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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.

Geoffrey

   

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