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    Re: When is the Autumnal Equinox?
    From: Geoffrey Kolbe
    Date: 2011 Sep 08, 19:23 +0100

    Paul Hirose wrote:
    >I'm not sure what Geoffrey is asking. I'm guessing he sees a
    >contradiction between the above definition of "equinox" and the
    >precession of the equinoxes.
    Not a contradiction, more a circular argument. The definition of the
    equinoctal points on the celestial sphere are those points where the
    ecliptic intersects the celestial equator. Thus the origin of the
    celestial sphere, the first point of Aries, zero degrees of celestial
    longitude, is the position of the sun on the celestial sphere as it
    crosses the equator from North to South and has zero declination.
    But, after a little digging, it would appear that the continuously
    moving feast that is the "dynamical equinox", as defined above, is
    not convenient for astronomical catalogues, so they fix an equinox at
    some epoch to create a "catalogue equinox" as the point of reference
    for the catalogue.
    I suspect that the definition which Paul quoted was a reference to a
    catalogue equinox for the Astronomical Almanac, whereas the dynamical
    equinox would be what you would lift out of the pages Nautical Almanac.

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