The equinoxes and the tip of the Earth were described beautifully by the 
Hungarian climatologist Milankovi\'{c} (btw that is an accented c at the 
end not tch as many books indicate).  

1.  On a cycle of 41 ky, the Earth's angle with the plane of the orbit changes
from 21.5o to 24.5o.  This only affects the amount of insolation that reaches
the high and mid latitudes.  It has no affect on the equinoxes.

2.  The equinoxes precess on a cycle of 21 ky.  Now the Earth is closest to
the sun during the southern hemisphere's summer, and furthest away during
the northern hemisphere summer.  The current difference is about 3 million
km.  At 10 ka, the northern hemisphere was closest to the sun during its 
summer, as it will be again in about 10 ky.

3.  The shape of the orbit changes on a 90 ky cycle.  It becomes more
oblate, then more circular, and then more oblate again.  This causes the
variation in maximum and minimum distances to the sun.  If the orbit is more
circular, the precession of the equinoxes causes less difference in insolation.
When the orbit is more oblate, the equinox precession is more noticeable. 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Bonnie Blackwell,                               bonn@qcvaxa.acc.qc.edu
Dept of Geology,                                (718) 997-3332
Queens College, City University of New York,    fax:  997-3349
Flushing, NY 11367-1597