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Name: Angela Jo S.
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: November 28, 2002


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?



Replies:
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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