupiter's shadow, or behind the planet's edge. There is a slight
    difficulty, though. Because of the finite size of the satellites, and the
    penumbra of the shadow-edge, the light doesn't switch off instantaneously
    (as a star does when occulted by the Moon), but dims over several seconds.
    You time the moment it extinguishes, but the better the light-collection of
    your telescope, the longer you will be able to see the dwindling speck of
    light. In an observatory, the moment of extinction, as seen in their big
    telescopes, would be what was recorded, and compared, later, with
    geographers' observations, but there needed to be some correction for the
    size of the field-telescope that was used. I don't know how that was done,
    in practice, and if anyone has information, I would be interested to find
    out.
    
    Modern predictions give timings that are defined by the geometry of the
    situation, as explained in the French ephemeris that Herbert refers to.
    
    And he asked-
    | 1)Sextants are expensive precise instruments, but they are still made of
    | brass and aluminum. Why does no one use invar to make them more
    | resistant to thermal expansion?
    
    A fair question, but I doubt if thermal expansion of the frame and index arm
    creates any real problem. As long as they are made of the same material, and
    it's a good conductor of heat, then they expand in the same way, the same in
    all directions, and so the measured angles remain unchanged. Much more
    sensitive to heat are the mountings and adjustments of the two mirrors,
    which can be affected by sunlight striking one side of the mount, while the
    other stays in shadow. That can cause a change in index error, which can
    always be checked and allowed for. There might well be a case for using
    invar for those small components.
    
    Bruce intrigued me with his mention of " It was on the bridge of a French
    bulk carrier that we were loading directly into, from our self-unloader, in
    Sept Iles bay."
    
    I haven't sailed in that area for 10 years or so, but I remenber it as being
    a rather deserted spot, and a useful small-craft anchorage, so that's a bit
    of a surprise. I can see it as providing useful shelter for trans-shipment.
    I wonder what cargo was being transferred? The local industry, in that part
    of Brittany, was collecting seaweed for fertiliser. And fishing, of course.
    And growing onions.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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