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I thought a bit about this, and got confused as to why we always claim the Sun is at the centre of the solar system since relativity tells us that there is no preferred frame of reference. A person sitting on a train is as justified in claiming that he is stationary as a person "standing still" outside the train. If they agree to measure things from the Earth's F.R then, yes, the train observer is the one who is really moving.

Now who in the world agreed to measure orbits relative to the Sun and quote it as objective fact? I think circular motion about one's own axis (spin) is not relative but this is circular motion about an axis outside the body. We could make things much easier for ourselves and treat each planet as a person who has 360 degree vision (so that we can ignore the need of spinning motion in comparing relative speeds).

I haven't done general relativity yet or slightly more advanced topics so it might turn out that accelerating frames of references are objective and unique in that sense when it comes to motion. Not sure!

Captain HD
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You are broadly right, in that one can use any physical entity at all, such as Earth or Mars or the Moon or a train or a human being or a jellyfish, and set up a system of coordinates in which that physical entity is not moving relative to the coordinates. So in the frame of reference represented by such coordinates, the chosen body is not moving but other things may be. In this sense you can pick anything at all (with non-zero rest mass) and say "that thing is not moving" and you would not be absolutely wrong. But on the other hand physical insight is not just about this fact. There is also the fact that some choices of point of view are more insightful than others because they render the maths and associated physics much more simple and elegant. In the case of the solar system, it is good practice to recognize that the whole set of motions is more elegantly described by adopting the frame in which the center of mass of the solar system is moving inertially. This is not to say that other points of view are wrong, it is just that they are not useful for thinking about the orbits etc.

The frame of reference in which Earth does not move is a good one to use for other purposes, such as the study of biology on Earth. It would be rather odd (and difficult) to treat all the cells of a plant, for example, from the point of view that they are whizzing along and around all the time at the velocity of Earth relative to the Sun. Better to say they are staying still and the Sun can move.

Andrew Steane
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