When we measure the motion of objects in the universe, we're usually measuring the 
motion of an object's center of mass – the location where the combined 
gravitational forces between all the parts of the object appear to come from.  In 
a planetary system, the motion of the center of mass can be significant and 
possibly measurable.

Jupiter orbits the Sun at a distance of 5.2 AU.  Jupiter's mass is 1.9 * 10^27 kg 
while the Sun's mass is 2.0 * 10^30 kg.  Imagine an astronomer living on a planet
10 parsecs away from the Sun, observing our solar system.

(a) At the time Jupiter is on the far side of the Sun, where is the center of mass 
of the Sun-Jupiter system, compared to the actual center of the Sun?

(b) At the time Jupiter is on the near side of the Sun, where is the center of 
mass of the Sun-Jupiter system, compared to the actual center of the Sun?

(c) What is the total distance that the Sun-Jupiter center of mass travels back 
and forth, as observed by the distant astronomer?