[Physics] If everything is relative to each other in this universe, why do we keep the Sun to be the reference point

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and study the solar system and universe relative to it and why not relative to the Earth?

Best Answer

When you're trying to understand the mechanics of a system it's usually convenient to choose coordinates that reflect the symmetry of the system. The solar system is roughly centrally symmetric because the Sun is by far the largest mass in it, and the coordinates that reflect this symmetry are polar coordinates with the Sun at the centre.

For example in these coordinates if the Earth was the only object apart from the Sun, the Earth's orbit would be (nearly) a ellipse. The presence of the other planets (mainly Jupiter) perturbs the Earth's orbit, but we can handle this by perturbation theory starting with the elliptical orbit and adding on the perturbations caused by the other planets.

So taking the Sun as a reference point is a reflection of the symmetry of the Solar system.

As noted in other answers, if we're describing the galaxy the Sun is no longer the best place to set the origin of our coordinate system, and we'd use polar coordinates centred on the centre of symmetry of the galaxy. Likewise to describe a galaxy cluster we'd choose the origin to be the centre of mass of the cluster. At the very largest scales the universe is isotropic and homogenous, so it doesn't matter where we place the origin.

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