Subject: If motion is relative, why is solar system considered heliocentric?

I have heard that all motion is relative, and that any point can be the 
stationary point that things move around. For example, if a car drives by a 
tree, you could validly assign the car the stationary position, and the road 
and the tree as moving.

If that is the case, how can we say that our solar system is definitely 
heliocentric? Isn't that just one way of describing the motion, and couldn't 
one choose earth, or any other point in the solar system, as the stationary 
point which the sun and other plantets move in realtion to?

If all this is correct, why is it so much more convienient to describe the 
motion of the planets and stars if we assume a heliocentric model?

I ask because I've been reading about the history of the heliocentric system in 
the popular science books "Foucault's Pendulum" and "Heavenly Intrigue" and 
can't see why the heliocentric system is 'correct' rather than just a 
convienient system of describing planetary motion.

Thanks for your consideration.

Re: If motion is relative, why is solar system considered heliocentric?