In article ,
Quadibloc wrote:
On Sep 19, 10:12=A0am, Sam Wormley wrote:

=A0 =A0Many definition will say the equinox occurs when the sun crosses t=
he
=A0 =A0celestial equator, but the correct definition is when the apparent
=A0 =A0longitude of the sun is 0=B0 or 180=B0.

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.

This definition is in mean orbital elements though, not osculating
orbital elements. Because of that, the center of the solar disk
doesn't cross the RA=0,Decl=0 point exactly around the March
equinox, but passes slightly beside it. Only a fraction of an arcsecond
beside it, but there's still a discrepancy. Therefore the "apparent
longitude is zero" and the "apparent declination is zero" definitions
of the moment of the March equinox will yield slightly different results.

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


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