Question

how earth rotates?

Answer 1

http://www.windows.ucar.edu/tour/link=/the_universe/uts/earth2.html

Answer 2

It just does. There is very little friction to slow it down, but there is some slowing down due to tides and the moon - about one second every ten years. The rotation has been going on since Earth was formed, as the original angular momentum was conserved.

Answer 3

The sources I've come across indicate a much slower rate of slowing than the one given above. One gives a value of 0.005 seconds per year, another says 2 milliseconds since 1820, yet another gives an increase in the length of the day of 0.0017 seconds (or 1.7 milliseconds) for the century. See "Tidal Acceleration" in Wikipedia for a description of the process itself, with links to other sources.