Re: why do the rotation speeds variey?

Hello Seth.

This is a good question. We can readily see that the objects in our solar 
system rotate. In fact we would be hard pressed to find an object that does 
not rotate. Rotation rates vary throughout the universe. Pulsars (spinning 
neutron stars) can have rotation rates as fast as 1/1000 of a second. The 
Milky Galaxy rotates once every 200 million years from the earth's point of 
view. 

Before we answer your question about why rotation rates vary, we should look 
at why objects rotate in the first place. There are probably several reasons 
why objects rotate. The most likely cause is gravity. As materials begin to 
collect due to gravity, the uneven distribution of mass causes the object to 
rotate. Let us take a look at our solar system. The nebula that our solar 
system formed out of would not have an even distribution of mater. As it 
collapsed due to gravity, it began to rotate and clump together. As the 
clumps gathered more dust and gas they would have had the same spin as all 
the other clumps. As a result of this spin each object had something called 
angular momentum. Momentum is the product of a body's velocity and its 
mass. Angular momentum is the product of a body's rotation and the 
distribution of its mass. The best example of angular momentum is with a 
figure skater. When a skater is spinning, they have angular momentum. If 
they change the position of their arms the distribution of their mass 
changes and so does their rate of spin. Angular momentum must be conserved. 
When the distribution of mass changes the rate of spin must also change to 
conserve angular momentum.
 
Mass is not distributed evenly in a planet. The heavier elements tended 
early on to settle in the core and the lighter material towards the surface. 
This probably explains why the smaller planets tend to rotate slower than 
the larger planets. This does not explain some of the exceptions to the 
trend however. Why do Venus and Uranus rotate backward? These are puzzles 
that we have not yet fully answered. We have some ideas; the most common one 
being that Venus and Uranus experienced a collision early in their 
formations altered their original motion.
 
There are probably other factors as well. This is just a basic answer. You 
should probable go see the physics teacher at your school to help explain 
this to you, or better yet take physics as a senior and find the answers for 
yourself.