From: rowe@pender.ee.upenn.edu (Mickey Rowe)
Newsgroups: k12.ed.science
Subject: changes in the earth's climate (was  Re: Evolution)
Date: 19 Dec 92 01:40:22 GMT
Reply-To: rowe@pender.ee.upenn.edu (Mickey Rowe)
Organization: University of Pennsylvania


In article <30918.2B3162C9@puddle.fidonet.org>
     Lowell.Herr@f32.n105.z1.fidonet.org (Lowell Herr) wrote:
 
>One cannot assume that the earth was always slowing down at the rate now
> observed.  ...

and

>Just the fact that we find tropical leaves in the arctic regions is reason
> enough to support the fact that the earth is not rotating on its axis as was
> once the case.  Perhaps some reader will have some ideas what caused this
> major change.

As somebody else has already mentioned, the continental positions are
constantly changing, so just because the leaves are "down there" now
doesn't mean that they were "down there" when they grew.  However, I
want to throw in a little bit more information about what real
geologists and astronomers have to say about both the orbit and the
revolution of the earth.  The following is excerpted from a recent
post to the USENET newsgroup talk.origins:

In article <7980@tekig7.PEN.TEK.COM> alanf@tekig6.PEN.TEK.COM 
     (Alan M Feuerbacher) wrote (among other things):

}   A theory that has proved resoundingly successful in
}explaining the ice ages was first advanced about 1911 by a
}Yugoslavian professor of mathematics, Milutin Milankovitch.
}He began work on a theory "capable of describing the cli-
}mates of earth, Mars, and Venus--today and in the past." To
}describe what Milankovitch did, we first must look at the
}earth's changing relationship with the sun.  A good explana-
}tion of the changes is here paraphrased from Time-Life's
}_Ice Ages_.  Unfortunately, the accompanying chart can't be
}reproduced here.
}
}   The 100,000 year stretch: The orbit of the earth grad-
}   ually stretches from nearly circular to an elliptical
}   shape and back again in a cycle of approximately
}   100,000 years.  This is called the orbit's eccentric-
}   ity.  During the cycle, the distance between earth and
}   sun varies by as much as 11.35 million miles.
}
}   The 41,000 year tilt: The earth's axis is never per-
}   pendicular to the plane of its orbit; over the course
}   of about 41,000 years the angle varies between 21.5
}   and 24.5 degrees.  Because of the tilt, the solar
}   radiation striking any point on earth fluctuates dur-
}   ing the yearly orbit, producing seasons.  When the
}   tilt is greatest, summers are hottest, winters are
}   coldest.
}
}   The 22,000 year wobble: Even while the shape of its
}   orbit and the tilt of its axis are changing, the earth
}   wobbles slowly in space, its axis describing a circle
}   once every 22,000 years.  Because of this movement,
}   known as axial precession or the precession of the
}   equinoxes, the distance between the earth and the sun
}   in a given season slowly changes. [Windsor  Chorlton,
}   _Ice  Ages_, pp. 106-7, Time-Life Books, Alexandria,
}   VA, 1983.]
}
}   These variations were calculated mathematically by sev-
}eral workers during the latter half of the 19th century.
}Milankovitch used the existing calculations of variations in
}eccentricity, precession and tilt to calculate how much
}solar radiation strikes the surface of the earth during each
}season and at each latitude.  

The original article was much longer.  I don't want to post the whole
thing, but I'll temporarily hang on to a copy and e-mail it to anyone
who asks.  Aside from fleshing out how the Milankovitch cycles were
demonstrated, the article gives a nice explanation of how dates
obtained from radiometry and the counting of layers of lake bottoms
are consistent with each other, and thus give us confidence in the
correctness of our understanding of the earth's long history.

More to the point of this thread, the post explains how periodicities
in variations of the earth's orbit explain why ice ages have come and
gone.  Another thing to remember (although Alan's article doesn't go
into it) is that the composition of the Earth's atmosphere has changed
quite a bit through time.  Earlier in the earth's history, the carbon
dioxide content was much higher than it is now, causing a more
significant greenhouse effect and therefore higher global average
temperatures. 

}Alan Feuerbacher
}alanf@atlas.pen.tek.com

>Lowell 

Mickey Rowe     (rowe@pender.ee.upenn.edu)