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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!

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  • $\begingroup$ The Sun moves around the center of mass of the Solar System just like the planets do. It just moves in a much smaller orbit because it is much more massive. $\endgroup$
    – G. Smith
    Commented Dec 9, 2020 at 21:53
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    $\begingroup$ Saying the planets orbit the Sun has little to do with preferred frames of reference, and everything to do with observation. $\endgroup$
    – Jon Custer
    Commented Dec 9, 2020 at 21:59
  • $\begingroup$ No one who understands relativity will say a heliocentric model is the only objective way to do things....it's just magnitudes easier to describe everything from the solar systems central star than from say one of the planets. And of course as G. Smith noted the center of the solar system isn't even at the center of the sun anyways. $\endgroup$
    – Triatticus
    Commented Dec 9, 2020 at 22:03
  • $\begingroup$ Hi @BowlOfRed . That link seems to have a bunch of answers that are longer and less straight-to-the-point than the one Andrew gave. In any case, I was satisfied by his answer and this can be closed thanks. $\endgroup$
    – Captain HD
    Commented Dec 9, 2020 at 22:58
  • $\begingroup$ @G.Smith I already knew that but my question was related to relativity rather than the exact spot of the Sun from it's... actually from our frame of reference lol. If it was from the F.R of the Sun then it would actually not orbit the centre of mass. Hmmm I don't want to get a headache and imagine what that would mean from the F.R of the Earth so let's just assume it (the Sun) really isn't orbiting a centre of mass. $\endgroup$
    – Captain HD
    Commented Dec 9, 2020 at 23:03

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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.

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  • $\begingroup$ This is a beautiful response! I wouldn't have said "not absolutely wrong", though. This is because it really is not wrong, just much easier to deal with mathematically. It would seem might teachers and the entirety of the internet has been too strong on their statements to such a degree that I had been given the impression that everyone believes the Sun is in a stationary frame of reference with respect to all the planets. This was actually so bad that I held this belief only up until today whilst thinking about relativity. It didn't even occur to me after learning basic relativity! $\endgroup$
    – Captain HD
    Commented Dec 9, 2020 at 22:54
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    $\begingroup$ Before GPS was invented, all celestial navigation of ships and aircraft was done "assuming" that the earth was stationary and the sun and stars moved around it once per day. Navigators didn't care where celestial objects "really" were in space, they only wanted to know which direction to look, to see them from the surface of the earth.. $\endgroup$
    – alephzero
    Commented Dec 10, 2020 at 2:39

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