[neilep (OP)]

Dearest Galaxiologists.

Thank ewe for all your hard work studying galaxies and stuff.

As a sheepy I of course know all there is to know about Galaxies, I've heard there's quite a lot of them. Maybe even more than 10 !!

I recently heard (without explanation) that the sun's orbit around the Milky way oscillates!!...what's that all about then ?
What causes the Sun to oscillate up and down in its orbit around the Milky Way galaxy ?..... completing a cycle of approximately 225-250 million years, whereas the planets in our solar system maintain relatively stable orbits around the Sun without such vertical oscillations?

I dunno !! (shrugging shoulders mode)




do you know ?




whajafink


Neil
xxxxxxxx









Sheepy gazed up with a grin
At the Sun's path, and its oscillation within
It dipped with a spin
Through the galaxy's din
Sheepy wondered, "What makes it swing?

[paul cotter]

Galactic potholes! Sorry neilep, I really have no idea and please excuse my frivolous remark.

[neilep (OP)]

Galactic potholes! Sorry neilep, I really have no idea and please excuse my frivolous remark.

Dear frivolous remark, ewe're exk'ewe'sed !!...Paul, love it chum...the more frivolity the better !!🐏

[Halc]

What causes the Sun to oscillate up and down in its orbit around the Milky Way galaxy ?

....  ]completing a cycle of approximately 225-250 million years, whereas the planets in our solar system maintain relatively stable orbits around the Sun without such vertical oscillations?

The galaxy is roughly a disk, and a disk, locally, is roughly a plane.  Any test mass in the neighborhood of a plane will oscillate back and forth through it (assuming it is sufficiently holy to let it through).  So that's why the sun goes up and down.  If it had no vertical motion near the center of that plane, it would not oscillate, but most stars have local motion.

As for the period, it is not well measured, but it goes up and down perhaps 4 times for each quarter-billion year orbit, tracing out a path like the edge of a tutu.


Another interesting fact:  A (reasonably large) plane of mass (say a million LY slice of swiss cheese) will attract objects at a fixed force, depending on the density of the plane mass.  That force is not a function of distance from the mass, assuming said distance is small compared to the size of the planar mass.  No n-squared rule.  Of course the force changes direction as you pass from one side of the plane to the other (presuming again that it has holes).

[neilep (OP)]

What causes the Sun to oscillate up and down in its orbit around the Milky Way galaxy ?

....  ]completing a cycle of approximately 225-250 million years, whereas the planets in our solar system maintain relatively stable orbits around the Sun without such vertical oscillations?

The galaxy is roughly a disk, and a disk, locally, is roughly a plane.  Any test mass in the neighborhood of a plane will oscillate back and forth through it (assuming it is sufficiently holy to let it through).  So that's why the sun goes up and down.  If it had no vertical motion near the center of that plane, it would not oscillate, but most stars have local motion.

As for the period, it is not well measured, but it goes up and down perhaps 4 times for each quarter-billion year orbit, tracing out a path like the edge of a tutu.


Another interesting fact:  A (reasonably large) plane of mass (say a million LY slice of swiss cheese) will attract objects at a fixed force, depending on the density of the plane mass.  That force is not a function of distance from the mass, assuming said distance is small compared to the size of the planar mass.  No n-squared rule.  Of course the force changes direction as you pass from one side of the plane to the other (presuming again that it has holes).

Thank You Hulk Halc


Thanks for the enlightening post! The tutu analogy is great, and your explanation of the galaxy's structure is really helpful. What is "test mass" ?

Now I want some Emmental !!

[Eternal Student]

Hi,

    Well I'm a bit slow, so I'm going to be the muppet that asks more questions and admits they don't understand all of this.

Another interesting fact:  A (reasonably large) plane of mass (say a million LY slice of swiss cheese) will attract objects at a fixed force, depending on the density of the plane mass.

