eli5: If every frame of reference is valid, why do people say the earth spins around the sun instead of the opposite, wouldn’t both views be equally valid?

In: Physics

Not entirely. If you are looking at the Earth and only the Earth, sure. But when you add any other planet in our solar system – the sun becomes the central entity that the Earth and that other planet both revolve around.

So if your reference is *only* the Earth and the Sun, sure, that can be a valid viewpoint. As soon as you add other planetary objects it becomes invalid.

Both points of view are equally valid in terms of yes, we can make the the numbers work. By extension you could create a model of the entire observable universe within an Earth-centric model.

The problem from a physics point of view is that there really isn’t anything special about the Earth in the universe, other than the fact that we’re here. If I wanted to understand the physics of a binary star system on the far side of the galaxy, using Earth as the basis of the mathematics would be cumbersome at best. The same problem exists within our own Solar System, if I wanted to understand the orbits of asteroids or planets, using Earth as the basis of math would be an unnecessary complication when the Sun is the gravitation heart of those equations.

By analogy, it would be like stores displaying the cost for products in CFA Francs in Oklahoma. I would have to convert my dollars to Francs and the shop keeper would have to do the reverse math, all when we live in a purely dollar economy otherwise, it’s a purely arbitrary and ridiculously artificial process that can be fraught with errors at every step, and has absolutely no benefit for anyone involved, so why do it?

If you’re only considering the sun and the earth, then for purposes of calculations and whatnot, both are equally valid.

When you start including more stuff into the mix, then things get complicated. If the earth is stationary, and it’s the sun that’s moving… then all the *other* planets in the solar system are also moving. Not only that, but the field of stars we see at night must *also* be moving… and not just moving, but moving at a tremendous, impossibly-fast speed given their distance to the earth. The only way that you can see the same starfield every night, filled with stars so far away, is if those stars are moving *faster than the speed of light*, and we know they aren’t, so…

No. Everyone’s frame of reference is valid, provided that their frame of reference doesn’t negate the truth that is scientific proof. Do our perceptions make it *look* like the sun rotates around us? Yes. But is it factual and something we can prove? No, due to other science proving it incorrect.

It’s like the old idea of respecting peoples’ opinions, but that doesn’t mean their opinion can’t be societally and culturally incorrect.

No.

Although many of the other comments are right that we can make the math work (with difficulty), that doesn’t mean all frames are equally valid.

In particular, the statement you’re talking about is usually about relativity. All velocities are relative so it doesn’t matter who you pick as “0 speed”.

But *acceleration* isn’t relative, it’s absolute. In that sense, the earth and sun reference frames are not the same. The earth is constantly accelerating (going in a circle) around the sun. We can directly measure that effect. The sun is *not* constantly going in a circle around the earth. We can force the math to model the motion that way but there’s no physics to back that up. There is no force out there causing the sun to move in a circle around the earth. There is a force causing the earth to go in a circle around the sun. Or at least a spacetime distortion that acts like a force (general relativity).

Yes.

However it’s much simpler to treat the largest mass as the central point because it translates well to discussions about the rest of our solar system.