# Earth moving around Sun?

1. Dec 25, 2004

### broegger

Hi, I have a question that and I am unable to come up with a consistent and simple answer to.

If we consider a coordinate system centered at the earth, then all planets, the sun and indeed the rest of the universe is rotating around the earth in a very complicated way (as measured in this system).

Now if we instead consider a coordinate system centered at the Sun, then the planets move in nice elliptical paths (as measured in this system).

Now, my question is: Is these descriptions in fact equivalent, but we prefer the latter because of its simplicity or is the latter the right description and the former the wrong? I know that the question of acceleration (because of the rotation around the sun) should settle the argument in favor of the Copernican view, but is there a more "direct" method? What if I took a trip from Earth to outerspace and looked down at the planets orbiting the Sun - what would that actually prove?

2. Dec 25, 2004

### Integral

Staff Emeritus
If both can successfully predict the location of a planet at a given time why does one have to be correct and the other wrong? The difference is the simplicity and universal application of one model vs the complexity and unique application of the other.

The model which generates the motion of the planets around the sun also works to generate the motion of all observed bodies in the universe. The model to generate the motion of the planets around the earth works only for the the planets around the earth. There is no general principle that can be applied anywhere else. Which makes it of limited use, in fact completly useless, because we have a much simpler general model.

3. Dec 25, 2004

### DaveC426913

Concurring with Integral, not only can the heliocentric model predict the positions of the planets, but it also has a excellent and simple mechanism for *why* they do what they do.

Occam's razor - a valid scientific principle: All other things being equal, the simplest explanation - the one that requires the fewest tinkerings to stuff the peg in the hole - is the preferred one.

The geocentric model AFAIK doesn't have a plausible mechanism that backs up the observations. Without that, we're merely cavemen making up stories about angry gods. We might as well accept psychic powers and ghosts as part of science too.

4. Dec 25, 2004

### rayjohn01

Equivalent views

The law of inertia is that objects in continuous steady motion are at rest and that they depart from this only under the application of a force.
The main force of Gravity in the solar system is the sun not the planets.
In the geocentric scheme the planets move in ways which are not consistent with this , the mass of the earth does not warrant such a view .
If you could find a suitable observation point outside of the solar system but with the same translational velocity ( in the Galaxy) you would see that the planets and sun orbit each other but with far less motion of the sun underlying it's far greater mass.
It is the same reason we would not claim that all the stars revolve around the earth even if they appear to do so.
We do not have to go to the stars to observe this -- just suspend two unequal weights on a bar from a string -- if they balance then they will rotate without tipping from the same point which will be closer to the greater mass.
Ray.