Why does the earth (or any planet for that matter) rotate around its axis?
If it didn't rotate, a better question would be "why not?".
Any energy put into the system, such as from collisions and mergers from the planetoids that formed Earth, that didn't completely balance all other energy put into the system combined, in both magnitude and direction, would result in a spin.
If it didn't, and the Earth (or other body) were'nt perfectly spherical, then its parent object would tidally impose a spin on it.
So, something with a rotation of 0.000000 would need to do a lot more explaining to do than something with a different value.
Most of the planets and their moons, and the sun all rotate and/or orbit in the same direction.
This is a result of the slight rotation of the original cloud of dust and gas that the solar system condensed from. Even a very tiny initial rotation gets vastly amplified as the system undergoes gravitational clumping. The classic analogy is the ice skater increasing her pirouette rate as she draws in her arms.
I think I see now. Thanks.
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