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    Re: Halley's lunar knowledge.
    From: George Huxtable
    Date: 2007 Nov 27, 09:46 -0000

    I am still having some difficulty in understanding what Mike Daly is telling
    us..
    
    I had pointed to predictions of the Moon's position, and particularly to the
    "Connaissance du Temps", available in Halley's time, and asked-
    
    | > Are those what Daly appears now to dismiss as "raw data"?
    
    And he has replied-
    
    | Of course they are raw data.  Halley did not go into his friendly,
    | neighbourhood navigator's supply shop and buy a printed copy of the
    | lunar information telling him when a particular star would be occulted
    | _in_the_future_ by the moon.  He had data that specified the position
    | information on the moon - in the past.  That's raw data.  He has to use
    | it along with a model of lunar motion to produce the information he needs.
    
    Well, that's exactly what he could do, back in 1698. He could buy a copy of
    Connaissance du Temps, for 1699, at Stationers' Hall, in London, which would
    provide him with the position of the Moon in the sky, in ecliptic latitude
    and longitude. I've little doubt that he did. I don't have a copy available
    to tell me whether it would supply occultations or not (they are rather
    local phenomena, which would have applied to Paris, not to mid-Atlantic, in
    any case). But Halley required close appulses, not occultations, as I have
    explained before. He was perfectly capable of deducing those from
    predictions of the Moon coordinates and his own detailed knowledge of star
    positions around the ecliptic. He had been doing that, routinely, for years.
    
    But Mike Daly seems to be insisting that only information on past positions
    of the Moon was available in Halley's time, and not predictions into the
    future (I have tried to discover some alternative meaning, without success,
    in his words quoted above, in case I'm accused once again of  "misquoting or
    convoluting" his meaning). If that is what he is claiming, it's nonsense.
    These, in "Connaissance" and elsewhere, were predictions, not history. It's
    perfectly true, as I've said before, that those lunar predictions in 1699
    were not up to the precision of Mayer's, in Maskelyne's 1767 almanac. They
    wouldn't have qualified for the longitude prize. There's little doubt that
    Halley, from his own observations, could do better. But Daly is telling us
    (isn't he?) that no such predictions existed.
    
    If, as it seems, he is describing those predictions in "Connaissance" as
    "raw data", that's simply abuse of language. Astronomer's real "raw data" of
    Moon observations had been collected for many years, sifted and assessed,
    fitted to the best model they could think of, and then that model was run to
    produce future predictions for coming years. At least, that's how we would
    describe their process in modern terms. "Raw data"?
    
    Halley's proposal was to bypass much of that process by examining data from
    one Saros cycle previous, if that detailed data had been available. It had
    some strengths, and some weaknesses, as I have discussed. It was superseded
    by better mathematicians than was Halley; Clairaut and then Mayer, who had
    Newton's understanding of the dynamics to build on.
    
    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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