Ask A Scientist Environmental Earth Science Archive

name         Angela Jo S.
status       educator
age          20s

Question -   What causes the tilt of the Earth's axis?  Is it the
result of the sun's and moon's gravity?  Why is it 23.5 degrees?
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Angela,

This is a question that has been pondered by many scientists in the
past and is to the present.  The locations and gravity of the other
planets, the Sun, and the Moon may have had some effect on the tilt
of the Earth. However, a recent theory says that the shape of the
Earth has had more of an effect on the tilt angle in the past.  The
shape may have been sufficiently altered by the massive ice sheets
that have formed during ice ages to cause a change in the tilt. At
least that is one theory.

You can demonstrate this effect by adding a small piece of bubble gum
to a ball (near its top but not at the pole itself); attempt
to spin the ball and see what effect it has on the rotation and tilt
of the ball.  As ice accumulated at the North Pole and over the Northern
Hemispheres during the ice ages, it may have changed the tilt in the
same way, but much more slowly with time.

The tilt of the Earth does precess with time now.  The tilt wobbles a
bit (just like the ball, but much more slowly), taking 23,000-26,000
years to make a complete cycle; 12,500 years from now (halfway through
this cycle), the Earth's poles will point in the opposite direction from
what they do now.  The tilt also changes from 21 degrees to 25 degrees
and back to 21 degrees over a 41,000 year cycle; we are about in the
middle of that cycle presently.


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