# The sun orbits the earth.

1. Oct 13, 2005

### Smurf

Now, the conventional way of viewing the solar system is that all the planets orbit the sun, but since you can declare anything at rest in space, can't you just as easily say that the sun is orbiting the earth, since it will appear that way if you hold the earth at rest?

2. Oct 13, 2005

### ZapperZ

Staff Emeritus
Hint: you can detect when you're accelerating without having to use another frame as a reference, but you cannot detect that you are either stationary or moving with a constant velocity without using another frame as a reference. Now look again at the scenario you have just described and see if you can find the answer here.

Zz.

3. Oct 13, 2005

### Smurf

My first thought is wether "acceleration" is the same as a change in velocity. But i'm not sure if circular motion counts as a change in velocity.

4. Oct 13, 2005

### ZapperZ

Staff Emeritus
Why not? A "velocity" is a vector. If it changes direction even if it maintains its speed, that is a change in velocity.

Furthermore, things just don't change direction on its own. Remember Newton's First Law. A circular motion requires the presence of a central FORCE. And when you have a net force acting on a body, that body undergoes an acceleration. An "acceleration" is, by definition, a rate of CHANGE of velocity.

Things a very consistent here.

Zz.

5. Oct 13, 2005

### Astronuc

Staff Emeritus
Smurf - someone let you out of GD?

I think you have spent way to much time there based on your OP.

6. Oct 13, 2005

### Smurf

Hey! I have over 2000 posts, and you don't get posts in GD!

I do read the other forums (well, the math ones at least).. I just never have something constructive to add so I don't post there. Isn't that obvious? Look how elementary my physics understanding is!

Look, the reason I'm inquiring about this is because I'm doing an essay competition where we write a logical defence of a known false hood, this is one of the ideas I'm researching to decide what to do.

7. Oct 13, 2005

### mezarashi

I think the reason we look at it from such a view is because there is high symmetry in the motion of the planets if we do so. If we take the Earth as the reference coordinate system, you will find the planets move in some very complex motion paths instead of circular (elliptical) orbits which are highly predictable and thus useful. As usual, the laws of physics would hold in any coordinate frame you choose. Try solving more problems in mechanics, and you'll see... why would you choose a coordinate system that makes solving your problems difficult? :P

I've heard about this many times. Now I'm just wondering. If I were on a rocket free falling towards Earth from a billion miles away, what kind of experiment could I do to tell me that I was in fact accelerating towards Earth and not just floating in space.

8. Oct 13, 2005

### Smurf

Sweet. I was right.

9. Oct 13, 2005

### Smurf

When you accelerate you experience G's.... i think.

Can that be right? Do we experience G's on earth because we're accelerating around the sun? Maybe we're just used to it we don't notice, how would one conduct an experiment to figure that out?

Last edited: Oct 13, 2005
10. Oct 14, 2005

### Geoff St. Germaine

Sure we do. I don't think that we're so used to it that we don't notice it, it is more that the force acting on a person is so much smaller than the force due to the earth's mass that we don't notice it. You could calculate the force straightforwardly and if there's a sensitive enough technique you could measure it. The only thing would be that you would need to take so many other things into consideration.

11. Oct 14, 2005

### Tide

Normally, we don't feel gravity when we're in free fall. However, the affect of that acceleration can be "felt" if you're large enough, like the entire planet, because there is a differential in the rate of falling or acceleration. It is called ... <drum roll> ... the Tide! :)

12. Oct 14, 2005

### mezarashi

Aha! That certainly would be a good way to detect acceleration due to gravity... given you have sensitive enough equipment =D