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    Re: When is the Autumnal Equinox?
    From: Paul Hirose
    Date: 2011 Sep 11, 15:32 -0700

    I wrote:
    > The notion of the ecliptic as the apparent path of
    > the Sun must be abandoned in faver of the mean plane of the orbit of the
    > Earth - Moon barycenter (center of gravity) about the solar system
    > barycenter.
    
    More precisely, a committee of the International Astronomical Union
    recommended this definition for the pole of the ecliptic: "the mean
    orbital angular momentum vector of the Earth-Moon barycenter in an
    inertial reference frame." That was adopted in 2006. Their paper
    discusses some of the issues with defining an ecliptic and equinox.
    http://syrte.obspm.fr/journees2005/s2_02_Hilton.pdf
    
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