# From what frame of reference does the Earth orbit the sun?

## Main Question or Discussion Point

I have been doing some study on the theory of relativity and it has got me thinking. We say that the Earth revolves around the Sun but from what reference frame? From the reference frame of Earch, we are stationery and the Sun simply spins. Why is any reference frame more accurate than another? Why can't we say that the Earth is stationery and the Sun simply rotates? I also ask this in regards to the Earth and Moon.
Any help would be appreciated.
Thanks.

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Why can't we say that the Earth is stationery and the Sun simply rotates?
Revolves.

We can. But it is difficult to use such a coordinate system. For example, there is an enormous fictitious force whirling the sun around the earth. It is far better to pick a coordinate system where the motion looks simple, and the sun-centered one is that system.

For example, there is an enormous fictitious force whirling the sun around the earth
I don't see why we need to adopt some enormous fictitious force, we should be able to use true forces since Einstien said that the laws of physics will remain constant from all reference frames.

Revolves.

We can. But it is difficult to use such a coordinate system. For example, there is an enormous fictitious force whirling the sun around the earth. It is far better to pick a coordinate system where the motion looks simple, and the sun-centered one is that system.
Yes, it is a question of classical mechanics. Newton's mechanics is approximately valid for inertial reference systems, wrt which motion makes sense physically (no assumption of fictitious forces). As a bonus the equations are simpler as well.

Special relativity relates to those reference systems of classical mechanics, see the intro of:
http://www.fourmilab.ch/etexts/einstein/specrel/www/

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In space-time there is no motion ... there are only static worldines .

russ_watters
Mentor
I don't see why we need to adopt some enormous fictitious force, we should be able to use true forces since Einstien said that the laws of physics will remain constant from all reference frames.
Yes, he did - but one of those laws is f=ma and so since the "m" of the sun is much larger than the "m" of the earth, it takes a much bigger "f" to give it the same "a" to make it travel in a circle around the earth. So you have to give it an extra force for which there is no cause - a fictitious force.

Acceleration can be thought of as a known velocity. It is not like an object traveling at a constant speed where it can say it is at rest or traveling at a constant speed at the same time. When under acceleration the person accelerating can prove that they are the one that is actually accelerating, so then it being in motion takes priority over it not being in motion. Still, I think it is strange that we don't feel any forces of acceleration while sitting on Earth. So, if you said that an object under acceleration was actually at rest then there would be a fictitious force that acted on all objects that are at "rest".

Dale
Mentor
Acceleration can be thought of as a known velocity.
No, the units are different.

When under acceleration the person accelerating can prove that they are the one that is actually accelerating
Yes.

Still, I think it is strange that we don't feel any forces of acceleration while sitting on Earth.
We do feel forces of acceleration while sitting on Earth. Take an accelerometer and you will see that we feel an upwards acceleration of magnitude g.

So, if you said that an object under acceleration was actually at rest then there would be a fictitious force that acted on all objects that are at "rest".
Yes, that fictitious force is gravity.

D H
Staff Emeritus