How do we orbit the sun when you can set your origin anywhere...

1. Oct 22, 2015

houlahound

As I understand it a coordinate origin is just chosen for convenience. So how can we state the earth revolves around the sun when you could make coordinate origin at any point in the universe??

2. Oct 22, 2015

andresB

In classical physics there is a distinction between inertial and the rest frame of reference. If you put the coordinate systems in the earth you have to introduce several fictitious forces to describe the movement of the objects from the earth point of view. As the sun being more close to an inertial system is more natural to say that the earth revolves around the sun. There are many other good reasons, I'm sure other people can provide a list.

However, the question is particularly interesting in general relativity. Some people think (with good reasons) that the question of heliocentrism is inconsequential
http://blogs.discovermagazine.com/c...s-the-earth-move-around-the-sun/#.ViiMpX4vfIU

3. Oct 22, 2015

phinds

You CAN choose your coordinate system anywhere you like, and in that coordinate system, the Earth will revolve around the sun. Also, in that coordinate system, the Sun revolves around the center of the Milky Way. These things represent physical reality.

4. Oct 22, 2015

houlahound

If I make my inertial frame earth I am at rest relative to it and the sun is moving around me...is it not.

Same for any point?

5. Oct 22, 2015

andresB

You are not free to choose what frame is inertial.

6. Oct 22, 2015

houlahound

But I am free to choose where I make the coordinate origin and have everything move relative to that point...I think.

7. Oct 22, 2015

PeroK

Yes, you are. Not least because you observe the movement of the sun, moon and stars from the Earth. So, whatever theory and calculations you use to explain that motion (e.g. the heliocentric view of the solar system), you then have to map that to an Earth-centric view in order to describe what you will observe from Earth.

Both coordinate systems are equally "valid". As is any other.

8. Oct 22, 2015

Nidum

The ancients had an extensive theory of Earth centered celestial mechanics and it was many years before the more correct modern day theory was developed .

https://en.wikipedia.org/wiki/Geocentric_model

Last edited: Oct 22, 2015
9. Oct 22, 2015

houlahound

So then why we claim one way or the other if both are equally valid, precisely my original question??

10. Oct 22, 2015

PeroK

The main reason for the heliocentric model is to explain the solar system. The simplest explanation is found by considering the sun at the centre and the planets moving in ellipses, under the force of the sun's gravity.

It's then tempting to say that this is a preferred or natural reference frame and that in reality the Earth orbits the sun. But, in fact, it's simply the reference frame in which the overall motion of the solar system is easiest to explain.

11. Oct 22, 2015

Staff: Mentor

I believe there is a difference between arbitrarily choosing the origin of your coordinate system, and saying that Earth revolving around the Sun is as equally valid as saying the Sun revolves around the Earth. The former is easy. You just assign coordinates to all observable objects and events relative to your origin. The latter is questionable at best. As AndresB said in post #2, you would have to invent some complicated fictitious forces to explain how the Sun revolves around the Earth. The explanation that the Earth revolves around the Sun according to the laws of gravity is much, MUCH simpler and intuitive, and is therefore the explanation of choice for science.

12. Oct 22, 2015

houlahound

Thanks I get that's the case ie the most commonsense approach.

I over interpreted the Ptolemy argument.

I guess the point is there is no "special" reference frame eg the earth the centre of the universe, anywhere you would like the centre to be is the centre.

I think I have improved my understanding.

Thank you all.

Moving reference frames always gave me a headache at college, Kepler is good enough for my math ability, don't want anything more.

13. Oct 22, 2015

Staff: Mentor

Changing the coordinate system does not change the actual motion - it is just a different way to calculate the same thing. If you choose a coordinate system that doesn't fit well with the actual motion (ie, with the sun at the center), it makes the calculations harder.

14. Oct 22, 2015

mathman

Motion problems are best addressed by using the center of mass of the bodies concerned as the origin for the system. For the solar system, this is essentially the center of the sun.

15. Oct 23, 2015

BobG

The choice of your coordinate system won't change what is actually happening. It changes your point of view.

You could think of it as lining up a pool shot. Looking down the pool stick through the cue ball to the 8-ball you want to pocket, it could be pretty tough to figure out where you have to hit the 8-ball. But if you line yourself up with the 8-ball and the pocket, you can see exactly where you need to hit the 8-ball. Then you stay focused on that spot and change your point of view back to the looking down the pool stick, through the cue ball to spot on the 8-ball you want to hit.

Some things are just easier to figure out in one coordinate system than the other.

16. Oct 23, 2015

houlahound

I just threw a heavy and light ball tied by a rope, one does orbit more than the other. Got nearly three ball "years" before my solar system crashed.

17. Oct 23, 2015

BobG

You mean they both "orbited" their combined center of mass? And the combined center of mass was closer to the larger one? Because they should both "orbit" at the same rate, since its the system that's rotating.

18. Oct 23, 2015

houlahound

Aaaah yep, that exactly what I meant :-)

19. Oct 26, 2015

rude man

Quite right. In fact, the Sun orbits the Earth, as the Ancients figured out a long, long time ago. Shame on Copernicus and Galileo for opposing Church doctrine!

20. Oct 26, 2015

MidiMagic

The earth is not an inertial reference frame. It is a constantly accelerating reference frame:

As the earth orbits the sun it is constantly accelerated toward the sun by the centripetal pull of gravity by the sun, causing it to follow the path of its orbit.

In addition, any reference point on the surface of the earth is also accelerated by the gravity of the earth and the rotation of the earth.

Add to that, the motion of the sun in orbit around the galaxy center.

If you set a fixed point in space, the earth would rapidly move away from it.

Actually, you can't set a fixed point in space because the universe is expanding.