    That bit I can understand.   It's a result most familiar to people who have studied electrostatics.    An infinite plane of positive charge creates an E field that is completely independant of the distance from the plane.    We have   E  =   σ/2ε = a constant value, directed away from the plane, e.g. to  "directly up" if you're above the plane and "directly down" if you're below the plane.



   We get a similar result for gravity (just with the gravitational field being toward the plane instead of away from it) noting that Coloumbs law for the repulsive force between two positive charges is identical to the gravitational law of attraction between two positive masses except for a change of sign (and the value of some constants).   

    I assume we're treating the entire Milky Way Galaxy as the large (approximated as infinite) plane of masses and not the much smaller solar system.  So, what I then don't understand is why that would make the sun oscillate up and down as described  but NOT the planets....  why would the planets be any different?

What causes the Sun to oscillate up and down in its orbit around the Milky Way galaxy ......  whereas the planets in our solar system maintain relatively stable orbits around the Sun without such vertical oscillations?[/size]

     Then it seemed there might be an explanation for that:   The plane of our solar system is tilted at an angle of about 60 degrees to the plane of the Milky Way.



   So most of the "oscillation up and down" that the planets should experience relative to the galactic plane as the whole solar system orbits around the milky way is actually going to look more like something else,  it would look like a wobble inwards and outwards to the sun.   Well, that's how it would look if you (the observer) orientated yourself in the plane of the solar system just for looking at the planets and how they move,  while orientating yourself in the plane of the galaxy to consider the motion of the sun around the galaxy.   I suppose there's some kind of logic for doing that, maybe.  I suppose the sun just wouldn't move at all if you centre and orientate yourself (the observer) in the plane of the solar system for that, so there's some sense in choosing to orientate yourself in the plane of the milky way just for that.

    If you did just keep yourself orientated in the plane of the milky way all the time, then I think the planets really would appear to oscillate up and down a bit and for much the same reasons as presented for the sun oscillating up and down.   However, I suppose that's a bit of moot point.... the planets would certainly appear to oscillate up and down in the galactic plane and very rapidly in comparison to the galactic rotation period just because that is exactly what their circular motion around the sun would look like when viewed "from the side".


Vertical Oscillation from circular motion viewed sideways.gif (304.07 kB . 729x640 - viewed 465 times)

Anyway, I don't know, that's just what I think.  I don't understand why the planets would be exempt from the same effects the sun was experiencing.   I suspect it's just a consequence of orientating yourself (the observer) in one plane for one thing but in a different plane for the other thing.

Best Wishes.

[Halc]

What is "test mass" ?

Any mass small in comparison to the thing that gravitationally shoving it around.  So our solar system is a test mass (effectively a particle) relative to the galaxy, and a planet is a test mass relative to the sun, although the planets do mass enough to move the sun away from the center of mass of the solar system.  That center of mass is within the sun somewhere around half the time on average.


I'll try to answer some of ES's questions.

An infinite plane of positive charge creates an E field that is completely independant of the distance from the plane.

Yes.  This is true under Newtonian mechanics, which suffices for something like our motion relative to the galaxy.  Under relativity, it is not possible to describe an infinite plane of material, so it doesn't hold.

I assume we're treating the entire Milky Way Galaxy as the large (approximated as infinite) plane of masses and not the much smaller solar system.

Yes, except the plane isn't very thin, so as we go up and down, more and more of the mass is one one side vs the other, meaning the force towards the plane is greatest the further we are from it.  The real trajectory is closer to a sin wave, whereas if the 'infinite plane' was really thin, it would be a series of parabolic curves and their inversions.

So, what I then don't understand is why that would make the sun oscillate up and down as described  but NOT the planets....  why would the planets be any different?

The planets are bound to the sun, so of course they go up and down (relative to the galaxy) with it.  What was said earlier is that planets don't have such oscillation relative to the solar system since the solar system is effectively a point mass, not a plane of uniform density.

