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    Re: Fall Equinox
    From: Bill B
    Date: 2008 Sep 21, 23:09 -0400

    George
    
    You've fooled before by writing under your nom de plume, Herbert Prince, but
    not this time.   (I've the same problem with movies and books--I remember
    the content but not the title, author, or actors names.) Once again it was
    Herbert's scholarly post.  URL for the post below (Herbert's post contains a
    link to the diagram I posted).
    
    http://www.i-DEADLINK-com/lists/navigation/0603/0106.html
    
    Just a snippet to whet the interest of others:
    
    "Conceptually, equinox implies that the declination of the sun equals
    zero. Hipparchus supposed this to happen in the equinoctial point, i.e.
    at zero longitude. When it was realized after Newton that the apparent
    Sun does not necessarily have to move through the equinoctial point each
    year, one had to make a decision whether to choose zero longitude or
    zero declination for the definition of equinox. Longitude was more
    practical.
    
    How come the Sun has latitude? Hint: The latitude of the Sun is always
    very roughly (give or take 0.2") that of the Moon, divided by 32000. "
    
    ==============================================
    
    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?
    >
    George replied
    >
    > 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.
    
    
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