Equinox

The solar radiation/orbital shadow boundary is close to splitting the
geographical axis as it passes the maximum solar diameter in its
orbital path around the central Sun -

http://upload.wikimedia.org/wikipedi...easonearth.png

Even if the longitudinal change in SR/OS boundary is more important
that the deviation of the Earth's orbital path from the central solar
diameter,this modification gets rid of the troublesome pseudo-dynamic
of variable axial tilt which requires the Earth to follow an
unorthodox path.

That is all.