Ask A Scientist Environmental Earth Science Archive

 
name         Adam B.
status       student
age          20s

Question -   Does the earth's 23.45 degree tilt change over time or is it always that 
number?
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Adam -

The earth precesses as it rotates - like a top that wobbles while it spins. Little by little 
the north star will move from being directly in line with the earth's north pole.  However it 
requires many years for one period of the earth's precession.  In our lifetime the 23.45 
degrees will change only an insignificant amount.

Have you noticed that sometimes a top will flip over and lay on its side changing its axis 
suddenly by 90 degrees?  There is a 50 year old theory (with little current support) that 
this happens to the earth every 6000 years.  It is an interesting thought, but I would not 
lose sleep waiting to see it!

Larry Krengel
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The axial tilt of Earth has changed throughout history.  The Earth wobbles like a spinning top 
on its axis.  This is called precession.
The period of Earth's precession is 26,000 years.  Presently the axis of Earth points toward 
Polaris, making it the "north star."  In about 12,000 years the bright star Vega will become 
the "north star" for a few thousand years.

Steven Miller
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Adam,

The Earth's tilt is slowly changing via two mechanisms.  One is a 41,000 year cycle that 
changes the tilt angle very slightly, between perhaps 23 and 24 degrees and back over the 
period of the cycle.  The other mechanism is the wobble of the axis over a 23,000 to 26,000 
year cycle, such that the north pole is pointing toward the North star, then away from it by 
46 degrees, then back toward it over the period of the cycle.  How these two cycles interact 
determines the actual tilt angle.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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