how the earth rotates by itself?
Momentum imparted to it million of years ago when the solar system formed.
One theory involves glancing collision with a large asteroid millions of years ago that also broke off material that eventually developed into the moon.
I hope this helps a little more. Thanks for asking.
the moon causing colision altered the spin both in velocity and angle giving us our 23deg. tilt among other things. the Moon is as a brake on rotation. Since its formation the rotation of the earth has slowed from 8 to 24 horus per rotation
I just bought the newest book on the moon subject, "The Big Splat" by Dana Mackenzie ISBN 0-471-51057-6
sun's gravity pull