Question

When determining the motion of the planets in the solar system, what is a good reference point to use? Explain

Answer 1

The center of the sun is a good reference point when discussing
the motions of the planets. 

When we examine the motions of the planets relative to the sun,
the nature of their elliptical orbits is clearly revealed.  We also
see the influence of the planets on other smaller bodies, like
their moons, and the comets and asteroids. 

We know that the sun is orbiting the center of the Milky Way Galaxy,
and dragging the whole solar system along with it.  But if we ignore
that, and study the motions of solar-system objects relative to the
sun, then the motions are a lot simpler, and easier to understand.

Answer 2

The sun. The planets actually move around the sun in a circumduction system, which is when their orbiting position around the sun while moving through space. The planets are continously moving at a rate concurrent with their proximity in regards to the sun as the sun's gravity pulls them while moving.