1st year Geo. question

["King, Norm" <nking.ucs@smtp.usi.edu>]

I'm just getting to my mail box.  118 messages.  Has anyone else noticed 
that it takes a long time just to read the mail, let alone join in the 
discussion?
I didn't respond earlier because I didn't have the numbers of the 
reference here at the office. Here's some scoop on the slowing of the 
earth's rotation.

The idea that the earth is slowing its rotation came originally from 
astronomical calculations.  However, John Wells of Cornell University 
found evidence in the fossil record to confirm it.  Wells noted that 
living corals grow in daily spurts, each one of which deposits a thin 
layer of skeletal material.  The result is that corals produce about 360 
growth lines per year (a few days didn't leave a record apparently 
because some days just weren't favorable for growth).  Seasons can be 
identified due to alternating intervals, averaging 360 lines, of thicker 
(roughly 180 of them) and thinner (also roughly 180 of them) growth 
lines.  

Wells then studied Devonian corals (370 million years old), and found 
nearly 398 growth lines per year.  Thus, there were more days per year at 
that time, so the earth was spinning faster back then.  He also found 
that 500 million years ago there were 412 days per year; 280 million 
years ago, there were 390 days per year; and 180 million years ago, there 
were 381 days per year. Make a graph of those data--one axis can be days 
per year, and the other axis can be millions of years ago.  Connect all 
of those points, and you'll get a nearly straight line that shows the 
gradual slowing of the earth's rotational speed.  A neat thing is that 
you can project that line into the future, and predict how many days per 
year there will be 100 million years from now!

Give it a try, and good luck!

Wells' article was published in 1963 in Nature, v. 197, p. 948©950. 

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Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu