# Does the Solar System spin around an axis and if so where is it?

1. Sep 24, 2014

### Darkan9el

Hi, Does the solar system spin around an axis as it travels through space, much like a planet rotates as it orbits the sun and if it does where is the axis point.

I know the solar system is orbiting the centre of the galaxy, I was just wondering if it behaved like a planet because things tend to follow the same rules.

2. Sep 24, 2014

### Bandersnatch

Hi Darkan9el, welcome to PF!

When we talk about the rotation of a rigid body, like a planet, it's easy to say where the axis of rotation is located, as no matter which point on or inside the planet you choose, it will rotate around the same axis.

With solar system, it's a bit more complicated, as the particles(i.e., planets, asteroids, the Sun, comets, gas) comprising the system are not rigidly bound together, so they can move in orbits whose axes of rotation each point somewhere else.

It is thus impossible to name a single axis that would fit all the celestial objects.

However, due to the dynamics of the formation of the solar system, all planets ended up orbiting in nearly the same orbital plane, very close to that of the Earth. As a first approximation then, it's possible to say something like "the line perpendicular to the ecliptic(Earth's orbital plane) passing through the Sun is approximately the axis around which most objects in the solar system orbit".
Such an axis points towards the constellation Draco, pretty much in the centre of the first loop of the Dragon's serpentine body, counting from the head. There are no particularly bright stars there. The star Polaris is, unsurprisingly, 23.5 degrees away from this point.

A bit better, more rigorous approach would be to calculate the total angular momentum of the solar system, by adding component vectors from each of the major bodies(Sun, planets) in the system. The direction of the resultant vector wouldn't differ from the previous approach by more than a few degrees, though.

3. Sep 24, 2014

### Darkan9el

Thank you for your reply, I was taking the solar system as a whole, as an entity a bit like if you have separate objects in an image then you group them and all of them rotate together but obviously all the different components move within this group and solar system group ends up a bit like a squashed transparent ellipse, that kind of occupies space like a bubble on water and creates eddy's as its moves, I don't know I've always perceived space as a kind of liquid, just not very dense.

One of the idea's I was looking at was does the sun move around the earth but in a less perceptible way, while the earth orbits the sun.

4. Sep 24, 2014

### Bandersnatch

Only in the sense of the two bodies orbiting their mutual centre of mass. It lies well within the Sun, but not at its centre.

I'm not sure I understand your previous statement. It looks like an analogy of the angular momentum approach.

5. Sep 24, 2014

### Barakn

I would think the barycenter (solar system's center of mass) would make a good point along the putative rotational axis. This point follows a straight line from the point of view of a distant observer in an inertial frame of reference.

6. Sep 24, 2014

### mathman

With respect to the center of the galaxy, the solar system moves in an approximately elliptic orbit.

7. Sep 24, 2014

### Barakn

Unless we can find a 200-million year old observer who is so far away from the Milky Way as to be outside of its gravity well, I think it's safe to ignore the sun's orbit.

8. Sep 25, 2014

### Dryson

Wouldn't our solar systems rotation be governed by a larger gravitational force that could in fact create a solar system rotation around the larger gravitational force of many solar systems? How much larger would that gravitational force need to be and how many other solar systems would rotate around it much like the planets of our solar system rotate around our sun?

9. Sep 25, 2014

### KL7AJ

Since 99% of the mass our our solar system is in the Sun itself, the rotation of the sun around its own axis would pretty well qualify. IN the big picture, our solar system is remarkably round. :)

10. Sep 25, 2014

### Bandersnatch

The Sun may have the most mass, but most of the angular momentum is in the planets.

11. Sep 25, 2014

### KL7AJ

I did not know that. Interesting if true!

12. Sep 25, 2014

### Bandersnatch

Well, let's calculate.

We'll compare the Sun's and Jupiter's contributions.

Let's use units of solar radii for distance, Jupiter's mass for mass and days for time.
Angular momentum is $L=I\omega$
Angular velocity is $\omega=2\pi/T$
Moment of inertia of a uniform ball is $I=2/5 mr^2$
The Sun is not a uniform ball, nor is it rotating uniformly across its volume, but by assuming unformity we'll get an easy to calculate ballpark result that's goint to set an upper bound on the real value.

In our chosen units, solar mass=1000, r=1, T=25
we get $L=2/5*1000*1^2*2\pi/25=~1000$

A point mass in orbit has got moment of inertia $I=mr^2$ (there's nothing to stop us from treating planets as point masses).
For jupiter, m=1 r=1000 T=4300
we get $L=1*1000^2*2\pi/4300=~1400$

So Jupiter's orbital motion alone contributes more to total angular momentum of the solar system than the Sun does.

13. Sep 25, 2014

### Barakn

That ignores the motion of the sun around the barycenter of the solar system about once every Jovian year or 11.86 Earth years. The distance of the sun from the barycenter varies from 0 to ~2 solar radii. Unfortunately this means the sun's orbital angular momentum around the barycenter is not constant. But if we assume the average distance2 is 1 solar radius2 (and a sun with point-like mass), then L = 1000 * 12 *2pi/12 = 524

14. Sep 26, 2014

### Bandersnatch

The result is in different units than the previous ones. Time should be in days, so you end up with $L=1000*1^2*2\pi/4300=1.4$ i.e., completely negligible contribution.

(just as is the contributrion of Jupiter's rotation)

15. Sep 27, 2014

### tony873004

Here's a simulation of the Sun going around the center of the solar system:
http://orbitsimulator.com/gravitySimulatorCloud/ssBarycenter.html

After you load the page, you need to press the Play button under the words "time step".

You can also switch the focus object to Earth, and watch Earth and Moon orbit the Earth/Moon barycenter.
You can also switch the focus object to Pluto and watch Pluto and its moons orbit their barycenter.
You might want to slow the time step and zoom in the screen width for Earth and Pluto.

16. Sep 27, 2014

### TumblingDice

This is pretty cool. And easy to observe the influence the planets and their orbits have on Sol. I watched out for a hundred years or so, enough to see the non-uniform, 'curly-Q' path for almost two cycles. I'm guessing that even the small stuff like moons, comets, meteors and asteroids would contribute small, but ever-changing influences to the system, so the Sun's path will never complete a fancy, 'Spirograph' type design. :p

17. Dec 5, 2014

### Subluminal

The simulation suggests some periodic changes to the force vector and the resultant acceleration on the sun as it revolves around the solar system. One has to wonder what effect all this has on its inner parts of varying density, and on the system creating sunspots and other magnetic fields. If the sun were completely axisymmetrical, it shouldn't produce a self-exciting dynamo (Cowling antidynamo theorem).

18. Dec 5, 2014

### Barakn

Bandersnatch - good call. I must have posted that calculation in haste. Subluminal - per Einstein the sun is following a straight line in space time so despite it's apparent wandering it is in free-fall. There will be tidal forces produced by the planets, but if you calculate their effect, they produce tides on the order of a millimeter on the sun.