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    Re: Fall Equinox
    From: George Huxtable
    Date: 2008 Sep 21, 10:14 +0100

    Bill wrote-
    
    | Further complicating matters, IF memory serves a scholarly review
    (complete
    | with diagrams) of the the relationship between the First Point of Aries,
    the
    | celestial equator, and the ecliptic pointed out the relationship is not as
    | simplistic as I might have expected.  As usual, when memory fails I blame
    | George Huxtable for the post. 
    |
    | If once again I am mistaken about the author, would the above-mentioned
    list
    | guru please give a reference to the date and subject of the post, or re
    | post?
    
    ==================
    
    Well, my memory is no better than is Bill's, and the indexing of my musings
    on Navlist is haphazard to say the least. So I can't recall the posting that
    Bill refers to: nor can I deny its authorship, though. I'm quite accustomed
    to taking the blame for any resulting confusion...
    
    But, of the cuff, I don't see where the complication lies, that Bill refers
    to. The First Point of Aries, that quaint name (and we need a better one)
    for the origin, at the Spring Equinox, of the coordinate system we use,  is
    in the direction  where the planes of the celestial equator and the ecliptic
    intersect; it's as simple as that.
    
    But complications arise in the slightly-different ways that those planes can
    be defined. We can usefully, for some purposes, smooth out the short-term
    wobbles in the direction of the Earth's spin axis (and hence in the plane of
    the equator). It then gives rise to the Mean Equinox, rather than the true
    Equinox-of-Date. Even the plane of the ecliptic is inconstant, undergoing
    its own very-slow precession. Indeed, everything is in a state of change,
    which makes it hard to nail-down a simple coordinate system.
    
    George.
    
    contact George Huxtable, now at george@hux.me.uk
    (switched from 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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