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    Re: Fix by Occultations
    From: George Huxtable
    Date: 2009 Feb 5, 21:06 -0000

    Bruce Hamilton wrote-
    
    "I have  attempted to see the occultations of Jupiter's moons, but that is a
    long term method. I don't mind watching the moons of Jupiter for long
    periods of time, but it's a bit of a guessing game. Did Io disappear or did
    I blink? Whereas the moon occulting a star seems to be a much more definite
    event."
    
    I don't understand Bruce's problem here. Yes, the Moon occulting a star is a
    more "definite" event in that it happens instantaneously, whereas Io,
    disappearing into Jupiter's shadow, takes a few seconds to extinguish. And
    the better the light collection from your telescope, the later will be your
    timing of its final twinkle. But what does "Did Io disappear or did I
    blink?" imply? If Io returned, then he blinked. If it didn't, then it
    immersed. It isn't necessary to watch Jupiter for a long time,  unless he is
    calibrating a clock with a large unknown error, because the immersions are
    closely predicted. Why does he call it "a long-term method?" True, there can
    be long gaps between immersions, especially near Sun conjunction when
    Jupiter is invisible for more than a month. But over much of the year
    several such timeable Jupiter events occur each week.
    
    The big problem about the Jupiter satellites is that they can't be seen from
    on board ship, because you need quite a lot of magnification, necessitating
    some sort of tripod. But with a simple tripod stand, and preferably an
    equatorial mount, then a cheapo hobby reflector, say 3-inch with
    magnification of 50, is more than adequate.
    
    In [7265], Bruce asked- "Has anyone had any luck putting the concept into
    practice?"
    
    That was how the longitudes of France were mapped, by surveyors setting
    their watches from Jupiter satellite events, long before the days when such
    events were predictable in time. What surveyors then had to do was to
    compare, after the event, with the local time the same event had been
    observed at the Paris observatory. Those were the observations that
    radically reduced the perceived size of France, causing King Louis XIV to
    complain that he had lost more territory to his astronomers than he ever had
    lost to his enemies.
    
    It later became a routine method for establishing longitudes of harbours and
    headlands worldwide, to put them on accurate charts.
    
    The book, "Spherical and Practical Astronomy", that Bruce described as "an
    amazing book", is what others may know better by the name of its author,
    Chauvenet. It is, indeed, amazing.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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