If the term 'absolute motion' has no meaning, then why do we say that the earth moves around the sun and not vice
Asked by: Akhilesh
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