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Re. Milankovich Cycles



<Snip>

>Although the three main cycles certainly play a role in forcing >climate 
>during ice ages and greenhouses, other factors, such as continental 
>positions and the uplift of major plateaus are probably more >important 
>in determining larger scale ice age to greenhouse transitions.

Agreed. I never intended to imply that the astronomical effects were the
factors influencing the weather/biota but that they _also_ contribute to the
net
effect . Geological forces were discussed on previous posts.


>>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.

>NO!  Precession is not variation in the the Earth-Sun distance.  
>Precession is variation in the direction that the axis of rotation 
>points due to the "wobble" of the axis.  It affects the seasonal
>timing of the aphelion and perihelion.  Because of this it, it affects
>the seasonal balance of radiation.  The radiational effects of >precession 
>and tilt are closely linked.


Yes and the seasonal timing of aphelion and perihelion _distances_ are
affected. A change in the orbital period caused by precession changes the
solistices and the distance from the sun where the new solistice occurs.
Obliquity affects the annual radiation  distribution falling on both
hemispheres and the intensity of their respective seasons. Though I 'll admit
your explanation was better,  the distances at the solistices are affected
and thus so are the earth-sun distances in general. 
>
>>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.

>Actually between about 21.5 and 24.5 degrees.  As I noted above >the 
>tilt and precession signals are closely linked because the variation
>in the direction the axis of rotation points is a function of both
>factors.

I was being general. No dispute there :-)



>>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).
>

>I am not sure that you have shown a direct influence of these 
>extraterrestrial cycles on biota.  Certainly, some of the cycles 
>have influence through agents such as sea-level and climate.

You're right. Sorry about that! I guess I went off on a tangent and started
to paint with a wider brush than I actually had.  The effects of these and
othe cyclic phenomena of course need to be studied further. I feel they do
exert an influence. I guess the question is;  How Much? and How often?



                                               Regards,
                                               Thomas R. Lipka
                                               Paleontological/Geological
Studies
                                               

I have thekeyboard form Hell!