Thats right. You, me and the rest of this lovely insignificant planet is actually influencing the motion of the Sun, however minuscule.

The physics is very simple, and the guy who first presented the ideas behind this was of course Isaac Newton. In his third law, Sir Isaac postulated that if you exert a force on someone or something, he/she or it will exert exactly the same force on you, only in the opposite direction. Thats why you should always obtain a firm position before pushing someone; You get pushed back with the exact same force as that with which you pushed, and if he’s not moving, you will be! The same thing goes with the Earth and the Sun, or any other celestial body orbiting another.

When the Earth is moving around the Sun, the Sun is actually pulling the Earth towards it, because of gravity. Following Newtons third law, the Earth must therefore be pulling the Sun the same amount. Well, you say, thats not true because we found out long time ago that the planets, and therefore also the Earth, are moving around the Sun, not the other way around or something in between. But, I say, because the Sun is so much heavier than the Earth, it is much harder to drag, and therefore moves less while taking on the same amount of force (a concept known as inertia). In the animation to the left, you can actually see how the bigger (heavier) object is moving slower and less than the smaller one. Now imagine that the bigger object is a million times more massive than the smaller (like the Sun is compared to the Earth), and it will hardly move…. but you’ll be surprised. Doing a quick calculation, using the reduced mass (woooot), shows that the Sun will be displaced nearly 500 kilometers!!!

Of course all the other planets (especially the big ones) are also displacing the Sun, and the calculations are thus more complex, but you get the idea. Actually, planets around other stars are dragging their stars too, which we can observe. So contrary to what you might believe from reading articles about newfound planets, we’ve usually don’t see planets around other stars (so called “exo-planets”), but infer their presence indirectly from the motion of the star around which they revolve.