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.
