Ask A Scientist© Astronomy Archive

Author:      m negus
What determines the present rotation speed of the Earth?  What started the
planets spinning?  Why is the Earth subject to a wobble?  Since the mass of
the Earth is changing...why not it is rotation?  Does mass/density/physical
size control rotation on axis?  Why do planets spin on their axes?

Response #:  1 of 1
Author      samuel p bowen
Lots of good questions.  Planets continue to rotate mostly because there are
very few forces that act to slow them down.  No one actually knows why the
planets got started spinning.  Most think the material that condensed was
originally spinning and the spin was not much changed during formation of
the planets.  Wobble will occur for any object that is not exactly a sphere. 
It has to do with the natural way that any solid body moves in space if it
is not spherical.  As an example try to spin a tennis racket about each of
its three axes and you will find some wobble about some axes.  The rotation
of a planet is only changed by forces that act perpendicular to the rotation
axis (actually called torques) for a planet in space there are not many of
these.  These answers are brief because of space (here).  Your questions are
good.