What causes the Sun to oscillate up and down in its orbit around the Milky Way galaxy

     Then it seemed there might be an explanation for that:   The plane of our solar system is tilted at an angle of about 60 degrees to the plane of the Milky Way.

The orbital tilt has to do with the angular momentum of our solar system and has effectively nothing to do with the solar system's path around the galaxy.

The wobble above/below the galactic plane gets to about 300 light years each way, whereas the wobble of planetary motion due to their tilted orbit about the sun is a few light hours at best.

[Janus]

Hi,

    Well I'm a bit slow, so I'm going to be the muppet that asks more questions and admits they don't understand all of this.

Another interesting fact:  A (reasonably large) plane of mass (say a million LY slice of swiss cheese) will attract objects at a fixed force, depending on the density of the plane mass.

    That bit I can understand.   It's a result most familiar to people who have studied electrostatics.    An infinite plane of positive charge creates an E field that is completely independant of the distance from the plane.    We have   E  =   σ/2ε = a constant value, directed away from the plane, e.g. to  "directly up" if you're above the plane and "directly down" if you're below the plane.



   We get a similar result for gravity (just with the gravitational field being toward the plane instead of away from it) noting that Coloumbs law for the repulsive force between two positive charges is identical to the gravitational law of attraction between two positive masses except for a change of sign (and the value of some constants).   

    I assume we're treating the entire Milky Way Galaxy as the large (approximated as infinite) plane of masses and not the much smaller solar system.  So, what I then don't understand is why that would make the sun oscillate up and down as described  but NOT the planets....  why would the planets be any different?

What causes the Sun to oscillate up and down in its orbit around the Milky Way galaxy ......  whereas the planets in our solar system maintain relatively stable orbits around the Sun without such vertical oscillations?[/size]

     Then it seemed there might be an explanation for that:   The plane of our solar system is tilted at an angle of about 60 degrees to the plane of the Milky Way.



   So most of the "oscillation up and down" that the planets should experience relative to the galactic plane as the whole solar system orbits around the milky way is actually going to look more like something else,  it would look like a wobble inwards and outwards to the sun.   Well, that's how it would look if you (the observer) orientated yourself in the plane of the solar system just for looking at the planets and how they move,  while orientating yourself in the plane of the galaxy to consider the motion of the sun around the galaxy.   I suppose there's some kind of logic for doing that, maybe.  I suppose the sun just wouldn't move at all if you centre and orientate yourself (the observer) in the plane of the solar system for that, so there's some sense in choosing to orientate yourself in the plane of the milky way just for that.

    If you did just keep yourself orientated in the plane of the milky way all the time, then I think the planets really would appear to oscillate up and down a bit and for much the same reasons as presented for the sun oscillating up and down.   However, I suppose that's a bit of moot point.... the planets would certainly appear to oscillate up and down in the galactic plane and very rapidly in comparison to the galactic rotation period just because that is exactly what their circular motion around the sun would look like when viewed "from the side".


Vertical Oscillation from circular motion viewed sideways.gif (304.07 kB . 729x640 - viewed 465 times)

Anyway, I don't know, that's just what I think.  I don't understand why the planets would be exempt from the same effects the sun was experiencing.   I suspect it's just a consequence of orientating yourself (the observer) in one plane for one thing but in a different plane for the other thing.

Best Wishes.

The main difference between the planets orbiting the Sun, and the Sun orbiting the galaxy is the mass distribution. With the Solar system, 99.95% of the total mass is concentrated in the Sun, While with the galaxy, it is distributed throughout the galaxy ( Even the super massive BH at the center only makes up a tiny fraction of the whole mass. Remove it and you would not see any noticeable change).   So with the solar system the planets, in effect orbit a centrally located mass, while the Sun orbits within a spread out mass.

[Eternal Student]

Hi,

   OK,  I am getting it now.   Thanks everyone.

[paul cotter]

Me too, I did not get it from Halc's original post and had intended to query it but I was distracted at the time by various conversations with the forum's beloved pet sheep.