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Q & A: Earth rotation

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Most recent answer: 09/27/2009
why the earth rotateÓ around itself
- essam (age 18)
There is a fundamental physical quantity called angular momentum that the earth possesses.  Angular momentum is conserved in the sense that if you don't apply any twisting moment to a spinning object it will continue to rotate forever.  When the earth was formed from its primeval constituents, they had some angular momentum relative to each other.  This angular momentum persists even today.


(published on 09/21/2009)

Follow-Up #1: Does the earth change rotation speed?

Has the angular velocity been constant since the formation of the Earth?(I feel it should be decreasing for some reason!)
- Nimish (age 17)
Mumbai, India
Yes.   There are several effects that cause changes in the earth's rotation rate, the largest of which is caused by the tidal effects of the moon.  Even so the rate of change is very small, 2.3 milliseconds per century.  That's pretty small by ordinary lifetime scales but there is geological evidence that the day was only about 22 hours long 700 million years ago.      See: 
A minor effect on the earth's rotation speed is the rearrangement of the distribution of land mass due to plate tectonic motion, ice caps at the poles, etc.  However, these are small in comparison to the moon-tide effect.  

By the way, due to conservation of angular momentum, if the earth's rotation speed slows down something else has to take up the lost angular momentum.   In the moon's case the actual orbit of the moon expands a tiny amount.  Since the angular momentum of an orbiting object is proportional to its mass times the product of its angular velocity times its radial distance squared, and the orbital period is related to the distance by Kepler's third law, the length of the lunar month and the earth-moon distance each change.
When Apollo II landed on the moon in 1969 they left some mirrors on the moon.  A laser beam shot from the earth bounces off these mirrors and returns to the earth.  Accurate timing measurements have revealed that the moon is spiraling away from Earth at a rate of 38 mm per year, in accordance with the spin down rate of the earth. 


(published on 09/27/2009)

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