# Orbit theory

1. Oct 12, 2012

### jackoblacko

I am under the impression that the earth orbiting the sun is really a matter of perspective and we can actually take a perspective in which the earth is stationary and the sun and everything else move in relation to it. And we can take any perspective we want but we just have to use fictional forces etc.

I am under the understanding that the earth orbits the sun no more then the sun orbits the earth. And I'm not talking about a Barycentre, that would be a third perspective, none of which is more true than the other.

Is this wrong?

2. Oct 12, 2012

### tom.stoer

With a star of infinite mass there is a unique perspective: the star is at rest (or is defining an inertial frame of reference), and the planet is orbiting the star. With a star of finite mass both the star and the plant are orbiting the center of mass (which again defines an inertial frame of reference in the absence of other forces)

Last edited: Oct 12, 2012
3. Oct 12, 2012

### jackoblacko

forgive me for feeling like you didn't address my question

4. Oct 12, 2012

### tom.stoer

I think I did

5. Oct 12, 2012

### jackoblacko

so is that a yes or no?

6. Oct 12, 2012

### tom.stoer

7. Oct 12, 2012

### jackoblacko

What are you assuming that I did when I read the answer?

8. Oct 12, 2012

### tom.stoer

OK, let's try again.

What I am saying is that indeed the two bodies are both orbiting the c.o.m.; therefore there is no preferred perspective "earth" or "sun", but there is a preferred perspective i.e. frame of reference defined by the c.o.m. which defines an inertial system (whereas earth and sun don't).

It's different if the sun would have infinite mass b/c then the rest frame of the sun is identical with the c.o.m. inertial frame.

The question with fictitious forces is difficult. In the most general sense (in general relativity) there is a class of equivalent frames, namely all frames defined by free falling bodies (in our case c.o.m., earth and sun and infinily many more). All description of the motion of earth and sun w.r.t. to these frames are equivalent. But you don't need any fictitous forces.

Then there is a different class af reference frames, namely all frames defined by accelerated bodies (i.e. not free falling bodies). Of course you can describe the motion of c.o.m., sun and earth w.r.t. to these frames, but now this description is physically different b/c in these frames you can feel acceleration, i.e. a force.

So I would say that we can indeed take any perspective we want (and transform all equations accordingly, using fictitious forces etc.), but there are two "classes of perspectives", one defined by free falling bodies (c.o.m, sun, earth, ...), and one defined by accelerated bodies (starting rockets, ...). These two are different!

9. Oct 12, 2012

### pervect

Staff Emeritus
It really rather depends on what you mean by "perspective". If we interpret "perspective" as "a description using generalized coordinates", I believe the answer would be yes, since one can assign generalized coordinates in any way one likes, though I haven't tried to construct any such description.

Such a "perspective" will be somewhat artificial. If you for instance use Fermi Normal coordinates, you'll find that your coordinate system with the "stationary Earth" covers only a small region of space-time. A coordinate system with a "stationary sun" will cover a greater region, one with a stationary barycenter will be even better.

10. Oct 12, 2012

### jackoblacko

Thank you! Way more helpful than your last couple tries ;p

I'm bright but uneducated, I don't think I understand this paragraph.

Now is mass relative because it changes when we move, or thats silly?

Does this paragraph support what I'm saying or go against it?

which is reality?

see this is why I tell people I am right, because you clearly state it so......

Now here are you suggesting I am wrong? Very unclear.

I feel like you are saying that it depends on how you look at it.

Isn't my question "Does it depend how you look at it or do we know for sure in reality the earth orbits the sun?"

I'm not be sarcastic or anything, its hard for me to understand your answers.

thx for your time

11. Oct 12, 2012

### jackoblacko

How else might we define perspective.

No of course not, theres no real world application according to schools.

Is this true and something I can't understand, or is it that Fermi Coo's, when applied to stationary earth would make the stars spin etc.

I realize some of my question are going to make sense because I don't understand what I am talking about but by asking them perhaps you can extract my level of knowledge

Thx for your time!!!!

12. Oct 12, 2012

### tom.stoer

The way looking at it depends on how we define 'preferred frames of reference'; and there's a big differenc between Newtonian mechanics and general relativity. So I think in order to proceed we first have to define the context, therefore my question is what is the context: 1) Newtonian mechanics or 2) general relativity?

13. Oct 12, 2012

### Mentz114

The Earth really is orbiting the sun in the sense that any observer who correctly measures the motions of the sun and Earth ( obviously using his own clocks and rulers) will conclude that the Earth is bound to the sun and travelling in an orbit around it.

Depends what you call 'real'.

Last edited: Oct 12, 2012
14. Oct 12, 2012

### jackoblacko

I may be confused but my understanding is if it depends on definition X or Y, then my belief that the truth is relative to perspective is correct.

I mean which one am I referring to? I am asking about reality, I don't know which that points too.

This relates to the observations of the double slit experiment, but I'll admit my understanding is 'cartoonish'.

15. Oct 12, 2012

### jackoblacko

But if this observers perspective is changed his observed results will change.

So if I look to the sky with a clock and a ruler, will I not conclude the earth is stationary and the sun is moving around it?

16. Oct 12, 2012

### Mentz114

In physics, theories are formulated so that the 'truths' are not dependent on perspective.

If you see a cow in the distance, does it bother you that it looks smaller than the one in the front yard ?

17. Oct 12, 2012

### jackoblacko

Does this exclude relativity and quantum physics?

18. Oct 12, 2012

### Mentz114

I don't know about QM, but the whole purpose of relativity theory is to enable theories to make the correct predictions independent of frame of reference. So, just the opposite of being excluded on those grounds.

One of the strongest motivations for SR was to resolve the apparent failure of Maxwell's equations in Gallilean relativity, for instance.

A charged particle moving in an electric field follows a certain path, and without using the relativistic version of EM, a moving frame would predict a different path, possibly resulting in a paradox ( did the electron go through the hole ?). With rhe relativistic version this cannot happen. So the truth ( eg the electron did go through the hole) is preserved in all frames.

I've been using sloppy language to avoid a wordstorm, but this is the nub of it.

19. Oct 12, 2012

### jackoblacko

I thought relativity meant: Car A travels at 20mph, car b at 30mph both relative to a stationary tree. Car b is going 10mph relative to car a. Relativity says car b is going both 30mph, and 10mph, and Xmph because the earth is moving, and its stationary etc.

This can be said with its size to as it shrinks relate to the tree as it moves etc.

Hopefully that wasn't complete gibberish to you.

20. Oct 12, 2012

### Mentz114

You need to find out more about relativity from a good source. I'm sure someone can recommend a suitable text or online resource.