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    Re: Ted Gerrard's book
    From: George Huxtable
    Date: 2007 Nov 20, 22:49 -0000

    Coralline Algae wrote-
    
    |I had no idea what the word appulse meant.   I found a definition
    | the approach of one body to another, so I  assume?  that the phrase
    | Halleys star appulses - it something like occultations?
    
    Yes. An occultation happens when the Moon passes in front of a star, so it
    appears to extinguish it. It's an instant event, so it can be timed very
    precisely. Halley would use occultations in determining the orbit of the
    Moon That required a known fixed location, such as from his home observatory
    in Islington, a London suburb, or later from Greenwich..
    
    An appulse is a near miss, when it comes close but doesn't cover the star.
    You can use the line between the horns, and extend it, to give guidance to
    the moment when the Moon has the same ecliptic longitude as the tabulated
    value for that star. But that ignores certain correction factors, which
    Halley may have been clever enough to allow for, but he never tells us about
    that.
    
    | I see in other messages on this list that occultations were used
    | as a sort of almanac, a way to mark a point in time,  when
    | clocks were still not accurate enough to for surveying or
    | navigation.  Here is where I will shamelessy speculate that
    | almanacs of that time may have stated something like there
    | will be an occultation of a specific ecliptic star by the moon at
    | approx midnight?
    
    Yes and no. Even in modern almanacs, you will find such predictions, as well
    as those for eclipses of Sun and Moon.  But it's a bit more complicated than
    that. Because the Moon is so close, the timing of such occultations depends
    significantly on where on Earth it is observed from (due to parallax).
    
    George.
    
    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.
    
    
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