I do not know the validity of what Cartes du Ciel is showing you, but it is
true that the Sun does not necessarily pass through the equinox (a direction
defined by the intersection of the plane of the ecliptic and the plane of
the Earth's true equator).

The Sun does not follow the path of the ecliptic exactly because of the way
the latter is defined. My understanding is that the ecliptic is determined
from the mean motion of the angular momentum vector of the Earth-Moon system
relative to the Solar System barycentre; the ecliptic plane is then
perpendicular to this vector. Some may qualify this ecliptic as "mean" to
distinguish it from the "true" ecliptic, which is the instantaneous
ecliptic, but in practice the former meaning is usually assumed. What
distinguishes the mean ecliptic from the true ecliptic is the absence of
short-term perturbations from other Solar System bodies.

Notice how this definition does not involve the motion of the Sun as seen
from the Earth, so it is not surprising to discover that the true path of
the Sun will deviate from the ecliptic thus defined. What does the time of
the equinox actually mean, then? For the vernal equinox it is the instant
when the apparent geocentric ecliptic longitude of the Sun is zero, apparent
meaning light-time delayed and aberrated position relative to the
true-of-date equatorial frame. At this time the RA and DEC are not
necessarily zero.

Using DE421 planetary ephemeris and IAU-2006 earth-orientation model, I find
the vernal equinox occurs at 11:43:37.6 UT1 (with a delta-T of 65.9
seconds). The ecliptic latitude at this time turns out to be -0.451
arcseconds, ie not zero!

Also, the times at which the Sun reaches zero RA and zero DEC are 11:43:33.0
and 11:44:02.9, respectively, which do not agree well with the results you
got from Cartes du Ciel. You will need to ascertain whether Cartes du Ciel
is using the correct reference frame and centre of observation: the
true-of-date equatorial frame and the geocentre, respectively. Maybe Cartes
du Ciel is showing the results for a topocentric location and/or for the
J2000 frame. In this case the position of the Sun could be even further
displaced from the ecliptic.


"Paul Forsdick" <pf006g3785@blueyonder.co.uk> wrote in message