QUESTION #118

# If the term 'absolute motion' has no meaning, then why do we say that the earth moves around the sun and not vice versa?

The term 'absolute motion' has no meaning in the following sense; if you were moving in a straight line, with constant velocity, and there were no windows to see the outside, there is no way you can tell what speed you are moving at (or, for that matter, whether you are moving at all) with any measurement. Thus, speed has only meaning relative to something else.

However, the term 'absolute acceleration' _does_ have a meaning. If you were on a roller coaster, even on one which has closed cars with no windows, you would still be able to tell you were moving -- you'd be thrown every which way, and you would even be able to feel the motion in your guts, given you were securely fastened in your seat. Now, when riding a roller-coaster, you definitely know it is YOU that is moving, and not your friend standing on the ground waving to you.

Now, another thing to be pointed out is that circular (or elliptic) motion inherently has an acceleration associated with it. Now, given the magnitude of the force between the earth and the sun is equal, the earth being much lighter, accelerates much more than the sun does, so that's why we can say the earth moves around the sun, and not vice versa.

On a side note, the sun is not stationary either. If there were no other planets but the earth (they make the overall motion of the sun pretty complicated) the sun would also be rotating around the center-of-mass of the sun-earth system. But, since the sun is way massive than the earth, the center of mass would (probably, I haven't calculated it) fall inside the sun.

Similarly the same kind of situation exists between the earth and the moon -- the two rotate around the center of mass of the earth-moon system, but this point lies well within the earth, so it makes sense to say the moon is moving around the earth, and not vice versa.
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey