# B Why not the sun revolves around the Earth?

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1. Mar 9, 2016

### hackhard

why was earth considered to revolve around the sun and nt the other way round?
why is it wrong to analyze planetary motion from earth frame?

2. Mar 9, 2016

### A.T.

It's not wrong, but the system's center of mass frame is closer to an inertial frame, making the math simpler.

3. Mar 9, 2016

### David Neves

I can't find it now but I remember reading somewhere about someone lamenting the supposed poor education of our kids by claiming that such-and-such percentage of kids couldn't "correctly" answer whether the Sun orbits the Earth, or the Earth orbits the Sun. However, of course, the statement "The Earth orbits the Sun" and "The Sun orbits the Earth" are equally true! They are each true in different reference frames. Of course the center of mass of the Earth-Sun system is very close to the center of the Sun. Also, it's a known fact that the Ptolemaic model is definitely not true. However, the real problem is that the public implicitly assumes that there is a universal frame of reference. Humans evolved on the surface of the Earth, where the ground you are walking on is an always present obvious frame of reference, so that shapes our psychology.

4. Mar 9, 2016

### hackhard

can you please explain how ?

5. Mar 9, 2016

### A.T.

Look up inertial and non-inertial frames of reference.

6. Mar 9, 2016

### hackhard

so analyzing motion from sun frame saves from worying about pseudo forces

7. Mar 10, 2016

### Guneykan Ozgul

Let me explain how. As David Neves explained sun and earth is in fact revolving around their common center of mass and to analyze this motion you also need to consider the position of the center of mass and position of sun and earth. However since the sun is much more massive than earth, the center of mass of this system is inside the sun. So we can say that the earth is revolving around the sun(not center of sun but very close to it). So even though the center of mass is not inertial we can approximate it because in that way we can take position of center of mass constant and this makes life easier because we don't deal with a function of time anymore but a constant.

8. Mar 11, 2016

### vanhees71

Note that the center-mass frame is not precisely the rest frame of the sun, although it's very close, because the mass of the sun is much larger than the mass of the earth, but more precise is to say that both the earth and the sun orbit around their common center of mass. The reason is that in this frame the two-body problem reduces to an effective one-body problem, namely that of a mass $\mu=m_1 m_2/(m_1+m_2)$ orbiting around the origin in a gravitational potential $$V(r)=-\Gamma m_1 m_2/r=-\Gamma \mu (m_1+m_2)/r.$$

9. Mar 13, 2016

### Ronie Bayron

If only that they can find a fixed reference in the universe, then we may able say what is moving around what. With respect to that reference, chances are the sun might be revolving around the earth or may be the earth. What are your thoughts on this?

Is it not right that massive objects revolves around the lighter ones in a vacuum? What are the odds?

Last edited: Mar 13, 2016
10. Mar 13, 2016

### hackhard

depends on choice of frame
isolated particles can appear to revolve and revolving particles can appear to purely translate when viewed from certain non-inertial frames

11. Mar 14, 2016

### Ronie Bayron

I think Galileo was half right for that fact--(not unless rebutted by someone)