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Milankovich Cycles

Milankovich cycles are changes in the orbital dynamics of the earth. 
There are three main cycles varying in periodicity of 100,000 ; 40, 00 and
20,000 year periods.  They DO play a role in forcing the earths climate
between Ice Ages and Greenhouses.

1) 100,000 Year changes in the eccentricity of the earths orbit. Though the
orbit is elliptical, the degree of ellipticity can vary to nearly circular.
This variation can result in seasonal extremes of hot or cold and can also
result in little seasonal change throughout the orbit.

2) 40,000 Year change in the earths precession. Basically, this is a
variation of the distance between the earth and the sun which can increase or
decrease the net Solar flux falling on the earth thereby heating or cooling
the planet. 

3) 20,000 Year variation in the EARTHS TILT (This is the one asked about).
The earths axial tilt is presently 23.5 degrees or so but  varies between 22
degrees and almost 24 degrees. 

Logically, there can be times when all three of these cycles are in phase and
will enhance further a particular climatic extreme or act to cancel out
effecst when 180 degrees out of phase.

I have read of larger M-cycles from 400,000 years to over 1,000,000 years as
well but cant loctate my source (Science I believe). Further there is a 26-33
Million year cyclicity in the orbit of our SOLAR SYSTEM whereby it oscillates
above and below the   galactic plane
and it was Rampino and Stothers who postulated this phenomenon as the cause
for a periodic shower of asteroids/comets bombarding the earth and causing
the several mass extinctions of the Phanerozoic. Finally, there is the ~250
Million Year orbit of the solar system aroud the galaxy, that's right kids, 1
Solar System Year equals 250 Million terrestrial years! 

With so many cyclic phenomena having a DIRECT influence on the birth,
evolution and death  (extinction) of life on earth it should be painfully
obvious by now that the earth is not an ISOLATED system of detached
evolutionary events. The concept of the death of the dinosaurs via an
extraterrestrial mechanism makes alot of sense (an is slowly being proven).

More fuel for a couple fires ;-)

                        Thomas R. Lipka
                        Paleontological/Geological Studies