There is a supermassive black hole in the center of our galaxy, which is lying dormant. Each star in our galaxy is affected by the pull of this black hole, which in turn affect the rotation of the planets. This is my theory, it could be wrong but i don't think it is. I just thought I would share.
By the way, in the next three million years our neighboring galaxy Andromeda will most likely collide with our galaxy. Hmmm...Isn't that a wonderful end to mankind?