On Sep 19, 10:12*am, Sam Wormley wrote:

* *Many definition will say the equinox occurs when the sun crosses the
* *celestial equator, but the correct definition is when the apparent
* *longitude of the sun is 0° or 180°.

The coordinate system in which the longitude of the Sun is zero at the
time of the Vernal Equinox, and 180 degrees at the time of the
Autumnal Equinox is *derived* from when the Sun crosses the celestial
equator.

Otherwise, precession means that there is one equinox in 1950.0
coordinates, and a different equinox in 2000.0 coordinates, and yet
another one in 2050.0 coordinates. Is that how you *really* want to
define the equinoxes?

You can buy star atlases in different coordinate systems; I did not
recall any IAU announcement of which coordinate system is currently
the "official" coordinate system for determining the equinox, but
maybe that's just me not paying attention.

John Savard