NavList:

Hate to beat a dead horse but it's just such details as this that lead to a better understanding.

Mr Prinz points out that the equinox occurs when Ecliptic longitude is zero, not when the declination
is zero.  That definition agrees with the AA (Astronomical Almanac).

The AA defines

Equinox - either of two points on the celestial sphere at which the ecliptic intersects the celestial equator;

also the time at which the Sun passes through either of these intersection points; ie., when the apparent

longitude of the SUN is 0D or 180D.

Ecliptic - the mean plane of the Earth's orbit around the Sun.

Celestial equator - the plane perpendicular to the Celestial ephemeris pole.

Celestial Ephemeris Pole - the reference pole for nutation and polar motion. ... This pole has no

nearly-diurnal  (daily) nutation with respect to space fixed or earth fixed coordinate systems.

Declination - referenced to celestial equator

So why does the Sun have some non-zero declination at the equinox?  It seems to me that this must be
because of the difference between the ecliptic (mean plane of the earth's orbit) and the apparent position
(instantaneous position as viewed from earth) of the Sun.

Is this it?

If so, the angle between the ecliptic and the plane of the apparent Sun must be the declination of the Sun
at the equinox. Is there a name for this?  I make this to be -0.34 arc seconds for this equinox.